Chapter XV: Introduction: Nematoda—anatomy—embryology—classification—ascaridae (5)

The sexes are separate, and are usually similar; the male of _D. gyrociliatus_ is, however, much smaller than the female. The generative organs occupy the greater part of the body-cavity; in the male the testes communicate, by means of the pair of seminal vesicles, with a median eversible apparatus. In the female the paired ovaries communicate with a median sac which serves as a spermatheca.

The development is simple:[296] the worm itself is more like a larval Polychaete than a full-grown worm. _Dinophilus_ is an extremely interesting form, and it has been suggested that, while still possessing certain Planarian characteristics, it may be looked upon as closely resembling the ancestor from which the Chaetopoda have arisen.

{244}_Protodrilus_ and _Polygordius_ are distinctly Annelidan in character. _Protodrilus_[297] is found in the mud of the "Pantano," an inlet of the sea near Messina; whilst of _Polygordius_[298] one species at least occurs on our shores, and several others in the Mediterranean and elsewhere. The worms are cylindrical, with many segments, but these segments are only indistinctly marked externally—by girdles of cilia in _Protodrilus_, or by faint grooves in _Polygordius_; but there are none of the characteristic Chaetopod bristles or chaetae. The small prostomium which overhangs the mouth is provided with a pair of ciliated pits, and carries a pair of tentacles, serving as sensory organs, which, in _Protodrilus_, are also respiratory. The anus is surrounded by glandular papillae in _Polygordius_, by means of which the animal can fix itself; these are represented in _Protodrilus_ by a couple of processes.

The nervous system lies entirely in the epidermis. The body-cavity is regularly segmented by transverse septa passing from the body-wall to the intestinal wall. The foregut presents a slight eversible portion in _Polygordius_, whilst in _Protodrilus_ it has a peculiar U-shaped muscular diverticulum on its ventral surface, corresponding with the similar apparatus in _Dinophilus_; it is capable of eversion, and aids the worm in burrowing, as well as in seizing and swallowing the mud. The vascular system is represented by a dorsal and a ventral vessel, neither of which, however, is contractile. In _Protodrilus_ the dorsal vessel divides into two branches in the first segment, each of which passes to the tip of the tentacle, and returning, joins its fellow to form the ventral vessel. In some species of _Polygordius_ there is a pair of vessels connecting the dorsal and ventral vessels in every segment, but no vessel to the tentacle. The blood is colourless in some species of _Polygordius_, but may be yellow (_P. neapolitanus_), red (_P. lacteus_), or green (_P. erythrophthalmus_). Paired nephridia, with distinct funnels, occur regularly throughout the body.

The sexes are separate in _Polygordius_, whilst _Protodrilus_ is hermaphrodite, bearing ova in the first seven segments and testes in the remaining segments. The genital cells are produced from the body-wall in every segment; their mode of discharge is unknown in the male _Polygordius_, though probably the nephridia {245}convey the spermatozoa to the exterior; but in the female the body-wall ruptures to allow the ova to escape, and then the animal dies. The development of _Polygordius_ has been made the subject of very careful study; the larva has long been known, and is a typical "trochosphere" of rather a depressed form. This "trochosphere" larva is of considerable importance, as it makes its appearance in sundry groups of animals in some form or another. Here, in _Polygordius_, it has the appearance of a couple of wide but low cones united together by their bases, which form the equator of the larva. This equator carries a double girdle of cilia, dividing the animal into a preoral and postoral region; for the mouth is placed on one side of the animal between the two girdles, while the anus lies at the apex of the postoral cone, and is surrounded by another girdle of cilia. The alimentary canal is divisible into three regions; it is separated from the body-wall by an extensive space, which contains cells destined to give rise to muscles and nephridia. A nervous system (apical plate) is present at the apex of the preoral cone. This little larva swims freely on the surface of the sea, moving, balancing, and feeding by means of the girdle of cilia. It soon increases in length by the active growth of the apex of the postoral cone, which becomes cylindrical and then segmented externally and internally. The greater part of the original larva remains of the same shape as before, and forms the head (prostomium and peristomium): small tentacles grow out of the preoral lobe, and after a gradual reduction in the relative size of the "head" by the growth of the segmented "body," the animal becomes worm-like and develops into a _Polygordius_.[299]

ORDER II. POLYCHAETA.

