Chapter XXI: Echinodermata (continued): Development and Phylogeny 601 (5)

The agitation of the flagellum determines a stream of water upwards along the outer walls of the funnel; and the food-particles brought along adhere to the outside of the funnel, and are carried by its streaming movement to the basal constriction, where they are swallowed by the plasma, which appears to form a swallowing vacuole at that point. Longitudinal fission is the ordinary mode of reproduction, extending up through the funnel. If the two so formed continue to produce a stalk, the result is the formation of a tree-like stem, whose twigs bear at the ends the funnelled cells, or "collar-cells" as they are usually called. In _Salpingoeca_, as in so many other Flagellates, each cell forms a cup or theca, often of most graceful vase-like outline, the rim being elegantly turned back. _Proterospongia_ (Fig. 75, p. 182) secretes a gelatinous investment for the colony, which is attached to solid bodies. In this species, according to Saville Kent, the central members of the colony retract their collar, lose their flagellum, become amoeboid, and finally undergo brood-formation to produce minute zoospores. This is the form which by its differentiation recalls the Sponges, and has been regarded as a {123}transition towards them; for the flagellate, nutritive cells of the Sponges are provided with a collar, which exists in no other group of Metazoa (see pp. 171, 181, and Fig. 70, p. 176). The most recent monographer of the family is Raoul Francé, but James-Clark and Saville Kent did the pioneering work.

Of the life-history of the Trichonymphidae,[136] all of which are parasitic in the alimentary canal of Insects, especially Termites or White Ants (Vol. V. p. 356), nothing is known. Some of them have a complete investment of motile flagella, like enormously long cilia, which in _Dinenympha_ appear to coalesce into four longitudinal undulating membranes. _Lophomonas_ inhabits the gut of the Cockroach and Mole-cricket. The Opalinidae have also a complete investment of cilia, which are short, and give the aspect of a Ciliate to the animal, which is common in the rectum of Amphibia, and dies when transferred to water. But despite the outward resemblance, the nuclei, of which there may be as many as 200, are all similar, and consequently this group cannot be placed among the Infusoria at all. _Opalina_ has no mouth nor contractile vacuole. It multiplies by dividing {124}irregularly and at intervals, resolving finally into 1-nucleate fragments, which encyst and pass into the water. When swallowed the cyst dissolves, its contents enlarge, and ultimately assume the adult form.[137]

_Maupasia_ has a partial investment of cilia, a single long flagellum and mouth, a contractile vesicle, and a single simple nucleus. It seems to find an appropriate place near the two above groups, though it is free, and possesses a mouth.

Among the Euglenaceae, _Euglena viridis_ is a very common form, giving the green colour to stagnant or slow-flowing ditches and puddles in light places, especially when contaminated by a fair amount of dung, as by the overflow of a pig-sty, in company with a few hardy Rotifers, such as _Hydatina senta_ (Vol. II. Fig. 106, p. 199) and _Brachionus_. _Euglena_ is about 0.1 mm. in length when fully extended, oval, pointed behind, obliquely truncate in front, with a flagellum arising from the pharyngeal pit. It shows a peculiar wriggling motion, waves of transverse constriction passing along the body from end to end, as well as flexures in different meridians. Such motions are termed "euglenoid." The front part is colourless, but under a low {125}power the rest of the cell is green, owing to the numerous chlorophyll bodies or chloroplasts. The outermost layer of the cytoplasm shows a somewhat spiral longitudinal striation, possibly due to muscular fibrils. The interior contains many laminated plates of paramylum, and a large single nucleus. At the front of the body at the base of the flagellum is a red "eye-spot" on the dorsal side of the pharynx-tube or pit, from which the flagellum protrudes. Wager has shown that this tube receives, also on its dorsal side, the opening of a large vacuole, sometimes called the reservoir, for into it discharges the contractile vacuole (or vacuoles). The eye-spot is composed of numerous granules, containing the vegetal colouring matter "haematochrome." It embraces the lower or posterior side of the communication between the tube and the reservoir. The flagellum has been traced by Wager through the tube into the reservoir, branching into two roots where it enters the aperture of communication, and these are inserted on the wall of the reservoir at the side opposite the eye-spot. But on one of the roots near the bifurcation is a dilatation which lies close against the eye-spot, so that it can receive the light reaction. _Euglena_ is an extremely phototactic organism. It shows various wrigglings along the longitudinal axis, and transverse waves of contraction and expansion may pass from pole to pole.[138]

Among the Chrysomonadaceae the genus _Zooxanthella_, Brandt, has already been described under the Radiolaria (p. 86), in the jelly of which it is symbiotic. It also occurs in similar union in the marine Ciliates, _Vorticella sertulariae_ and _Scyphidia scorpaenae_, and in _Millepora_ (p. 261) and many Anthozoa (pp. 373 f., 396).

Of the Chlamydomonadidae, _Sphaerella_ (_Haematococcus_, Ag.) _pluvialis_ (Fig. 43), and _S. nivalis_, in which the green is masked by red pigment, give rise to the phenomena of "red snow" and "bloody rain." The type genus, _Chlamydomonas_, is remarkable for the variations from species to species in the character and behaviour of the gametes. Sometimes they are equal, at other times of two sizes. In some species they fuse immediately on approximation, in the naked active state; in others, they encyst on approaching, and unite by the emission of a fertilising tube, {126}as in the Algal Conjugatae. _Zoochlorella_ is symbiotic in green Ciliata (pp. 153 f., 158), Sponges (p. 175), _Hydra_ (p. 256), and Turbellaria (Vol. II. p. 43).

Of the Volvocidae, _Volvox_ (Fig. 44) is the largest and most conspicuous genus. Its colony forms a globe the size of a pin's head, floating on the surface of ponds, drains, or even puddles or water-barrels freely open to the light. It has what may be called a skeleton of gelatinous matter,[139] condensed towards the surface into a denser layer in which the minute cells are scattered. These have each an eye-spot, a contractile vacuole, and two flagella, by the combined action of which the colony is propelled. Delicate boundary lines in the colonial wall mark out the proper investment of each cell. The cells give off delicate plasmic threads which meet those of their neighbours, and form a bond between them. In that half of the hemisphere which is posterior in swimming, a few (five to eight) larger cells ("macrogonidia" of older writers) are evenly distributed, protruding as they increase in size into the central jelly. These as they grow segment to form a new colony.

