Chapter XXXVI: Section IV: STICHOCYRTIDA, Haeckel, 1862, Monogr. d. Radiol., p. 280, 312 (11)

Shell ovate, slightly compressed, with numerous (forty to fifty or more) short and thin, bristle-shaped, radial spines on the margin. Peristome obliquely truncate, with three curved spines; the dorsal odd spine subvertical, nearly as long as the radius, and much stronger than the two parallel, nearly horizontal, lateral spines.

_Dimensions._--Length of the shell 0.15, breadth 0.11; length of the peristome 0.1.

_Habitat._--North Pacific, Station 231, depth 2250 fathoms.

24. _Challengeron willemoesii_, n. sp. (Pl. 99, fig. 13).

Shell broadly-ovate or nearly circular, lenticular, with numerous (fifty to sixty) short and stout, conical, radial spines on the margin, increasing in size towards the aboral pole; the spines are alternately larger and smaller, the largest about half as long as the radius. Peristome helmet-shaped or half conical, longer than the radius of the shell, with two pairs of conical, curved, {1660}divergent teeth; the terminal teeth are the halves of a forked dorsal tooth and diverge laterally; the lower teeth are nearly horizontal and directed ventrally.

_Dimensions._--Diameter of the shell 0.13 to 0.15; length of the peristome 0.07 to 0.1.

_Habitat._--Tropical Pacific, Stations 274 to 276, depth 2350 to 2750 fathoms.

25. _Challengeron wyvillei_, n. sp. (Pl. 99, fig. 15).

Shell ovate, lenticular, strongly compressed, with numerous (forty to fifty) short, conical, radial spines on the margin; the spines are larger on the aboral side and spinulate. Peristome narrow, with two parallel, lateral, lamellar, forked teeth, about as long as the radius; the upper branch of the forks is sabre-shaped, obliquely truncate, subvertical, ciliate, and much broader than the lower nearly horizontal branch, which is directed towards the ventral side and curved upwards.

_Dimensions._--Length of the shell 0.12 to 0.16, breadth 0.1 to 0.14; length of the peristome 0.08 to 0.11.

_Habitat._--Tropical Atlantic, Station 347, depth 2250 fathoms.

Subfamily 2. PHARYNGELLIDA, Haeckel.

_Definition._--#Challengerida# with a pharynx, or an internal prominent mouth tube.

Genus 696. _Entocannula_,[313] Haeckel, 1879, Sitzungsb. med.-nat. Gesellsch. Jena, Dec. 12, p. 5.

_Definition._--#Challengerida# with a pharynx, without teeth on the mouth, and without marginal spines.

The genus _Entocannula_ and the two following genera represent together the subfamily Pharyngellida, differing from the preceding Lithogromida in the development of a peculiar pharynx, an internal, cylindrical, or funnel-shaped tube, which is conspicuous, leads from the mouth into the shell-cavity, and is provided with an inner and an outer opening. The Pharyngellida are much rarer than the Lithogromida; only ten species have been observed of the former, fifty of the latter. _Entocannula_ is the simplest form of the Pharyngellida, having a smooth shell, without aboral teeth and marginal spines; it differs from _Lithogromia_ only in the possession of the pharynx. One remarkable species is covered entirely with extremely thin, flinty hairs, and may represent therefore a peculiar genus, _Trichogromia_.

{1661}1. _Entocannula circularis_, n. sp.

Shell circular or nearly circular, lenticular, strongly compressed. Pharynx cylindrical, one-third or one-fourth as broad as the shell, twice as broad as long, its outer and inner apertures of equal breadth. Surface smooth.

_Dimensions._--Diameter of the shell 0.20 to 0.32, of the pharynx 0.07 to 0.09.

_Habitat._--North Pacific, Station 250, depth 3050 fathoms.

2. _Entocannula subglobosa_, n. sp.

_Challengeria bromleyi_, (_partim_), John Murray, 1879, _in schedulis_
Chall. Coll.

Shell circular, nearly spherical, slightly compressed. Pharynx funnel-shaped, truncate, conical, its outer opening twice as broad as the inner. Surface smooth.

_Dimensions._--Diameter of the shell 0.25 to 0.28, of the inner mouth 0.05.

_Habitat._--South Pacific, Station 289, depth 2550 fathoms.

3. _Entocannula infundibulum_, n. sp. (Pl. 99, fig. 19).

_Challengeria bromleyi_ (_partim_), John Murray, 1879, _in litteris_,
Narr. Chall. Exp., vol. i. p. 226, pl. A, fig. 5.

Shell ovate, truncate at the mouth, slightly compressed. Pharynx funnel-shaped, its outer aperture two to three times as broad as the inner. Surface smooth.

_Dimensions._--Length of the shell 0.22 to 0.28, breadth 0.2 to 0.25; inner mouth 0.04 to 0.08.

_Habitat._--South Atlantic, Station 318, depth 2040 fathoms.

4. _Entocannula hirsuta_, n. sp. (Pl. 99, fig. 20).

_Trichogromia hirsuta_, Haeckel, 1881, Manuscript.

Shell ovate. Pharynx cylindrical, one-fourth as broad as the shell, its inner and outer apertures of equal breadth. The whole surface of the shell, and mainly the outer mouth, is densely covered with thin, curved, siliceous bristles.

_Dimensions._--Length of the shell 0.27 to 0.36, breadth 0.2 to 0.3; mouth 0.05 to 0.08.

_Habitat._--North Atlantic, Færöe Channel (Gulf Stream), John Murray, depth 600 fathoms.

Genus 697. _Pharyngella_,[314] n. gen.

_Definition._--#Challengerida# with a pharynx, and with one or more teeth on the mouth, but without marginal spines.

The genus _Pharyngella_ differs from the preceding _Entocannula_, its ancestral form, in the development of one or more teeth on the peristome, and exhibits therefore the same relation to it that _Challengeria_ bears to _Lithogromia_. Only a few species of this genus have been observed, all in the Atlantic.

{1662}1. _Pharyngella monoceros_, n. sp.

Shell ovate, slightly compressed, with a single large tooth on the dorsal corner of the peristome. The tooth is cylindro-conical, half as long as the shell, straight or slightly curved, and lies in the main axis of the shell. Pharynx cylindrical, straight, half as long as the shell-radius. This species is very similar to _Challengeria tritonis_ (Pl. 99, fig. 5), and to the common cosmopolitan _Challengeria naresii_, but differs generically from them in the possession of the pharynx.

_Dimensions._--Diameter of the shell 0.25 to 0.28; length of the tooth 0.14.

_Habitat._--Tropical Atlantic, Station 348, depth 2450 fathoms.

2. _Pharyngella gastrula_, n. sp. (Pl. 99, fig. 18).

Shell ovate. Peristome with two parallel, vertical, straight, pointed teeth, which are half as long as the shell-radius. Pharynx of the same length, cylindrical or funnel-shaped, its outer aperture scarcely broader than the inner.

_Dimensions._--Length of the shell 0.3 to 36, breadth 0.24 to 0.3; teeth and pharynx 0.07 to 0.09.