ANATOMY OF NEREIS.—In order to obtain a general idea of a Polychaete worm, it is well to study a concrete example, and for this purpose the common _Nereis_ serves excellently. Several species (see p. 315) occur more or less commonly on our coasts, and the general remarks will apply to one as well as to another.

_Nereis pelagica_ Linnaeus reaches a length of 5 to 6 inches, and is about ¼ inch across. It is convex above, nearly flat below. Its {246}colour is brown or bronze. The worm, which is to be found in shallow water, is made up of a considerable number of rings or segments, constituting the "trunk" or "body," terminated at each end by modified segments known as "head" and "tail" (Fig. 122). The segments composing the trunk are all alike, except for small proportional differences, and it will be convenient to describe a "typical segment" before referring to the head or tail.

A TYPICAL BODY SEGMENT carries on each side a muscular lobed outgrowth, bearing bundles of bristles or "chaetae," and filamentous sensory organs known as "cirri." To this lateral locomotor organ Huxley gave the name "parapodium" (Fig. 124). Each parapodium or foot consists of a basal portion, supporting a dorsal and a ventral process, the "notopodium" (_ntp_) and "neuropodium" (_nrp_) respectively, each of which is bilobed. The lobes are very vascular and glandular, and probably serve as respiratory organs or "gills."

The chaetae, or bristles, of each bundle project from the mouth of a great sac, the lips of which are particularly prominent in _Nereis_. Each chaeta arises from a single cell situated at the bottom of the sac. The chaetae of _Nereis_, as of many other Polychaetes, are of a kind usually termed compound or "jointed," each being composed of a long stalk and a small "appendix" articulated in a cup at its {247}extremity (Fig. 123). The shape of the cup varies; it is in some cases of equal height all round, or it is higher on one side than on the other. Further, the appendix may be short and curved, or more elongate and spear-like; it is generally notched or finely toothed on one side.

In addition to these projecting locomotor chaetae, there is embedded in each of the two chaetigerous lobes a much stouter and dark-coloured, needle-shaped bristle known as an "aciculum," whose point only just projects beyond the surface. This aciculum extends into the interior of the body much farther than do the locomotor chaetae, and it is to it that the muscles serving to move the whole bundle of chaetae are attached. The acicula thus serve as an internal skeleton to the parapodium. The shape of the parapodium, the relative lengths of cirri and lobes, the shape and arrangement of the chaetae, are all employed as specific characters.

{248}The HEAD consists of a preoral portion above the mouth, the "prostomium," and a postoral region surrounding the mouth, the "peristomium" (Fig. 125). The prostomium varies in shape in different species of _Nereis_; but it always carries on its dorsal surface two pairs of eyes. From its narrower anterior end there arises a pair of short, somewhat conical, sensory processes known as the "prostomial tentacles." A second pair of processes springs from the under surface, and rather to the side of the prostomium; these are known as the "palps," and in _Nereis_ are much more conspicuous than the tentacles; each is composed of two parts, a large basal piece and a smaller terminal joint, capable of being withdrawn into the former. The palps are highly muscular, and though they are sensory organs, act also as great lateral lips.

The peristomium is in many species of _Nereis_ (as in _N. pelagica_) considerably larger than the trunk segments; it carries at its anterior edge four filiform cirri on each side, which are directed forwards and used as feelers. They are arranged in couples; a more anterior couple of dorsal and ventral cirri, and a more posterior couple of dorsal and ventral cirri.

The TAIL.—As the most anterior segment is perforated by the mouth, and is modified as described above, so the last or anal segment, which carries the anus, differs from the rest. It is more or less elongated, cylindrical, and without parapodia or chaetae. It retains, however, its pair of ventral cirri, which are very long.