{127}

The divisions are only in two planes at right angles, so that the young colony is at first a plate, but as the cells multiply the plate bends up (as in the gastrulation of the double cellular plate of the Nematode _Cucullanus_, Vol. II. p. 136), and finally forms a hollow sphere bounded by a single layer of cells: the site of the original orifice may be traced even in the adult as a blank space larger than exists elsewhere. Among the cells of the young colony some cease to divide, but continue to grow at an early period, and these are destined to become in turn the mothers ("parthenogonidia") of a new colony; they begin segmenting before the colony of which they are cells is freed. The young colonies are ultimately liberated by the rupture of the sphere as small-sized spheres, which henceforth only grow by enlargement of the sphere as a whole, and the wider separation of the vegetative cells. Thus the vegetative cells soon cease to grow; all the supply of food material due to their living activities goes to the nourishment of the parthenogonidia, or the young colonies, as {128}the case may be. These vegetative cells have therefore surrendered the power of fission elsewhere inherent in the Protist cell. Moreover, when the sphere ruptures for the liberation of the young colonies, it sinks and is doomed to death, whether because its light-loving cells are submerged in the ooze of the bottom, or because they have no further capacity for life. When conjugation is about to take place, it is the cells that otherwise would be parthenogonidia that either act as oospheres or divide as "spermogonia" to form a flat brood of minute yellow male cells ("sperms"). These resemble vegetative cells, in the possession of an eye-spot and two contractile vacuoles, but differ in the enormously enlarged nucleus which determines a beaked process in front. After one of these has fused with the female cell ("oosphere") the product ("oosperm") encysts, passes into a stage of profound rest, and finally gives rise to a new colony. The oospheres and sperm-broods may arise in the same colony or in distinct ones, according to the species.

Before we consider the bearings of the syngamic processes of _Volvox_, we will study those presented by its nearer allies, which have the same habitat, but are much more minute. Three of these are well known, _Stephanosphaera_, _Pandorina_, and _Eudorina_, all of which have spherical colonies of from eight to thirty-two cells embedded at the surface of a sphere, and no differentiation into vegetative cells and parthenogonidia (or reproductive cells).

_Stephanosphaera_ has its eight cells spindle shaped, and lying along equidistant meridians of its sphere; in vegetative reproduction each of these breaks up in its place to form a young colony, and the eight daughter-colonies are then freed. In conjugation, each cell of the colony breaks up into broods of 4, 8, 16, or 32 small gametes, which swim about within the general envelope, and pair and fuse two and two: this is "isogamous," "endogamous" conjugation. In _Pandorina_ (Fig. 45) the cells are rounded, and are from 16 to 32 in each colony. The vegetative reproduction in this, as in _Eudorina_, is essentially the same as in _Stephanosphaera_. In conjugation the cells are set free, and are of three sizes in different colonies, small (S), medium (M), and large (L). The following fusions may occur: S × S, S × M, S × L, M × M, M × L. Thus the large are always female, as it were, the medium may play the part of male to the large, female to the small; the small are males to the medium and to the large. The medium {129}and small are capable, each with its like, of equal, undifferentiated conjugation; so that we have a differentiation of sex far other than that of ordinary, binary sex. _Eudorina_, however, has attained to "binary sex," for the female cells are the ordinary vegetative cells, at most a little enlarged, and the male cells are formed by ordinary cells producing a large flat colony of sixty-four minute males or sperms. In some cases four cells at the apex of a colony are spermogonia, producing each a brood of sperms, while the rest are the oospheres. The transition to _Volvox_ must have arisen through the sterilisation of the majority of cells of a colony for the better nutrition of the few that are destined alone for reproduction.

_Volvox_, as we have seen, has attained a specialisation entirely comparable to that of a Metazoon, where the segmentation of the fertilised ovum results in two classes of cells: those destined {130}to form tissues, and condemned to ultimate death with the body as a whole, and those that ultimately give rise to the reproductive cells, ova, and sperms. But this is a mere parallelism, not indicating any sort of relationship: the oospores of the Volvocaceae show that tendency to an encysted state, in which fission takes place, that is so characteristic of Algae, and these again show the way to Cryptogams of a higher status. Thus, _Volvox_, despite the fact that in its free life and cellular differentiation it is the most animal of all known Flagellates, is yet, with the rest of the Volvocaceae, inseparable from the Vegetable Kingdom, and is placed here only because of the impossibility of cleaving the Flagellates into two.

The Dinoflagellata (Figs. 46, 47) are often of exceptionally large dimensions in this class, attaining a maximum diameter of 150 µ (1/160") and even 375 µ (1/67") in _Pyrocystis noctiluca_. The special character of the group is the presence of two flagella; the one, filiform, arises in a longitudinal groove, and extending its whole length projects behind the animal, and is the conspicuous organ of motion: the other, band-like, arises also in the longitudinal groove, but extends along a somewhat spiral transverse groove,[140] and never protrudes from it in life, executing undulating movements that simulate those of a girdle of cilia, or a continuous undulating membrane (Fig. 46). This appearance led to the old name "Cilioflagellata," which had of course to be abandoned when Klebs discovered the true structure.[141] There is a distinct cellulose membrane, sometimes silicified, to the ectoplasm, only interrupted by a bare space in the longitudinal groove, whence the flagella take origin. This cuticle is usually hard, sculptured, and divided into plates of definite form, bevelled and overlapping at their junction; occasionally the cell has been seen to moult them.

A large vacuolar space, traversed by plasmic strings, separates the peripheral cytoplasm from the central, within which is the large nucleus. There are in most species one or more chromatophores, coloured by a yellowish or brownish pigment, which is a mixture of lipochromes, distinct from diatomin. In a few species the presence of these is not constant, and these species {131}show variability as to their nutrition, which is sometimes holozoic. Under these conditions the cell can take in food-particles as bulky as the eggs of Rotifers and Copepods, by the protrusion of a pseudopod at the junction of the two grooves. As in most coloured forms an eye-spot is often present, a cup-shaped aggregation of pigment, with a lenticular refractive body in its hollow. A contractile vacuole, here termed a "pusule," occurs in many species, communicating with the longitudinal groove by a canal. Nematocysts (see p. 246 f.) are present in _Polykrikos_, trichocysts (see p. 142) in several genera.