_Habitat._--North Atlantic, Færöe Channel (Gulf Stream), John Murray, depth 600 fathoms.

3. _Pharyngella invaginata_, n. sp.

Shell circular, slightly compressed or nearly spherical. Peristome with two parallel, vertical, straight, pointed teeth, which are about as long as the shell-radius. Pharynx of the same length, funnel-shaped; its outer aperture twice as broad as the inner.

_Dimensions._--Diameter of the shell 0.2 to 0.22, teeth and pharynx 0.05.

_Habitat._--Tropical Atlantic, Station 347, depth 2250 fathoms.

4. _Pharyngella gastræa_, n. sp. (Pl. 99, fig. 17).

Shell broad, ovate or nearly circular, lenticular, strongly compressed. Peristome with two parallel, pointed teeth, which are nearly as long as the shell-radius, slightly curved, and obliquely inclined over the mouth. Pharynx half as long, cylindrical, obliquely descending; its outer and inner apertures of equal breadth.

_Dimensions._--Diameter of the shell 0.3 to 0.33, teeth 0.12, pharynx 0.06.

_Habitat._--South Atlantic, Stations 328 to 334, surface.

Genus 698. _Porcupinia_,[315] Haeckel, 1879, Sitzungsb. med.-nat. Gesellsch. Jena, Dec. 12, p. 5.

_Definition._--#Challengerida# with a pharynx, with teeth on the mouth and marginal spines.

The genus _Porcupinia_ differs from the preceding _Pharyngella_ in the development {1663}of marginal spines, and exhibits therefore the same relation to it that _Challengeron_ bears to _Challengeria_. Only two species have been observed; the first of these bears, besides the large marginal spines, numerous flinty bristles on the surface of the shell.

1. _Porcupinia aculeata_, n. sp.

Shell ovate, one and one-third times as long as broad, slightly compressed, covered with short, bristle-shaped spines over the whole surface, similar to _Entocannula hirsuta_ (Pl. 99, fig. 20). The keeled margin is armed with numerous long and stout radial spines, which are alternately larger and smaller, as in _Challengeron moseleyi_ (Pl. 99, fig. 14); the larger about as long as the radius, the smaller half as long. Peristome with two straight, parallel teeth. Pharynx funnel-shaped, half as long as the radius, its outer aperture twice as broad as the inner.

_Dimensions._--Length of the shell 0.24, breadth 0.18; peristome 0.05.

_Habitat._--Tropical Atlantic, Station 347, depth 2250 fathoms.

2. _Porcupinia cordiformis_, n. sp. (Pl. 99, fig. 16).

Shell heart-shaped, nearly triangular, with a deep, concave bosom at the broad oral base, and a bunch of five to nine divergent spines at the blunt aboral apex, the middle spine (in the main axis) larger than the others. Peristome with two parallel, short, triangular teeth, which are obliquely inclined over the mouth (their upper convex edge nearly horizontal). Pharynx cylindrical, short, curved, and obliquely descending.

_Dimensions._--Diameter of the shell 0.2 to 0.25, of the pharynx 0.05 to 0.08.

_Habitat._--South Atlantic, Station 332, depth 2200 fathoms.

Family LXXIX. #MEDUSETTIDA#, n. fam. (Pls. 118-120).

_Definition._--PHÆODARIA with a simple, ovate, hemispherical or cap-shaped lattice-shell of peculiar alveolate structure, and with hollow articulate feet surrounding the widely open mouth. Central capsule excentric, placed in the aboral half of the shell-cavity.

The family #Medusettida# represents a peculiar group of remarkable PHÆODARIA, which are similar to the Challengerida and Tuscarorida in the general form of the monaxonian shell and its apophyses, but differ from both families in the strange alveolate structure of the shell and of the hollow articulate feet surrounding its mouth. All the forms of this family here described are new, and not a single species was known before the discoveries of the Challenger. The majority seem to be inhabitants of the deep-sea, and occur mainly in those places where other families of PHÆODARIA (Aulacanthida, Aulosphærida, Coelodendrida) are also well represented; they are, however, nowhere so common as these latter families. Their long feet and the small delicate {1664}shell are extremely fragile, so that complete and well-preserved specimens are rare. Some species are among the most admirable forms of Radiolaria, and are similar to small elegant Medusæ, _e.g._, _Medusetta_ (Pl. 120, figs. 1-4), _Gazelletta_ (Pl. 118, fig. 1), and _Gorgonetta_ (Pl. 119).

The shell of the Medusettida is usually very small and delicate, in comparison to the long, often exceedingly large, feet, which depend from the margin of its mouth. From the size and the weight of these feet, we must suppose that the shell floats in the water in the same position as in the NASSELLARIA, so that the main axis is vertical, and the open mouth with the feet on its lower pole. The form of the shell exhibits the same varieties as the similar umbrella of the Medusæ. It is usually more or less hemispherical, sometimes flatter, cap-shaped, at other times more highly vaulted, campanulate or ovate. The similarity with the umbrella of a Medusa is so great, that in many species the large lower opening on the mouth of the shell is surrounded by a prominent ring or diaphragm, comparable to the velum of the Craspedotæ or Hydromedusæ (Pl. 120, figs. 3-4).

The structure of the shell, which we term shortly "alveolate," is very remarkable, and seems to be different from that of the other PHÆODARIA. Its reticulate appearance seems to indicate at first sight the usual lattice-shell, pierced by numerous very small pores (Pl. 118, figs. 1-3). But as soon as we make the shell dry, air always enters into its thin walls, and each apparent pore is found to be a small alveole or a separate compartment, which contains a small bubble of air (Pl. 120, figs. 11, 16). The thin wall of the shell is therefore double, composed of two parallel, very thin lamellæ of silica, which are little distant from one another, and are connected by a network of small rods or septa. This network is rarely regular, with square meshes (Pl. 118, fig. 3), or with hexagonal or triangular meshes (Pl. 116, fig. 2); it is usually irregular, with polygonal meshes (Pl. 120, figs. 11-16). The size of the meshes or alveoles is usually smallest on the aboral apex of the shell, and increases gradually towards the mouth. The largest alveoles are around the mouth, on the velum-shaped peristome, and here the marginal alveoles of the shell pass directly over into the separate alveoles or hollow compartments of the articulate feet (Pl. 118, fig. 1). Probably each alveole is a separate compartment, filled up by jelly, and has a small opening or pore on its inside; this pore is very distinct in the marginal alveoles of some larger species (Pl. 116, fig. 1_a_; Pl. 120, fig. 14), but I have not been able to recognise it everywhere. Moreover, it often happens that the alveoles are apparently completely closed. In some smaller species the alveolate structure seems to be present only in the peripheral part of the shell, whilst the central part is solid.

The outer convex surface of the shell is usually smooth, sometimes papillate (Pl. 116, fig. 1), at other times studded with small, conical, radial spines (Pl. 118, fig. 1; Pl. 120, figs. 4-11). Prolonged cylindrical spines or tubes are rarely {1665}scattered over the surface (Pl. 116, figs. 1, 2). In some genera a long vertical or oblique spine is developed from the apex of the shell, and is comparable to the "apical horn" of the NASSELLARIA (Pl. 118, figs. 2, 3; Pl. 120, figs. 1-4).