Division is usually oblique, dividing the body into two dissimilar halves, each of which has to undergo a peculiar growth to reconstitute the missing portion, and complete the shell. The incomplete separation of the young cells leads to the formation of chains, notably in _Ceratium_ and _Polykrikos_, the latter dividing transversely and occurring in chains of as many as eight. The process of division may take place when the cell is active, or in a cyst, as in _Pyrocystis_ (Fig. 47). Again, encystment may precede multiple fission, resulting in the formation of a brood of minute swarmers. It has been suggested that these are capable of playing the part of gametes, and conjugating in pairs.[142]

The Dinoflagellates are for the most part pelagic in habit, floating at the surface, and when abundant tinge the water of fresh-water lakes or even ponds red or brown. _Peridinium_ (Fig. 46) and _Ceratium_ (the latter remarkable for the horn-like backward prolongations of the lower end) are common genera both in the sea {132}and fresh-waters. _Gymnodinium pulvisculus_ is sometimes parasitic in _Appendicularia_ (Vol. VII. p. 68). _Polykrikos_[143] has four transverse grooves, each with its flagellum, besides the terminal one. Many of the marine species are phosphorescent, and play a large part in the luminosity of the sea, and some give it a red colour.

Several fossil forms have been described. _Peridinium_ is certainly found fossil in the firestone of Delitzet, belonging to the Cretaceous. A full monograph of the group under the name "Peridiniales" was published by Schütt.[144]

The Cystoflagellates contain only two genera,[145] _Noctiluca_, common at the surface of tranquil seas, to which, as its name implies, it gives phosphorescence, and _Leptodiscus_, found by R. Hertwig in the Mediterranean. _Noctiluca_ is enormous for a Flagellate, for with the form of a miniature melon it measures about 1 mm. (1/25") or more in diameter. In the depression is the "oral cleft," from one end of which rises, by a broad base, a large coarse flagellum, as long as the body or longer and transversely striated. In front of the base of the flagellum are two lip-like {133}prominences, of which one, a little firmer than the other, and transversely ridged, is called the tooth; at the junction of the two is a second, minute, flagellum, usually called the cilium. Behind these the oral groove has an oval space, the proper mouth; behind this, again, the oral groove is continued for some way, with a distinct rod-like ridge in its furrow. The whole body, including the big flagellum, is coated by a strong cuticular pellicle, except at the oblong mouth, and the lips and rod are mere thickenings of this. The cytoplasm has a reticulate arrangement: the mouth opens into a central aggregate, from which strands diverge branching as they recede to the periphery, where they pass into a continuous lining for the cuticular wall, liquid filling the interspaces. The whole arrangement is not unlike that found in many plant-cells, but the only other Protists in which it occurs are the Ciliata _Trachelius_ (Fig. 56, p. 153) and _Loxodes_. The central mass contains the large nucleus. _Noctiluca_ is an animal feeder, and expels its excreta through the mouth. The large flagellum is remarkable for the transverse striation of its plasma, especially on the ventral side. The cuticle may be moulted as in the Dinoflagellates. As a prelude to fission the external differentiations disappear, the nucleus divides in the plane of the oral groove, and a meridional constriction parts the two halves, the new external organs being regenerated. Conjugation occurs also, the two organisms fusing by their oral region; the locomotive organs and pharynx disappear; the conjoined cytoplasms unite to form a sphere, and the nuclei fuse to form a zygote or fertilisation nucleus. This conjugation is followed by sporulation or brood-formation.[146]

{134}The nucleus passes towards the surface, undergoes successive fissions, and as division goes on the numerous daughter-nuclei occupy little prominences formed by the upgrowth of the cytoplasm of the upper pole. The rest of the cytoplasm atrophies, and the hillocks formed by the plasmic outgrowths around the final daughter-nuclei become separate as so many zoospores (usually 256 or 512); each of these is oblong with a dorsal cap-like swelling, from the edge of which arises a flagellum pointing backwards; parallel to this the cap is prolonged on one side into a style also extending beyond the opposite pole of the animal.[147] In this state the zoospore is, to all outward view, a naked Dinoflagellate, whence it seems that the Cystoflagellates are to be regarded as closely allied to that group. _Leptodiscus_ is concavo-convex, circular, with the mouth central on the convex face, 1-flagellate, and attains the enormous size of 1.5 mm. (1/16") in diameter.

The remarkable phosphorescence of _Noctiluca_ is not constant. It glows with a bluish or greenish light on any agitation, but rarely when undisturbed. A persistent stimulus causes a continuous, but weak, light. This light is so weak that several teaspoonsful of the organism, collected on a filter and spread out, barely enable one to read the figures on a watch a foot away. As in other marine phosphorescence, no rise of temperature can be detected. The luminosity resides in minute points, mostly crowded in the central mass, but scattered all through the cytoplasm. A slight irritation only produces luminosity at the point touched, a strong one causes the whole to flash. Any form of irritation, whether of heat, touch, or agitation, electricity or magnetism, is stated to induce the glow. By day, it is said, _Noctiluca_, when present in abundance, may give the sea the appearance of tomato soup.

The earliest account of Noctiluca will be read with interest. Henry Baker writes in _Employment for the Microscope_:[148]—"A curious Enquirer into Nature, dwelling at Wells upon the Coast of Norfolk, affirms from his own Observations that the Sparkling of Sea Water is occasioned by Insects. His Answer to a Letter wrote to him on that Subject runs thus, 'In the Glass of Sea Water I send with this are some of the Animalcules which cause the Sparkling Light in Sea Water; they may be seen by holding {135}the Phial up against the Light, resembling very small Bladders or Air Bubbles, and are in all Places of it from Top to Bottom, but mostly towards the Top, where they assemble when the Water has stood still some Time, unless they have been killed by keeping them too long in the Phial. Placing one of these Animalcules before a good Microscope, an exceeding minute Worm may be discovered, hanging with its Tail fixed to an opake Spot in a Kind of Bladder, which it has certainly a Power of contracting or distending, and thereby of being suspended at the Surface, or at any Depth it pleases in the including Water.'"

"The above-mentioned Phial of Sea Water came safe, and some of the Animalcules were discovered in it, but they did not emit any Light, as my Friend says they do, upon the least Motion of the Phial when the Water is newly taken up. He likewise adds, that at certain Times, if a Stone be thrown into the Sea, near the Shore, the Water will become luminous as far as the Motion reacheth: this chiefly happens when the Sea hath been greatly agitated, or after a Storm." Obviously what Mr. Sparshall, Baker's correspondent, took for a worm was the large flagellum.

The chief investigators of this group have been Huxley, Cienkowski, Allman, Bütschli, and G. Pouchet, while Ischikawa and Doflein have elucidated the conjugation.

{136}CHAPTER VI

PROTOZOA (_CONTINUED_): INFUSORIA (CILIATA AND SUCTORIA)

IV. Infusoria.

_Complex Protozoa, never holophytic save by symbiosis with plant
commensals, never amoeboid, with at some period numerous short cilia, of
definite outline, with a double nuclear apparatus consisting of a large
meganucleus and a small micronucleus (or several),[149] the latter alone
taking part in conjugation (karyogamy), and giving rise after conjugation
to the new nuclear apparatus._

The name Infusoria was formerly applied to the majority of the Protozoa, and included even the Rotifers. For the word signifies organisms found in "infusions" of organic materials, including macerations. Such were made with the most varied ingredients, pepper and hay being perhaps the favourites. They were left for varying periods exposed to the air, to allow the organisms to develop therein, and were then examined under the microscope.[150] With the progress of our knowledge, group after group was split off from the old assemblage until only the ciliate or flagellate forms were left. The recognition of the claims of the Flagellates to independent treatment left the group more natural;[151] while it was enlarged by the admission of the Acinetans (_Suctoria_), which had for some time been regarded as a division of the Rhizopoda.

{137}I. CILIATA

_Infusoria, with a mouth, and cilia by which they move and feed; usually with undulating membranes, membranellae, cirrhi, or some of these._ Genera about 144: 27 exclusively marine, 50 common to both sea and fresh water, 27 parasitic on or in Metazoa, the rest fresh water. Species about 500.

We divide the Ciliata thus:[152]—

(I.) Mouth habitually closed, opening by retraction of its circular or
slit-like margin; cilia uniform

Order 1. GYMNOSTOMACEAE.

_Lacrymaria_, Ehrb.; _Loxodes_, Ehrb.; _Loxophyllum_, Duj.; _Lionotus_,
Wrez.; _Trachelius_, Schrank; _Amphileptus_, Ehrb.; _Actinobolus_, St.;
_Didinium_, St.; _Scaphiodon_, St; _Dysteria_, Huxl.; _Coleps_, Nitzsch.;
_Dileptus_, Duj.; _Ileonema_, Stokes; _Mesodinium_, St.

(II.) Mouth permanently open, usually equipped with one or more
undulating membranes, receiving food by ciliary action (TRICHOSTOMATA,
Bütschli)

(_a_) Cilia nearly uniform, usually extending over the whole body,
without any special adoral wreath of long cilia or membranellae; mouth
with one or two undulating membranes at its margin or extending into
the short pharynx.

Order 2. ASPIROTRICHACEAE.

_Paramecium_, Hill; _Colpoda_, O. F. Müll.; _Colpidium_, St.;
_Leucophrys_, Ehrb.; _Cyclidium_, Cl. and L.; _Lembadion_, Perty;
_Cinetochilum_, Perty; _Pleuronema_, Duj.; _Ancistrum_, Maup.;
_Glaucoma_, Ehrb.; _Uronema_, Duj.; _Lembus_, Cohn; _Urocentrum_,
Nitzsch; _Icthyophtheirius_, Fouquet.

(_b_) Strong cilia or membranellae forming an adoral wreath, and
bounding a more or less enclosed area, the "peristome," at one point of
which the mouth lies.

(i.) Body more or less equally covered with fine cilia; adoral wreath
an open spiral

Order 3. HETEROTRICHACEAE

_Spirostomum_, Ehrb.; _Bursaria_, O. F. Müll.; _Stentor_, Oken;
_Folliculina_, Lamk.; _Conchophtheirus_, St.; _Balantidium_, Cl. and
L.; _Nyctotherus_, Leidy; _Metopus_, Cl. and L.; _Caenomorpha_,
Perty; _Discomorpha_, Levander; _Blepharisma_, Perty.

(ii.) Body cilia limited in distribution or absent; peristome
anterior, nearly circular, sinistrorse.

Order 4. OLIGOTRICHACEAE.

_Halteria_, Duj.; _Maryna_, Gruber; _Tintinnus_, Schrank;
_Dictyocystis_, Ehrb.; _Strombidium_, Cl. and L. (= _Torquatella_,
Lank.).

(iii.) Peristome extending backwards along the ventral face, which
alone is provided with motile cirrhi, etc.; dorsal cilia fine,
motionless.

Order 5. HYPOTRICHACEAE.

{138}_Stylonychia_, Ehrb.; _Kerona_, O. F. Müll.; _Oxytricha_, Ehrb.;
_Euplotes_, Ehrb.; _Stichotricha_, Perty; _Schizotricha_, Gruber.

(iv.) Body cilia reduced to a posterior girdle, or temporarily or
permanently absent; peristome anterior, nearly circular, edged by the
adoral wreath,[153] bounded by a gutter edged by an elevated rim or
collar.

Order 6. PERITRICHACEAE.

_Lichnophora_, Cl.; _Trichodina_, Ehrb.; _Vorticella_, L.;
_Zoothamnium_, Bory; _Carchesium_, Ehrb.; _Epistylis_, Ehrb.;
_Opercularia_, Lamk.; _Vaginicola_, Lamk.; _Pyxicola_, Kent;
_Cothurnia_, Ehrb.; _Scyphidia_, Lachmann; _Ophrydium_, Bory;
_Spirochona_, St.