The mouth of the shell, or the large opening on the lower pole of its vertical main axis, is usually widely open, circular or polygonal, more rarely constricted. In many species the peristome is broadened and its margin developed in the form of a horizontal diaphragm, which constricts the entrance into the shell-cavity, and is comparable to the velum of the Hydromedusæ (Pl. 120, figs. 1-4). Sometimes the margin of the velum is fringed and bears numerous small dependent filaments of silica, comparable to the marginal tentacles of many Medusæ (Pl. 118, figs. 1, 3; Pl. 120, fig. 16). Rarely the mouth is strongly constricted and prolonged into a narrow, long, cylindrical tube, comparable to a proboscis; its opening is usually fringed (Pl. 116, figs. 1, 2).

The hollow articulate feet, or the large radial apophyses of the peristome, are the most remarkable organs of the Medusettida, and occur in the same form in no other family of Radiolaria. Their number is variable from three to twelve or more, and is here employed for the distinction of genera, as it is rather constant; the most frequent number is four (_Medusetta_) or six (_Gazelletta_). The feet are usually cylindrical, much longer than the shell, and more or less curved; more rarely they are straight or slightly compressed. They are always articulate, and divided by numerous transverse parallel septa into hollow compartments or alveoles; the transversely striated appearance of the feet is brought about by these septa. The alveoles have usually the form of short discoidal cylinders, and are broader than long (Pl. 120, figs. 5-13); rarely they are longer than broad (fig. 15). The septa between the alveoles are always perforated by a small opening and usually this opening is prolonged into a shorter or longer tubule, comparable to the siphuncle in the septa of the chambered shells of Cephalopoda; as in these latter, the succeeding siphons are arranged in a continuous series, which lies either in the axis of the foot or parallel to it (Pl. 120, figs. 8, 13, 15). The siphons are directed towards the distal end of the foot, so that, _e.g._, the siphon which arises from the septum between the ninth and tenth alveole, projects into the cavity of the latter. The communicating cavities of all the alveoles are filled up by jelly. In the dry shells, which are cleaned by hot mineral acids or by fire, and in which, therefore, the jelly is destroyed, the cavities become easily filled by air-bubbles (Pl. 120, figs. 8, 11, 15). Whilst the alveoles of each foot always form a long simple series, this series becomes doubled on the base of the larger feet, where they are broadened, and pass over into the compartments of the peristome (Pl. 122, figs. 10, 11, 14). The terminal alveole of each foot is closed.

The feet of the Medusettida are rarely quite simple and smooth (Pl. 120, figs. 1, 5, 7, 10, &c.). Usually they are armed with numerous spines or with elegant pencils of spathillæ. These secondary appendages or branches are also hollow, with a {1666}thin wall of silex; but they seem usually not to communicate with the alveoles of the foot directly; the cavities of the foot and its appendages are separated by a thin, solid septum. In _Medusetta_ the convex outer edge of each curved foot is studded with a series of simple alveolate branches (Pl. 120, figs. 2-4). In _Gazelletta_, the feet are usually armed with verticillate or alternate spines, which are either simple or branched (Pl. 120, figs. 11-15); sometimes each branch is elegantly arborescent (Pl. 118, fig. 1). Sometimes the distal end of each branch is armed with a spathilla or a coronet of recurved teeth. The branches attain their highest development in the admirable _Gorgonetta_ (Pl. 119). The twelve feet of this most interesting genus are differentiated into two different and alternating groups, six descending and six ascending. The six upper or ascending feet are arborescent, and each branch is armed at the distal end with a spathilla (figs. 1, 2). The six lower or descending feet are covered with most elegant arborescent pencils or anchor-bearing trees; each terminal branch of a tree is armed with two spathillæ, a smaller terminal and a larger below it (figs. 3, 4). The distal ends of the feet are rarely simple, usually they are forked or branched, or armed with peculiar spathillæ; and sometimes these terminal branches are very large (Pls. 118, 119).

The _central capsule_ of the Medusettida is usually subspherical or somewhat lenticular, and hidden in the aboral or upper part of the shell-cavity, whilst its oral or lower part is filled up by the phæodium (Pl. 120, figs. 2, 9, 10, 11). Sometimes the phæodium is very large and prominent at the aperture of the mouth (Pl. 118, figs. 2, 3; Pl. 119, fig. 1). The nucleus is usually ellipsoidal, half as large as the central capsule, and includes numerous nucleoli. The membrane of the central capsule seems in all Medusettida to possess only one opening, the astropyle or main-opening with a radiate operculum and proboscis, placed at the lower or oral pole (Pl. 120, fig. 2). I have never been able to observe any secondary opening or parapyle. The free space between the shell wall and the capsule is filled up by the calymma, which also includes the phæodium. The jelly of the calymma is probably in direct continuity with the jelly which fills up the alveoles of the shell-wall and of the articulate feet.

_Synopsis of the Genera of Medusettida._

I. Subfamily Euphysettida. { Three equal feet, 699. _Cortinetta_.
Peristome of the shell with {
a corona of three or four { Four equal feet, 700. _Medusetta_.
feet. Apex of the shell {
(usually) with an apical { One large and three
horn. { small feet, 701. _Euphysetta_.

{ Six descending feet, 702. _Gazelletta_.
{
II. Subfamily Gazellettida. { Six descending and six
Peristome of the shell with { ascending alternating
six to twelve or more feet. { feet, 703. _Gorgonetta_.
Apex of the shell (usually) {
without apical horn. { Numerous (ten to twenty
{ or more) descending
{ feet, 704. _Polypetta_.

{1667}Genus 699. _Cortinetta_,[316] n. gen.

_Definition._--#Medusettida# with three articulate feet on the peristome.

The genus Cortinetta comprises Medusettida which exhibit the minimum number of feet, three, and which therefore may be compared to the tripodal NASSELLARIA (_Cortina_, _Cortiniscus_, _Tripodiscus_, &c.). This similarity is the greater, as in the few observed species an apical horn is developed (as also in the following genus). The three feet are in the two species observed of equal size and similar form, and equidistant, so that they cannot be distinguished as an odd caudal and two paired lateral feet, as in the similar NASSELLARIA.

1. _Cortinetta tripodiscus_, n. sp. (Pl. 117, fig. 7).

Shell campanulate, covered with numerous curved ascending bristles, with a prominent annular velum on the peristome, similar to that of _Medusetta craspedota_. (Pl. 120, fig. 4). Feet equal, widely divergent and equidistant, somewhat longer than the shell, slightly curved, and covered on their convex outer edge with a series of short lateral branches, the three ramules of which bear a spathilla. Apical horn conical, straight, about as long as the shell, also covered with curved bristles.

_Dimensions._--Length of the shell 0.14, breadth 0.11; length of the feet 0.16, of the apical horn 0.11.

_Habitat._--Central Pacific, Station 271, depth 2425 fathoms.

2. _Cortinetta cortiniscus_, n. sp.