The Ciliata have so complex an organisation that, as with the Metazoa, it is well to begin with the description of a definite type. For this purpose we select _Stylonychia mytilus_, Ehrb. (Fig. 49), a species common in water rich in organic matter, and relatively large (1/75" = ⅓ mm.). It is broadly oval in outline, with the wide end anterior, truncate, and sloping to the left side behind; the back is convex, thinning greatly in front; the belly flat. It moves through the water either by continuous swimming or by jerks, and can either crawl steadily over the surface of a solid or an air surface such as an air bubble, or advance by springs, which recall those of a hunting spider. The boundary is everywhere a thin plasmic pellicle, very tender, and readily undergoing diffluence like the rest of the cell. From the pellicle pass the cilia, which are organically connected with it, though they may be traced a little deeper; they are arranged in slanting longitudinal rows, and are much and variously modified, according to their place and function. On the edge of the dorsal surface they are fine and motionless, probably only sensory (_s.h._); except three, which protrude well over the hinder end (_c.p._), stout, pointed, and frayed out at the ends, and possibly serving as oars or rudders for the darting movements. These are distinguished from simple cilia as "cirrhi."

{139}

At the right hand of the frontal area there begins, just within the dorsal edge, a row of strong cilium-like organs (Fig. 49, _per_); these, on careful examination, prove to be transverse triangular plates, which after death may fray into cilia.[154] They are the "adoral membranellae." This row passes to the left blunt angle, and there crosses over the edge of the body to the ventral aspect, and then curves inwards towards the median line, which it reaches about half-way back, where it passes into the pharynx (_m_). It forms the front and left-hand boundary of a wedge-shaped depression, the "peristomial area," the right-hand boundary being the "preoral ridge" or lip (_l_), which runs nearly on the median line, projecting downward and over the depression. This ridge bears on its inner and upper side a row of fine "preoral cilia" (_poc_) and a wide "preoral undulating membrane" (_p.om_), which extends horizontally across, below the peristomial area. The roof of this area bears along its right-hand edge an "internal undulating membrane" (_g_), and then, as we pass across to the left, first an "endoral membrane" and then an "endoral" row of cilia. In some allied genera (not in _Stylonychia_), at the base and on the inner side of each adoral membranella, is a "paroral" cilium. {140}All these motile organs, with the exception of the preoral cilia, pass into the pharynx; but the adoral membranellae soon stop short for want of room. There are some seventy membranellae in the adoral wreath.

The rest of the ventral surface is marked by longitudinal lines, along which the remaining appendages are disposed. On either side is a row of "marginal cirrhi" (_mc._), which, like the membranellae, may fray out into cilia, but are habitually stiff spine-like, and straight in these rows; these are the chief swimming organs. Other cirrhi, also arranged along longitudinal rows, with so many blank spaces that the arrangement has to be carefully looked for, occur in groups along the ventral surface. On the right of the peristome are a group which are all curved—the "frontal cirrhi" (_f.c._). Behind the mouth is a second group—the "abdominal cirrhi" (_a.c._), also curved hooks; and behind these again the straight spine-like "caudal" or "anal" cirrhi (_c.c_), which point backwards. These three sets of ventral cirrhi are the organs by which the animal executes its crawling and darting movements. Besides the mouth there are two other openings, both indistinguishable save at the very moment of discharge; the anus (_an_) which is dorsal, and the pore of the contractile vacuole, which is ventral.

The protoplasm of the body is sharply marked off into a soft, semi-fluid "endoplasm" or "endosarc," and a firmer "ectoplasm" or "ectosarc." The former is rich in granules of various kinds, and in food-vacuoles wherein the food is digested. The mode of ingestion, etc., is described below (p. 145). The ectoplasm is honeycombed with alveoli of definite arrangement, the majority being radial to the surface or elongated channels running lengthwise; inside each of these lies a contractile plasmic streak or myoneme. The contractile vacuole (_cv_) lies in this layer, a little behind the mouth, and is in connexion with two canals, an anterior (_e_) and a posterior, from which it is replenished.

The nuclear apparatus lies on the inner boundary of the ectoplasm; it consists of (1) a large "meganucleus" formed of two ovoid lobes (_N_, _N_), united by a slender thread; and (2) two minute "micronuclei" (_n_, _n_), one against either lobe of the meganucleus.

_Stylonychia_ multiplies by transverse fission, the details of which are considered on pp. 144, 147.

The protoplasm of Ciliata is the most differentiated that we {141}find in the Protista, and we can speak without exaggeration of the "organs" formed thereby.

The form of the body, determined by the firm PELLICLE or plasmic membrane, is fairly constant for each species, though it may be subject to temporary flexures and contractions. The pellicle varies in rigidity; where the cilia are abundant it is proportionately delicate, and scarcely differs from the ectoplasm proper, save for not being alveolate. In the Peritrichaceae it is especially resistant and proof against decay. In _Coleps_ (Gymnostomaceae) it is hardened and sculptured into the semblance of plate-armour, and the prominent points of the plates around the mouth serve as teeth to lacerate other active Protista, its prey; but, like the rest of the protoplasm, this disappears by decay soon after the death of the _Coleps_. Where, as in certain Oligotrichaceae, cilia are absent over part of the body, the pellicle is hardened; and on the dorsal face and sides of _Dysteria_ it even assumes the character of a bivalve shell, and forms a tooth-like armature about the mouth.

From the pellicle protrude the CILIA, each of which is continued inwards by a slender basal filament to end in a "basal granule" or "blepharoplast." The body-cilia are fine, and often reversible in action, which is exceptional in the organic world. They may be modified or combined in various ways. We have seen that in _Stylonychia_ some are motionless sensory hairs. The cirrhi and setae sometimes fray out during life, and often after death, into a brush at the tip, and have a number of blepharoplasts at their base. The same holds good for the membranellae and undulating membranes. They are thus comparable to the "vibratile styles" of Rotifers (Vol. II. p. 202) and the "combs" or "Ctenophoral plates" of the Ctenophora (p. 412 f.).[155]

{142}

The ECTOSARC has a very complex structure. Like other protoplasm it has a honeycombed or alveolate structure, but in this case the alveoli are permanent in their arrangement and position. Rows of these alveoli run under the surface; and the cilia are given off from their nodal points where the vertical walls of several unite, and wherein the basal granule or blepharoplast is contained. Longitudinal threads running along the inner walls of the alveoli of the superficial layer are differentiated into muscular fibrils or "myonemes," to which structures so many owe their marked longitudinal striation on the one hand, and their power of sudden contraction on the other. The appearance of transverse striation may be either due to transverse myonemes, or produced by the folds into which the contraction of longitudinal fibrils habitually wrinkles the pellicles, when it is fairly dense (Peritrichaceae); circular muscular fibrils, however, undoubtedly exist in the peristomial collar of this group. Embedded in the ectosarc are often found TRICHOCYSTS,[156] analogous {143}to the nematocysts of the Coelenterata (p. 247), and doubtless fulfilling a similar purpose, offensive and defensive. A trichocyst is an oblong sac (4 µ long in _Paramecium_) at right angles to the surface, which on irritation, chemical (by tannin, acids, etc.) or mechanical, emits or is converted into a thread several times the length of the cilia (33 µ), often barbed at the tip. In the predaceous Gymnostomaceae, such as _Didinium_, the trichocysts around (or even within) the mouth are of exceptional size, and are ejected to paralyse, and ultimately to kill, the active Infusoria on which they feed. In most of the Peritrichaceae they are, when present, limited to the rim around the peristome, while in the majority of species of Ciliata they have not been described. Fibrils, possibly nervous,[157] have been described in the deepest layer of the ectosarc in Heterotrichaceae.

The innermost layer of the ectosarc is often channelled by a system of canals,[158] usually inconspicuous, as they discharge continuously into the CONTRACTILE VACUOLE; but by inducing partial asphyxia (_e.g._ by not renewing the limited supply of air dissolved in the drop of water on the slide under the cover-glass), the action of the vacuole is slackened, and these canals may be more readily demonstrated. The vacuole, after disappearance, forms anew either by the coalescence of minute formative vacuoles, or by the enlargement of the severed end of the canal or canals. The pore of discharge to the surface is visible in several species, even in the intervals of contraction.[159] The pore is sometimes near that of the anus, but is only associated with it in Peritrichaceae, where it opens beside it into the vestibule or first part of the long pharynx, often through a rounded reservoir (Fig. 60, _r_) or elongated canal.

The ENDOSARC, in most Ciliates well differentiated from the ectosarc, is very soft; though it is not in constant rotation like that of a Rhizopod, it is the seat of circulatory movements alternating with long periods of rest. Thus it is that the food-vacuoles, after describing a more or less erratic course, come to discharge their undigested products at the one point, the ANUS. {144}In a few genera (_Didinium_, for instance) the course from mouth to anus is a direct straight line, and one may almost speak of a digestive tract. In _Loxodes_ and _Trachelius_ (Fig. 56) the endosarc, as in the Flagellate _Noctiluca_ (Fig. 48, p. 133), has a central mass into which the food is taken, and which sends out lobes, which branch as they approach and join the ectoplasm. The endosarc contains excretory granules, probably calcium phosphate, droplets of oil or dissolved glycogen, proteid spherules, paraglycogen grains, etc.

The NUCLEAR APPARATUS lies at the inner boundary of the ectoplasm. The "meganucleus" may be ovoid, elongated, or composed of two or more rounded lobes connected by slender bridges (_Stentor_, _Stylonychia_). The "micronucleus" may be single; but even when the meganucleus is not lobed it may be accompanied by more than one micronucleus, and when it is lobed there is at least one micronucleus to each of its lobes.[160] The meganucleus often presents distinct granules of more deeply staining material, varying with the state of nutrition; these are especially visible in the band-like meganuclei of the Peritrichaceae (Figs. 51, 60). At the approach of fission it is in many cases distinctly fibrillated.[161] But all other internal differentiation, as well as any constriction, then disappears; and the ovoid or rounded figure becomes elongated and hour-glass shaped, and finally constricts into two ovoid daughter-meganuclei, which, during and after the fission of the cell, gradually assume the form characteristic of the species. The micronuclei (each and all when they are multiple) divide by modification of karyokinesis (or "mitosis") as a prelude to fission: in this process the chromatin is resolved into threads which divide longitudinally, but the nuclear wall {145}remains intact. If an Infusorian be divided into small parts, only such as possess a micronucleus and a fragment of the meganucleus are capable of survival. We shall see how important a part the micronuclei play in conjugation, a process in which the old meganuclei are completely disorganised and broken up and their débris expelled or digested.

The MOUTH of the Gymnostomaceae is habitually closed, opening only for the ingestion of the living Protista that form their prey. It usually opens into a funnel-shaped PHARYNX, strengthened with a circle of firm longitudinal bars, recalling the mouth of an eel-trap or lobster-pot ("Reusenapparat" of the Germans); and this is sometimes protrusible. In _Dysteria_ the rods are replaced by a complicated arrangement of jaw- or tooth-like thickenings, which are not yet adequately described. We have above noted the strong adoral trichocysts in this group.

In all other Ciliates[162] the "mouth" is a permanent depression lined by a prolongation of the pellicle, and containing cilia and one or more undulating membranes, and when adoral membranellae are present, a continuation of these. In some species, such as _Pleuronema_ (Fig. 57), one or two large membranes border the mouth right and left. In Peritrichaceae the first part of the pharynx is distinguished as the "vestibule," since it receives the openings of the contractile vacuole or its reservoir and the anus. The pharynx at its lower end (after a course exceptionally long and devious in the Peritrichaceae; Figs. 51, 60) ends against the soft endosarc, where the food-particles accumulate into a rounded pellet; this grows by accretion of fresh material until it passes into the endosarc, which closes up behind it with a sort of lurch. Around the pellet liquid is secreted to form the food-vacuole. If the material supplied be coloured and insoluble, like indigo or carmine, the vacuoles may be traced in a sort of irregular, discontinuous circulation through the endosarc until their remains are finally discharged as faeces through the anus. No prettier sight can be watched under the microscope than that of a colony of the social Bell-animalcule (_Carchesium_) in coloured water—all producing food-currents brilliantly shown up by the wild eddies of the pigment granules, and the vivid blue or crimson colour of {146}the food-vacuoles, the whole combining to present a most attractive picture. Ehrenberg fancied that a continuous tube joined up the vacuoles, and interpreted them as so many stomachs threaded, as it were, along a slender gut; whence he named the group "Polygastrica."