Shell campanulate, subspherical, thorny, with a prominent annular velum on the peristome. Feet equal, very large, widely divergent, about three times as long as the shell and studded with arborescent branches, similar to those of _Gazelletta drymonema_ (Pl. 118, fig. 1). Apical horn slender, conical, straight, thorny, twice as long as the shell.

_Dimensions._--Length of the shell 0.18, breadth 0.16; length of the feet 0.5 to 0.6, of the apical horn 0.33.

_Habitat._--Central Pacific, Station 274, depth 2750 fathoms.

Genus 700. _Medusetta_,[317] n. gen.

_Definition._--#Medusettida# with four equidistant articulate feet of equal size on the peristome.

The genus _Medusetta_ comprises those forms of the family which bear on the peristome four equal radial feet, and therefore exhibit a striking similarity to many {1668}small Medusæ (_e.g._, _Codonium_, _Tiara_, _Sarsia_, &c.). This similarity is the greater, as usually the four feet are elegantly curved and the entrance in the shell-cavity partly closed by a marginal diaphragm, comparable to the velum of the Craspedotæ or Hydromedusæ. In all the species observed the apex of the shell bears a slender vertical or curved spine, comparable to the apical horn of the NASSELLARIA. In most species the feet bear on their convex outside a series of teeth or branches, which are also alveolate.

1. _Medusetta codonium_, n. sp. (Pl. 120, fig. 1).

Shell ovate, smooth, twice as broad as the constricted mouth. Apical horn about as long as the shell, nearly straight, smooth. Feet cylindrical, smooth, irregularly curved, about as long as the shell.

_Dimensions._--Length of the shell 0.08, breadth 0.05.

_Habitat._--Tropical Atlantic, Station 347, depth 2250 fathoms.

2. _Medusetta tiara_, n. sp.

Shell campanulate, smooth, about as long as broad, and three times as broad as the constricted mouth, which is surrounded by a horizontal broad ring, like the velum of a Hydromedusa. Apical horn conical, straight, half as long as the shell. Feet divergent, irregularly curved, twice as long as the shell.

_Dimensions._--Length of the shell 0.07, breadth 0.06.

_Habitat._--North Atlantic (Færöe Channel), John Murray, surface.

3. _Medusetta minima_, n. sp.

Shell slenderly ovate, smooth, about twice as long as broad, twice as broad as the constricted mouth. Apical horn straight, vertical, of the same length. Feet somewhat longer, very thin, slightly curved at the base, in the distal half nearly parallel and descending vertically.

_Dimensions._--Length of the shell 0.06, breadth 0.03.

_Habitat._--South Atlantic, Station 332, depth 2200 fathoms.

4. _Medusetta quadrigata_, n. sp. (Pl. 120, fig. 2).

Shell ovate, campanulate, smooth, little broader than the wide open mouth. Apical horn longer than the shell, more or less curved, smooth. Feet cylindrical, about as long as the shell, strongly curved and somewhat spirally revoluted, with a series of short curved teeth on the abaxial margin.

_Dimensions._--Length of the shell 0.07 to 0.09, breadth 0.04 to 0.06.

_Habitat._--North Pacific (off Japan), Stations 231 to 237, surface.

{1669}5. _Medusetta tetranema_, n. sp. (Pl. 120, fig. 3).

Shell campanulate, conical, smooth, nearly twice as broad as the constricted mouth, without prominent velum. Apical horn somewhat shorter, straight, conical, thorny. Feet strongly curved inwards, convergent, twice as long as the shell, each with four to six strong, ascending, alveolate branches on the abaxial margin.

_Dimensions._--Length of the shell 0.07 to 0.09, breadth 0.05 to 0.07.

_Habitat._--Tropical Atlantic, Station 352, surface.

6. _Medusetta spiralis_, n. sp.

Shell hemispherical, thorny, with wide open mouth. Apical horn straight, vertical, thorny, twice as long as the shell; feet cylindrical, three times as long as the shell, spirally convoluted around the vertical axis of the shell, with a series of recurved teeth on the abaxial margin.

_Dimensions._--Length of the shell 0.05, breadth 0.08.

_Habitat._--Central Pacific, Stations 271 to 272, surface.

7. _Medusetta craspedota_, n. sp. (Pl. 120, fig. 4).

Shell campanulate or nearly spherical, spiny, twice as broad as the constricted mouth, which is surrounded by a funnel-shaped truncate velum. Apical horn conical, straight, spiny, half as long as the shell. Feet strongly curved, longer than the shell, with a series of short conical spines on the abaxial side.

_Dimensions._--Length of the shell 0.06, breadth 0.06.

_Habitat._--North Pacific, Station 239, surface.

Genus 701. _Euphysetta_,[318] n. gen.

_Definition._--#Medusettida# with four articulate feet on the peristome, one odd very large, and three small or rudimentary feet.

The genus _Euphysetta_ agrees with the preceding _Medusetta_ in the possession of four articulate feet; but whilst these in the latter are of equal size and similar shape, in the former a single odd foot only is developed, and very large, whilst the three others are rudimentary and very small. _Euphysetta_ exhibits therefore the same relation to _Medusetta_ that the Euphysidæ (_Euphysa_, _Steenstrupia_, &c.) bear to the Sarsiadæ (_Codonium_, _Sarsia_), among the Hydromedusæ (compare my System der Medusen, 1879, vol. i. p. 12).

{1670}1. _Euphysetta staurocodon_, n. sp. (Pl. 118, fig. 2).

Shell ovate, nearly as broad as long, smooth, with an oblique, slender, conical, apical horn of the same length. The larger odd foot cylindrical, curved, somewhat longer than the shell, with three diverging branches at the distal end. The three smaller feet very short, representing the form of a regular rectangular cross.

_Dimensions._--Length of the shell 0.15, breadth 0.14.

_Habitat._--Tropical Atlantic, Station 348, depth 2450 fathoms.

2. _Euphysetta hybocodon_, n. sp.

Shell ovate, one and one-third times as long as broad, smooth, with an oblique, stout, conical horn of half the length. The larger odd foot cylindrical, curved, twice as long as the shell, with a few pairs of short lateral branches and forked distal end. The three smaller feet half as long as the shell, thorny.

_Dimensions._--Length of the shell 0.16, breadth 0.12.

_Habitat._--Tropical Atlantic, Station 347, depth 2250 fathoms.

3. _Euphysetta amphicodon_, n. sp. (Pl. 118, fig. 3).

Shell subspherical, smooth, with a short, oblique, conical, apical horn. The larger odd foot cylindrical, nearly straight, as long as the shell, with a few short lateral branches, at the distal end forked. The three smaller feet were in one specimen observed forked, in another (the specimen figured) replaced by nine small thorns.

_Dimensions._--Length of the shell 0.11, breadth 0.13.

_Habitat._--South Atlantic, Station 332, depth 2200 fathoms.

Genus 702. _Gazelletta_,[319] John Murray, 1876 (_in litteris_).

_Definition._--#Medusettida# with six articulate feet on the peristome.