We owe to Miss Greenwood[163] a full account of the formation and changes of the food-vacuoles in _Carchesium polypinum_. The vacuole passes steadily along the endosarc for a certain time after its sudden admission into it, and then enters on a phase of quiescence. A little later the contents of the vacuole aggregate together in the centre of the vacuole, where they are surrounded by a zone of clear liquid; this takes place in the hollow of the meganucleus, in this species horseshoe-shaped. The vacuole then slowly passes on towards the peristome, lying deep in the endosarc, and the fluid peripheral zone is absorbed. {147}For some time no change is shown in the food-material itself: this is the stage of "storage." Eventually a fresh zone of liquid, the true digestive vacuole, forms again round the food-pellet, and this contains a peptic juice, of acid reaction. The contents, so far as they are capable of being digested, liquefy and disappear. Ultimately the solid particles in their vacuole reach the anal area of the vestibule, and pass into it, to be swept away by the overflow of the food-current. The anus is seated on a transverse ridge about a third down the tube, the remaining two-thirds being the true pharynx.

FISSION is usually transverse; but is oblique in the conical Heterotrichaceae, and longitudinal in the Peritrichaceae. It involves the peristome, of which one of the two sisters receives the greater, the other the lesser part; each regenerates what is missing. When there are two contractile vacuoles, as in _Paramecium_, either sister receives one, and has to form another; where there is a canal or reservoir divided at fission, an extension of this serves to give rise to a new vacuole in that sister which does not retain the old one. In some cases the fission is so unequal as to have the character of budding (_Spirochona_). We have described above (p. 144) the relations of the nuclear apparatus in fission.

Several of the Ciliata divide only when encysted, and then the divisions are in close succession, forming a brood of four, rarely more. This is well seen in the common _Colpoda cucullus_. In the majority, however, ENCYSTMENT is resorted to only as a means of protection against drought, etc., or for quiet rest after a full meal (_Lacrymaria_).

Maupas[164] has made a very full study of the LIFE-CYCLES of the Ciliata. He cultivated them under the usual conditions for microscopic study, _i.e._ on a slide under a thin glass cover supported by bristles to avoid pressure, preserved in a special moist chamber; and examined them at regular intervals.

{148}

The animals collect at that zone where the conditions of aeration are most suitable, usually just within the edge of the cover, and when well supplied with food are rather sluggish, not swimming far, so that they are easily studied and counted. When well supplied with appropriate food they undergo binary fission at frequent intervals, dividing as often as five times in the twenty-four hours at a temperature of 65-69° F. (_Glaucoma scintillans)_, so that in this period a single individual has resolved itself into a posterity of 32; but such a rapid increase is exceptional. At a minimum and a maximum temperature multiplication is arrested, the optimum lying midway. If the food-supply is cut off, encystment occurs in those species capable of the process; but when there is a mixture of members of different broods of the same species, subject to the limitations that we shall learn, conjugation ensues. Under the conditions of Maupas' investigations he found a limit to the possibilities of continuous fissions, even when interrupted by occasional encystment. The individuals of a series ultimately dwindle in size, their ciliary apparatus is reduced, and their nuclear apparatus degenerates. Thus the ultimate members of a fission-cycle show a progressive decay, notably in the nuclear apparatus, which Maupas has aptly compared to "senility" or "old age" in the Metazoan. If by the _timely_ mixture of broods conjugation be induced, these senile degenerations do not occur.[165] In _Stylonychia {149}mytilus_ the produce of a being after conjugation died of senility after 336 fissions; in _Leucophrys_ after 660.

Save in the Peritrichaceae (p. 151) CONJUGATION takes place between similar mates, either of the general character and size of the species, or reduced by fissions, in rapid succession, induced by the same conditions as those of mating. The two mates approach, lying parallel and with their oral faces or their sides (_Stentor_) together, and partially fuse thereby; though no passage of cytoplasm is seen it is probable that there is some interchange or mixture.[166]

{150}

The meganucleus lengthens, becomes irregularly constricted, and breaks up into fragments, which are ultimately extruded or partially digested. The micronucleus enlarges (Fig. 52, A) and undergoes three successive divisions, or, strictly speaking, two fissions producing four nuclei, of which one only undergoes the third. The other three nuclei of the second fission degenerate like the meganucleus.[167] Of the two micronuclei of this last division one remains where it is as a "stationary" pairing nucleus, while its sister passes as a "migratory" pairing-nucleus into the other mate, and fuses with its stationary pairing-nucleus. Thus in either mate is formed a "zygote-nucleus," or "fusion-nucleus." All these processes are simultaneous in the two mates; and the migratory nuclei cross one another on the bridge of junction of the two mates (Fig. 52, C). Each mate now has its original cytoplasm (subject to the qualification above), {151}but its old nuclear apparatus is replaced by the fusion-nucleus. This new nucleus undergoes repeated fissions; its offspring enlarge unequally, the larger being differentiated as mega-, the smaller as micro-nuclei. The mates now separate (Fig. 52, F, G), and by the first (or subsequent) fission of each, the new mega- and micro-nuclei are distributed to the offspring. _Colpidium colpoda_ offers the simplest case, on which we have founded our diagram showing the nuclear relations. During conjugation the oral apparatus often atrophies, and is regenerated; and in some cases the pellicle and ciliary apparatus are also "made over."

In the Peritrichaceae the mates are unequal; the larger is the normal cell, and is fixed; the smaller, mobile, is derived from an ordinary individual by brood-divisions, which only occur under the conditions that induce conjugation (Fig. 60). Here, though the two pairs of nuclei are formed, it is only the migratory {152}nuclei that unite, the stationary ones aborting in both mates. During the final processes of conjugation the smaller mate is absorbed into the body of the larger, and so plays the part of male there. But this process, though one of true binary sex, is clearly derived from the peculiar type of equal reciprocal conjugation of the other Infusoria.

The Ciliata are almost all free-swimming animals with the exception of most of the Peritrichaceae, and of the genera we now cite. _Folliculina_ forms a sessile tube open at either end; and _Schizotricha socialis_ inhabits the open mouths of a branching gelatinous tubular stem, obviously secreted by the hinder end of the animal, and forking at each fission to receive the produce. A similar habit to the latter characterises _Maryna socialis_; all three species are marine, and were described by Gruber.[168] _Stentor_ habitually attaches itself by processes recalling pseudopodia, and often forms a gelatinous sheath.

The majority of the Oligotrichaceous Tintinnidae inhabit free chitinous tests often beautifully fenestrated, as in _Dictyocystis_.

Many genera are parasitic in the alimentary canal of various Metazoa, but none appear to be seriously harmful except _Ichthyophtheirius_, which causes an epidemic in fresh-water fish. Quite a peculiar fauna inhabit the paunch of Ruminants. _Nyctotherus_ and _Balantidium_ are occasionally found in the alimentary canal of Man.[169]

The Gymnostomaceae are predaceous, feeding for the most part on smaller Ciliates. We have described the peculiar character of the mouth and pharynx in this group, and the mail-like pellicle of _Coleps_ (Fig. 54). _Loxophyllum_ is remarkable for the absence of cilia from one of the sides of its flattened body, and the tufts of trichocysts studding its dorsal edge at regular intervals. _Actinobolus_ has numerous tentacles, exsertile and retractile, each bearing a terminal tuft of trichocysts, which serve to paralyse such active prey as _Halteria_. _Ileonema_ has one tentacle overhanging the mouth; and _Mesodinium_ has four short sucker-like projections around it.[170] It has only two girdles {153}of cilia, which are stout and resemble fine-pointed cirrhi. In _Dysteria_ the cilia are exclusively ventral, and the naked dorsal surface has its pellicle condensed into a bivalve shell; a posterior motile process ("foot") and a complex pharyngeal armature add to the exceptional characters of the genus.

The Aspirotrichaceae are well known to every student of "Elementary Biology" by the "type" _Paramecium_ (Fig. 55), so common in infusions, especially when containing a little animal matter. _P. bursaria_ often contains in its endosarc the green symbiotic Flagellate _Zoochlorella_. _Colpoda cucullus_, very frequent in vegetable infusions, usually only divides during encystment, and forms a brood of four. _Pleuronema chrysalis_ (Fig. 57) is remarkable for its habit of lying for long periods on its side and for its immense undulating membrane, forming a lip on the left of its mouth; _Glaucoma_ has two, right and left.

{154}

The Heterotrichaceae present very remarkable forms. _Spirostomum_ is nearly cylindrical, and, a very giant, may attain a length of 4 mm. (1/6"). _Stentor_ can attach itself by its hinder end, which is then finely tapered and prolonged into a few pseudopodia; its body is trumpet-shaped, with a spiral peristome forming a coil round its wide end, and leading on the left side into the mouth. Many species when attached secrete a gelatinous sheath or tube. _S. polymorphus_ is often coloured green by _Zoochlorella_ (p. 125); _S. coeruleus_[171] and _S. igneus_ owe their names to the brilliant pigment, blue or scarlet, deposited in granules in lines between the conspicuous longitudinal myonemes. From their large size and elongated meganucleus accompanied by numerous micronuclei, these two genera have frequently been utilised for experiments on regeneration. In _Metopus sigmoides_ the peristomial area forms a dome above its wreath of membranellae; and in _M. pyriformis_ this is so great as to form the larger part of the cell, which is top-shaped, tapering behind to a point. _Caenomorpha_ (Fig. 58) has the same general form, with a peg-like tail, and possesses a girdle of cirrhi.[172] The converse occurs in {155}_Bursaria_; the cell is a half ellipse, something like a common twin tobacco-pouch when closed: a deep depression thus occupies the whole ventral surface, and opens by a wide slit extending along the anterior end. The peristomial area occupies the dorsal side of the pocket so formed, and the mouth is in the hinder left-hand corner. _Blepharisma_ sp. is parasitic in the Heliozoon _Raphidiophrys viridis_ (Fig. 20, 1, p. 74).

Among Oligotrichaceae, _Halteria_, common among the débris at the bottom of pools in woods containing dead leaves, is remarkable for an equatorial girdle of very long fine setae, and for its rapid erratic darting movements, alternating with a graceful bird-like hover. The Tintinnidae are mostly marine, pelagic, with the general look of a stalkless _Vorticella_; some have a latticed chitinous shell.[173]

{156}

Among Peritrichaceae, _Vorticella_ (Fig. 60) and its allies have long been known as Bell-animalcules to every student of pond-life. The body has indeed the form of an inverted bell, closed at its mouth by the "peristome," or oral disc; this is a short, inverted truncate cone set obliquely so that its wide base hardly projects at one side, but is tilted high on the other; the edge of the bell is turned out into a rim or "collar," separated from the disc by a deep gutter. The collar, habitually everted, or even turned down, contracts over the retracted disc when the animal is retracted (E^2), which is brought about by any sort of shock, or when it swims freely backwards. For the latter purpose a posterior ring of cilia (or rather membranellae) is developed round the hinder end of the bell (A, _cr_, E^3). The cilia of the adoral wreath are very strong, united at the base into a continuous membrane, and indeed themselves partake of the composite nature of membranellae. The wreath forms more than one turn of a right-handed spiral, the innermost turn ending abruptly on the disc, the outer leading down into the mouth at the point where the disc is most tilted and the groove deepest.[174] The pharynx (_p_) is long, and contains an undulating membrane (_u.m_) on its inner side projecting out through the mouth, and numerous cilia; it leads deep into the body (_p_). The first part is distinguished as the "vestibule" (_v_), as into it opens the anus, and the contractile vacuole (_c.v._), the latter sometimes opening by a reservoir (_r_). The body contains in the ectoplasm {157}myonema-fibrils which, by their contraction, withdraw the disc, and at the same time circular fibrils close the peristome over it. In the type-genus the pellicle is continued into a long, slender elastic stalk (_s_), of which the longitudinal myoneme fibrils of the ectoplasm converge to the stalk, and are prolonged into it as a spirally winding fibre, sometimes transversely striated.[175] The effect of the contraction of this is to pull the stalk into a helicoid spiral (like a coil-spring), with the line of insertion of the muscle along the inner side of the coils, which is, of course, the shortest path from one end to the other (Fig. 60, B).

The members of the Vorticellidae are very commonly attached to weeds or to various aquatic Metazoa, each species being more or less restricted in its haunts. _Vorticella_, the type, is singly {158}attached to a contractile stalk; fission takes place in the vertical plane, and one of the two so formed retains the original stalk, while the other swims off (Fig. 60, E^1-E^3), often to settle close by, so that the individuals are found in large social aggregates, side by side, fringing water-weeds with a halo visible to the naked eye, which disappears on agitation by the sudden contraction of all the stalks. _Carchesium_ and _Zoothamnium_ differ from _Vorticella_ in the fact that the one daughter-cell remains attached by a stalk coming off a little below the body of the other, so as to give rise to large branching colonies.