The genus _Gazelletta_ is by far the most common of all Medusettida, and some species seem to be widely distributed, or even cosmopolitan. The number of species, too, is in this genus much larger than in all the other genera of this family. It differs from the latter in the possession of six feet, which number is very constant in the majority of species; in some species, however, single specimens are not rarely found which possess five or seven feet instead of six. Usually the six feet are rather regularly formed and disposed, and diverge downwards; but sometimes there is more or less {1671}irregularity in their shape and arrangement. In the majority of species the feet are very long, curved, and armed in a very various manner, so that four subgenera may be distinguished, according to the armatures.

Subgenus 1. _Gazellarium_, Haeckel.

_Definition._--Feet simple, smooth, without spines and without terminal branches.

1. _Gazelletta hexanema_, n. sp. (Pl. 120, fig. 5).

Shell hemispherical, smooth. Feet very long, cylindrical, straight, smooth, widely divergent, without terminal branches.

_Dimensions._--Length of the shell 0.03 to 0.04, breadth 0.06 to 0.07; length of the feet 0.1 to 0.2.

_Habitat._--Cosmopolitan; Mediterranean, Atlantic, Indian, Pacific, surface.

2. _Gazelletta macronema_, n. sp. (Pl. 120, figs. 7, 8).

Shell hemispherical, smooth. Feet very long, cylindrical, smooth, in the basal part horizontally expanded, in the distal part strongly curved downwards, without terminal branches.

_Dimensions._--Diameter of the shell 0.1 to 0.12, length of the feet 1.2 to 1.6.

_Habitat._--Central Pacific, Stations 270 to 274, surface.

3. _Gazelletta orthonema_, n. sp. (Pl. 120, fig. 10).

Shell hat-shaped, thorny. Feet cylindrical, straight, smooth, widely divergent, without terminal branches.

_Dimensions._--Diameter of the shell 0.12 to 0.16, length of the feet 0.3 to 0.5.

_Habitat._--Central Pacific, Stations 270 to 274, surface.

4. _Gazelletta cyrtonema_, n. sp. (Pl. 120, fig. 9).

Shell hat-shaped, thorny. Feet cylindrical, smooth, widely diverging at the base, strongly curved and convergent in the distal part, often semicircular, without terminal branches.

_Dimensions._--Diameter of the shell 0.11 to 0.13, length of the feet 0.4 to 0.6.

_Habitat._--North Atlantic, Station 354, Canary Islands, surface.

Subgenus 2. _Gazellidium_, Haeckel.

_Definition._--Feet smooth, without lateral spines, but branched at the distal end, or with a bunch of terminal spines.

{1672}5. _Gazelletta furcata_, n. sp.

Shell flat, cap-shaped, smooth. Feet cylindrical, nearly straight, at the distal end forked, with two divergent, spinulate, terminal branches.

_Dimensions._--Length of the shell 0.12, breadth 0.3; length of the feet 0.6 to 0.8.

_Habitat._--Western Tropical Pacific, Station 224, surface.

6. _Gazelletta bifurca_, n. sp. (Pl. 120, fig. 6).

Shell cap-shaped, thorny. Feet nearly straight, widely divergent, cylindrical, smooth, at the distal end twice forked, with four divergent, terminal branches, each of which bears a spathillum with six to eight recurved teeth (sometimes, as in the specimen figured, five or six terminal branches instead of four).

_Dimensions._--Diameter of the shell 0.1, length of the feet 0.3 to 0.4.

_Habitat._--South Pacific, Station 288, surface.

7. _Gazelletta penicillata_, n. sp.

Shell campanulate, spiny. Feet cylindrical, strongly curved, smooth, with a bunch of twelve to twenty or more curved spines at the distal end.

_Dimensions._--Diameter of the shell 0.08, length of the feet 0.6.

_Habitat._--Central Pacific, Station 266, surface.

Subgenus 3. _Gazellonium_, Haeckel.

_Definition._--Feet spiny, armed with simple or branched lateral spines, and with terminal branches of the same shape.

8. _Gazelletta pectinata_, n. sp.

Shell flat, cap-shaped, thorny, three times as broad as high. Feet straight or slightly curved, cylindrical, widely divergent, with four longitudinal rows of short and simple conical spines, which are directed towards the distal end, and about as long as a single joint of the foot.

_Dimensions._--Length of the shell 0.16, breadth 0.5; length of the feet 1.5 to 2.0, breadth 0.03 to 0.05.

_Habitat._--North Atlantic, Færöe Channel (John Murray), Hebrides (Moebius).

9. _Gazelletta pinnata_, n. sp.

Shell hat-shaped, thorny. Feet cylindrical, strongly curved, pinnate, with two opposite longitudinal rows of simple conical spines, which are slightly curved, directed towards the distal end and opposed in pairs on about each third or fourth joint.

_Dimensions._--Diameter of the shell 0.3, length of the feet 1.2 to 1.5.

_Habitat._--South Atlantic, Station 325, depth 2650 fathoms.

{1673}10. _Gazelletta studeri_, n. sp. (Pl. 120, fig. 15).

Shell flat, cap-shaped, thorny. Feet widely divergent, nearly straight and horizontally expanded, with two alternate longitudinal rows of lateral spines, which arise perpendicularly from the foot, are simple, conical, slightly curved, and alternate on the succeeding joints. Ends of the feet constricted, with four to six spines in one row. Dedicated to Professor Studer.

_Dimensions._--Diameter of the shell 0.15, length of the feet 0.6 to 0.8.

_Habitat._--South Atlantic (east of Patagonia), Station 318, surface.

11. _Gazelletta schleinitzii_, n. sp. (Pl. 120, figs. 11, 12).

Shell hat-shaped, thorny. Feet cylindrical, nearly straight and horizontally expanded, with four longitudinal rows of equal lateral spines, which arise perpendicularly from the foot and bear at the end three short divergent teeth. The end of the arms bears three forked spines. I name this elegant species in honour of Captain Schleinitz, commander of the German ship "Gazelle," who conducted the deep-sea soundings during the years 1874 to 1876.

_Dimensions._--Diameter of the shell 0.12, length of the feet 0.6 to 0.8.

_Habitat._--Tropical Atlantic, Station 347, depth 2250 fathoms.

12. _Gazelletta trispathilla_, n. sp. (Pl. 120, fig. 13).

Shell flat, cap-shaped, smooth. Feet cylindrical, nearly straight, widely divergent, with longitudinal rows of alternate spines, which arise perpendicularly from the foot and bear at the end three short divergent branches, each with a spathilla of six short teeth.

_Dimensions._--Diameter of the shell 0.2 to 0.25, length of the feet 1.2 to 1.5.

_Habitat._--North Pacific, Station 235 to 238, surface.

13. _Gazelletta robusta_, n. sp. (Pl. 120, fig. 14).

Shell flat, cap-shaped, thorny. Feet cylindrical, very stout, slightly curved, covered with short irregularly branched spines; their ends with a bunch of similar spines.

_Dimensions._--Diameter of the shell 0.6, length of the feet 2.5 to 3.3.

_Habitat._--Central Pacific, Stations 263 to 274, surface.

Subgenus 4. _Gazellusium_, Haeckel.

_Definition._--Feet spiny, armed with simple or branched lateral spines, and with larger terminal branches of a different shape.

{1674}14. _Gazelletta dendronema_, n. sp. (Pl. 120, fig. 16).

Shell hemispherical, thorny, with a broad, alveolate velum which bears irregular hollow thorns on the inside and on the free margin (fig. 16). Feet divergent, irregularly curved, with scattered arborescent lateral spines, which are irregularly branched, and bear at the distal end of each branch a small spathilla with four to six teeth. The distal ends of the feet are dichotomously branched, with stouter simple fork-branches.

_Dimensions._--Length of the shell 0.17, breadth 0.36; length of the feet 0.5 to 0.7.

_Habitat._--North Pacific, Station 252, surface.

15. _Gazelletta melusina_, n. sp. (Pl. 118, fig. 1).

Shell campanulate, spiny. Feet divergent, strongly curved, with scattered arborescent lateral spines, which are richly and dichotomously branched, with thin, simple, terminal branches. The distal ends of the feet bear three or four much larger and stouter branches, which are again dichotomously branched.

_Dimensions._--Length of the shell 0.2, breadth 0.15; length of the feet 0.5 to 0.8.

_Habitat._--South Pacific, Station 293, depth 2025 fathoms.

Genus 703. _Gorgonetta_,[320] n. gen.

_Definition._--#Medusettida# with twelve articulate feet on the peristome, six ascending and six alternate descending.

The genus _Gorgonetta_ is the most highly developed of all hitherto observed Medusettida, and belongs to the most remarkable forms of Radiolaria. Its general shape is very similar to that of a larva of a _Geryonia_ or _Carmarina_, in which six interradial larval tentacles are directed upwards, six alternating perradial permanent tentacles downwards. The six ascending feet are in all the four species observed smaller, more or less branched, and armed with spathillæ. The six descending feet are much larger and covered with very elegant pencils, the single threads of which bear a double spathilla. The distal ends of the feet are forked or branched.

1. _Gorgonetta mirabilis_, n. sp. (Pl. 119, figs. 1-4).

Shell cap-shaped, flatly vaulted, or nearly hemispherical, about twice as broad as long, with smooth surface. Six ascending feet widely divergent, about twice as long as the breadth of the shell, nearly straight, arborescent, with numerous curved branches, each of which bears at the distal end a spathilla with four to six short recurved teeth (fig. 2). Six descending feet about twice as {1675}long as the six alternate ascending, nearly straight, covered with numerous elegant pencils. Each pencil is richly branched, with fifty to one hundred or more thread-like ramules, and each terminal ramule has two cruciate spathillæ, a larger proximal and a smaller distal (figs. 3, 4); the four recurved anchor-teeth of the proximal spathilla are twice as large as those of the distal. The distal end of each descending foot bears three or four very stout terminal branches, each of which is about as long as the shell and again trifurcate at the distal end.

_Dimensions._--Length of the shell 0.2 to 0.3, breadth 0.4 to 0.5.

_Habitat._--Tropical Atlantic, Stations 346 to 348, surface.

2. _Gorgonetta geryonia_, n. sp.

Shell hemispherical, twice as broad as long, with short scattered thorns on the surface. Twelve feet of similar shape to those in the preceding species, but differing in the following characters:--the six ascending feet are much smaller, about as long as the breadth of the shell, and each bears only six to twelve short branches; the six descending feet are about three times as long as the shell, more or less curved; their pencils much smaller, and the two spathillæ of each pencil-thread bear five anchor-teeth. The distal ends are more richly and irregularly branched.

_Dimensions._--Length of the shell 0.4, breadth 0.7.

_Habitat._--Indian Ocean (Cocos Islands), Rabbe, surface.

3. _Gorgonetta carmarina_, n. sp.

Shell flat, cap-shaped, three times as broad as long. Twelve feet similar in shape to those of the two preceding species, but more irregular in form and arrangement, and differing in the following characters:--the six ascending feet are nearly horizontally expanded, one and a half times as long as the breadth of the shell, with few short branches, each of which bears a spathilla with six recurved anchor-teeth. The six descending feet are about five times as long as the shell, irregularly curved, studded with numerous small pencils, each thread of which bears a double spathilla with six teeth. The distal ends are irregularly branched.

_Dimensions._--Length of the shell 0.2, breadth 0.7.

_Habitat._--South Pacific, Station 318, depth 2040 fathoms.

4. _Gorgonetta bisenaria_, n. sp.

Shell campanulate, similar to that of _Gazelletta melusina_ (Pl. 118, fig. 1), with smooth surface. Six ascending feet about as long as the shell, nearly straight, similar to those of _Gorgonetta mirabilis_ (Pl. 119, fig. 1). Six descending feet also similar to those of the latter, but shorter, about twice as long as the shell, strongly curved, studded with numerous small pencils, each thread of which has a double spathilla with three teeth. The distal end of each foot bears a whorl of six to eight irregular forked branches.

_Dimensions._--Length of the shell 0.5, breadth 0.4.

_Habitat._--Central Pacific, Station 263, surface.

{1676}Genus 704. _Polypetta_,[321] n. gen.

_Definition._--#Medusettida# with numerous (ten to twenty or more) articulate feet.

The genus _Polypetta_ comprises those Medusettida in which the number of feet is very large and not limited, usually twenty to thirty or more. The few specimens of this genus that have been observed are rather different; and it may be that only the two first species described in the following lines are true Medusettida; they possess the usual distinctly alveolate feet, and are derived from the similar _Medusetta_ or _Gazelletta_ simply by multiplication of the feet. The two other species, however (figured in Pl. 116, figs. 1, 2, as _Porospathis_), belong perhaps to another family of PHÆODARIA (Castanellida?); their shell-structure is peculiar and their feet not distinctly alveolate; they may therefore represent a peculiar genus _Porospathis_ (Haeckel, Sitzungsb. med.-nat. Gesellsch. Jena, Dec. 12, 1879, p. 5).

1. _Polypetta polynema_, n. sp.

Shell campanulate, thorny, about as long as broad, with slightly constricted mouth, similar to that of _Gazelletta orthonema_ (Pl. 120, fig. 10). Shell-wall hollow, with irregular polygonal alveoles. Peristome with a corona of eighteen divergent, curved, cylindrical feet of unequal size; six larger primary on the margin of the shell, the other twelve secondary, between the former, somewhat above the margins at different heights. The feet are two to three times as long as the shell, irregularly curved and distinctly alveolate, without appendages.

_Dimensions._--Length of the shell 0.12, breadth 0.11.

_Habitat._--Central Pacific, Station 266, surface.

2. _Polypetta alveolata_, n. sp.

Shell campanulate, thorny, about as long as broad, very similar to that of _Medusetta craspedota_ (Pl. 120, fig. 4). Shell-wall hollow, with subregular hexagonal alveoles. Peristome prolonged into a short funnel-shaped proboscis, twice as long as the similar velum of the latter. The velum is surrounded by a corona of eight larger and eight to twelve smaller feet; the larger are about as long as the shell, divergent, irregularly curved, thorny, and distinctly alveolate; the smaller feet are scattered above the latter and form an immediate transition to the small spines of the shell-surface.

_Dimensions._--Length of the shell 0.08, breadth 0.09.

_Habitat._--South Pacific, Station 284, surface.

{1677}3. _Polypetta tabulata_, n. sp. (Pl. 116, figs. 2, 2_a_, 2_b_).

_Porospathis tabulata_, Hæckel, 1879, Sitzungsb. med.-nat. Gesellsch.
Jena, Dec. 12.

Shell subspherical, with elegantly panelled or tabulate surface, composed of small triangular plates, which are separated by a network of prominent crests; at each nodal point of the network a small conical thorn arises (figs. 2_a_, 2_b_). Each triangular plate seems to contain a hollow alveole, which opens into the shell-cavity (?). On the surface of the shell are scattered numerous (thirty to forty or more), cylindrical, hollow, radial spines, which are straight or slightly curved, not distinctly alveolate, and increase in size towards the mouth. The aboral spines are scarcely one-third as long as the radius of the shell, whilst the oral spines are three times as long as its diameter. The proboscis of the constricted mouth is a hollow cylinder, longer than the shell-radius, and armed with a corona of fifteen to twenty slender bristle-shaped teeth. The position of this singular species in this family is doubtful.

_Dimensions._--Diameter of the shell 0.1 to 0.11, length of the peristome 0.07, breadth 0.02.

_Habitat._--Central Pacific, Station 271, depth 2425 fathoms.

4. _Polypetta mammillata_, n. sp. (Pl. 116, figs. 1, 1_a_).

_Porospathis mammillata_, Hæckel, 1879, Sitzungsb. med.-nat., Gesellsch.
Jena, Dec. 12.

Shell subspherical, densely covered with numerous mammiliform hollow papillæ, between which are scattered single longer, hollow, radial spines; these are irregularly curved, cylindrical, without distinct alveoles; the twelve longest are two to three times as long as the shell, divergent in the oral direction, and surround the base of the peristome in two irregular alternate circles, each with six spines. The hollow papillæ of the surface, between which are placed numerous circular pores (or dimples?), open into the shell-cavity by a small pore (fig. 1_a_); they are perhaps reduced radial spines. The constricted peristome is conical and prolonged into an inversely conical proboscis, which is about as long as the radius of the shell; its dilated mouth is surrounded by a corona of slender, divergent, bristle-shaped teeth. The position of this species in the family Medusettida is doubtful; perhaps it is more closely allied to the Castanellida.

_Dimensions._--Diameter of the shell 0.12; length of the proboscis 0.08, breadth 0.03; length of the shorter spines 0.1, of the longest spines 0.3.

_Habitat._--North Pacific, Station 252, surface.

Family LXXX. #CASTANELLIDA#, Haeckel, 1879 (Pl. 113).

_Castanellida_, Haeckel, 1879, Sitzungsb. med.-nat. Gesellsch. Jena, Dec.
12, p. 5.

_Definition._--PHÆODARIA with a spherical or subspherical shell, exhibiting ordinary lattice-work, with circular or roundish pores. Radial spines without circles of basal pores. Mouth of the shell large, usually circular and armed with teeth. Central capsule excentric, placed in the aboral half of the shell-cavity.

{1678}The family #Castanellida# represents a common and widely distributed group of PHÆODARIA, which posses a very simple and uniform shell, viz., a simple lattice-sphere with radial spines and one simple mouth. It may therefore be easily confounded with the Coscinommida, or those #Sphæroidea# in which the shell is also a simple lattice-sphere armed with radial spines (_Astrosphærida simplicia_, p. 209). Indeed the shell of both groups is very similar, and differs in one important point only; the Castanellida constantly possess one larger opening in the shell-wall, the shell-mouth, which is either smooth or armed with a corona of teeth; in the Coscinommida, however, such a mouth is never present. The living specimens of both groups, and those shells in which the soft body is preserved, are very easily distinguished, since the shell encloses in the Coscinommida the central capsule and the transparent calymma only, whilst the latter, besides, in the Castanellida contains a voluminous dark brown or green mass of phæodella, the characteristic phæodium. A closer examination of the central capsule reveals in all Castanellida the typical operculum, the astropyle, with the proboscis of the PHÆODARIA, which is never present in any #Sphæroidea#.

The Castanellida are easily distinguished also from those similar PHÆODARIA in which the shell is also a lattice-sphere; the lattice-work is constantly quite simple, as in the similar Coscinommida, never composed of separated tangential pieces (as in the Aulosphærida), or of porcellanous structure, with basal circles of pores (as in the Circoporida), or of diatomaceous structure (as in the Challengerida). The gigantic Orosphærida, which also in part posses a simple lattice-sphere, differ from the Castanellida in the absence of the peculiar shell-mouth.

Though the Castanellida belong to the most common PHÆODARIA, and though the number of individuals, floating on the surface of the tropical seas, is extraordinarily great, their variety of forms is very small; the six genera distinguished in the following system differ only in very slight characters, and the majority of the species are very similar, and often hardly distinguishable. The seven species figured in Pl. 113 exhibit the most striking differences which I could distinguish among all the species observed. The shell usually has the characteristic appearance of a chestnut, a sphere covered with very numerous short radial spines or bristles. In the majority of species a certain number of longer thin radial spines is scattered over the surface; these are usually simple, rarely branched. The mouth of the shell, corresponding to the proboscis of the central capsule, and placed in the same radius, is either a quite simple larger opening with a smooth margin (figs. 3, 5, 7) or is armed with a corona of teeth (figs. 1, 6).

The size of the shell varies between 0.2 and 0.8 mm., and is usually between 0.3 and 0.5 mm. Its form is in the majority of species a geometrical sphere; rarely it is somewhat irregular, slightly ellipsoidal (prolonged in the axis of the mouth and proboscis), or polyhedral (by conical protuberances from the bases of the radial spines). {1679}The ideal fundamental form of the shell, however, is constantly monaxonian, since the place of the shell-mouth and of the proboscis of the capsule (both lying in one axis of the sphere), causes a principal axis to be different from all others. The longer radial spines, quite variable in number and disposition, are never arranged regularly or symmetrically, and therefore determine no constant axes.

The lattice-work of the spherical shell offers also but slight differences. It is usually more or less irregular, with circular or subcircular pores of unequal sizes. More rarely the network is perfectly regular, with equidistant circular pores of equal size, and sometimes these are surrounded by hexagonal frames (fig. 6), as in many #Sphæroidea#. More rarely the pores are perfectly irregular, roundish or sometimes polygonal, of unequal sizes and dissimilar forms (fig. 2). The size of the pores is very variable between 0.01 and 0.05, usually between 0.015 and 0.025. They are commonly somewhat broader than the bars between them, more rarely smaller. The substance of the shell-wall is in the the majority of Castanellida hyaline, transparent (as in the similar #Sphæroidea#); in some larger forms, however, a network of fine axial filaments is visible in the lattice (fig. 2), and sometimes this seems to be hollow, with an axial canal (as in the large Orosphærida). Such an axial filament or fine axial canal is usually visible in the larger radial spines. The shell-wall of the Castanellida, however, never exhibits that characteristic structure (with tangential needles in a cement-like fundamental substance), which we find in all Circoporida. The latter differ also in the constant presence of circles of large pores around the bases of the radial spines, which are never found in the Castanellida. The shell becomes usually stained more or less purple by carmine, and when burned by fire, becomes of a brown colour.

The inner surface of the shell is always quite smooth, the outer surface armed with numerous short and thin radial bristles. Sometimes these bristles or "by-spines" are very short, but never perfectly wanting. Usually their length is about one-third or one-fourth of the radius of the shell, often less, rarely more. They arise from the nodal points between every three or four neighbouring spines, and are commonly very thin and straight, pointed, with slightly thickened conical basis. Rarely they are directed not quite radially, but more or less obliquely.

The larger radial spines, which arise between the thin bristles or "by-spines" and may be opposed to the latter as "main-spines," are wanting in two genera only, in _Castanarium_ and _Castanella_ (Pl. 113, fig. 6). They are simple in _Castanidium_ (figs. 2, 5, 7) and _Castanissa_ (fig. 1), irregularly branched in _Castanopsis_ and _Castanura_ (figs. 3, 4). Their number and disposition is never regular nor symmetrical (as in the majority of #Sphæroidea#) and varies even in each single species. In general the number of main-spines varies between ten and forty (rarely less or more) usually between twenty and thirty. Their length is commonly about equal to the diameter of the shell, often somewhat shorter, rarely considerably longer. Their {1680}thickness is in the majority of species about equal to the diameter of one or two pores of the lattice. Sometimes they are much stronger, rarely thinner.

The form of the radial main-spines is not very variable, usually it is cylindrical or cylindro-conical, rarely angular, prismatic or pyramidal. Sometimes they are straight (perfectly radial), at other times more or less irregularly curved. Usually they are simple, rarely branched; the branches are always very irregular and exhibit a remarkable tendency to unite by concrescence (figs. 3, 4). Some thicker forms of spines exhibit a remarkable structure, the surface being covered with small dimples and spinules between them (fig. 1); sometimes the dimples are hexagonal (fig. 5_a_). In the axis of the thicker spines there is often visible the same axial filament or funicle which we find in many other PHÆODARIA, more rarely a hollow axial canal.

The mouth of the shell is usually circular, more rarely polygonal or irregularly roundish (fig. 2). It is either quite smooth (fig. 3, 5, 7) or armed with a corona of strong teeth (figs. 1, 4, 6). These teeth are different from the other spines of the shell; they are much thicker than the radial bristles, and shorter than the main-spines. Their number varies between three and twenty, usually between five and seven. They are usually parallel to the main axis, which is determined by the excentric position of the mouth and the centre of the sphere (figs. 1, 6). The teeth are usually conical, more or less irregular in size and form, more rarely of equal size. The corona of teeth is usually sharply separated from the surrounding lattice-work of the shell, but has no influence on its regular spherical form.

The _central capsule_ of the Castanellida immediately determines beyond doubt their true PHÆODARIAN nature; it lies excentrically in the cavity of the enclosing shell, and is surrounded at the oral pole by the large and dark phæodium. The latter covers the radiate operculum of the capsule membrane and the tubular proboscis, which arises from it and is directed towards the shell mouth. Usually the diameter of the spherical central capsule is about one-third of the shell diameter, rarely one-quarter only, and in a few cases it attains nearly its half. The calymma, therefore, is larger than the capsule, and fills up the whole cavity of the shell which is not occupied by the latter. The greater part of the calymma again is filled up by the dark green or brown, sometimes black phæodella, the roundish granules which compose the voluminous non-transparent phæodium. Sometimes the latter proceeds through the mouth and is partially placed outside the shell (fig. 5).

The astropyle, or the main-opening of the central capsule, is placed on the oral pole of its main axis, while its radiate operculum and the tubular proboscis arising from it possess the same structure as in the other PHÆODARIA. But the two lateral parapylæ or accessory openings which are present in the majority of the latter seem to be absent in the Castanellida as well as in the Medusettida and the Challengerida. I could never observe a trace of them. The proboscis is a small cylindrical tube and is directed towards the {1681}mouth of the shell, therefore placed in its main axis; it is usually quite hidden in the dark phæodium, which occupies the oral half of the shell-cavity, whilst the capsule lies in the aboral half. The large nucleus of the central capsule and its double membrane present the same characters as in the other PHÆODARIA.

_Synopsis of the Genera of Castanellida._

-------------------------------------------------------------------------
No long radial main-spines (in addition to the constant short bristles
of the shell-surface).
-------------------------------------------------------------------------
Mouth smooth, 705. _Castanarium_.

Mouth dentate, 706. _Castanella_.
-------------------------------------------------------------------------
Long radial main-spines scattered between the short constant bristles
of the shell-surface.
-------------------------------------------------------------------------
{ Mouth smooth, 707. _Castanidium_.
Main spines simple. {
{ Mouth dentate, 708. _Castanissa_.

{ Mouth smooth, 709. _Castanopsis_.
Main spines branched. {
{ Mouth dentate, 710. _Castanura_.

Genus 705. _Castanarium_,[322] Haeckel, 1879, Sitzungsb. med.-nat. Gesellsch. Jena, Dec. 12, p. 5.

_Definition._--#Castanellida# without radial main-spines, with a smooth mouth.

The genus _Castanarium_ is the simplest form of all the Castanellida, and may be regarded as the common ancestral form of this family. The simple spherical shell has a smooth, toothless mouth and no larger radial spines; it is densely studded with the simple, radial bristles, which are common to all Castanellida. The shell therefore is very similar to those species of the Astrosphæride _Acanthosphæra_, which represent the subgenus _Rhapidococcus_ (compare above, p. 210, Pl. 26, fig. 3); it differs from the latter in the possession of the shell-mouth, which is absent in all #Sphæroidea#.

1. _Castanarium darwini_, n. sp.

Pores regular, circular, hexagonally framed, three times as broad as the bars. Radial bristles half as long as the radius.

_Dimensions._--Diameter of the shell 0.3 to 0.4, of the pores 0.03.

_Habitat._--South Atlantic (east of Patagonia), Station 318, depth 2040 fathoms.

{1682}2. _Castanarium hookeri_, n. sp.

Pores regular, circular, hexagonally framed, twice as broad as the bars. Radial bristles one-third as long as the radius.

_Dimensions._--Diameter of the shell 0.55, of the pores 0.025.

_Habitat._--Indian Ocean, Cocos Islands (Rabbe), surface.

3. _Castanarium lyelli_, n. sp.

Pores regular, circular, without hexagonal frames, of the same breadth as the bars. Radial bristles twice as long as the diameter of one pore.

_Dimensions._--Diameter of the shell 0.24, of the pores 0.015.

_Habitat._--Central Pacific, Station 272, depth 2600 fathoms.

4. _Castanarium lubbocki_, n. sp.

Pores irregularly roundish, about of the same breadth as the roundish, non-crested bars. Radial bristles one-third as long as the radius.

_Dimensions._--Diameter of the shell 0.36, of the pores 0.01 to 0.02.