INTERNAL ANATOMY.—In correspondence with the external {249}metamerism there is an internal repetition of parts. For, except in the anterior segments, where the powerful protrusible pharynx is situated, the body-cavity or "coelom" is divided into a series of chambers, by means of muscular septa inserted, on the one hand, into the body-wall at the level of the grooves between the external segments, and, on the other, into the wall of the alimentary canal. Each of these coelomic chambers contains a pair of nephridia, a portion of the intestine, of the vascular system, and of the nervous system, as will be seen in Fig. 124.

The epidermis, which forms the outer part of the _body-wall_, consists of a single layer of cells, covered externally by a thin, tough cuticle. The latter is usually stated to consist of the chemical substance known as chitin, but since the cuticle differs from true chitin by dissolving in caustic potash after a time, Eisig[300] has suggested that its substance is merely a stage in the formation of chitin. The epidermis contains gland-cells, which are especially abundant on the lobes of the parapodia. Below the epidermis lies the circular coat of muscles by whose contraction the worm diminishes its diameter: it is interrupted on each side at the junction of the parapodium with the body. Deeper still lie the longitudinal muscles, which form four great bundles, two dorsal, separated by the insertion of a small mesentery and dorsal blood-vessel, and two ventral bundles separated in the middle line by the nerve-cords. These longitudinal muscles, by their contraction, bend the worm from side to side, and are continuous from segment to segment. A very characteristic muscle, present in all the Polychaeta, is an obliquely transverse sheet of fibres passing from the body-wall at the side of the nerve-cord to the parapodium, where it spreads out and serves to move the parapodium (Fig. 124). All these muscles consist of smooth fibres, as in the earthworm.

The ALIMENTARY CANAL may be divided into the following four regions:—(1) buccal or eversible region, (2) pharynx, carrying the great jaws, (3) oesophagus, (4) intestine.

The first two regions constitute an "introvert" (Lankester[301]). When fully everted the whole of the buccal region is turned inside out, and the terminal aperture leads directly into the pharynx, which is not everted but merely protruded. {250}Throughout the following chapters the word "buccal" region is used for that part—if any—which is thus everted (Figs. 125, 126).

Both the buccal and pharyngeal regions are wrapped round by several coats of muscle, to form apparently a single muscular organ (Fig. 127, _sh_), which occupies about eight segments in a condition of complete introversion. The septa are absent from the anterior part of the body.

The buccal region is lined with chitin, which is specially thickened at certain definite spots, forming small "denticles" or "paragnaths" (Fig. 125), which have a different arrangement in the various species.

The cavity of the pharynx is narrow and the walls thick and muscular; each side wall carries a large, dark, chitinous "jaw" (Fig. 127, J), which is hollow at the base, into which the muscles serving to move it are inserted, whilst the apex is solid, curved, and more or less notched. These two great jaws are used not only for tearing prey, but for seizing it; for when the pharynx is entirely protruded the two jaws are wide apart, and when retraction takes place they come together and grasp the prey.

Eversion of the apparatus is partly effected by protractor muscles (Fig. 126, A, _p_) and partly by the pressure of the coelomic fluid, compressed by the muscles of the body-wall; the eversion is stopped at a certain stage by a sheet of muscular tissue or "diaphragm" (Fig. 127, _diaph_) inserted round the buccal region and attached to the body-wall in the second segment. The introversion is effected partly by the contraction of this diaphragm and partly by the action of powerful retractor muscles (Fig. 126, _r_) inserted into the hinder end of the pharynx and passing to the body-wall (these are removed in Fig. 127). The {251}movement of the jaws themselves and of the wall of the apparatus is due to other muscles.

The _oesophagus_ is quite short; into it opens a pair of sacculated diverticula or glands. Then follows the _intestine_, which extends through the rest of the body as a thin-walled tube, slightly dilated at the insertion of the septa.

The VASCULAR SYSTEM consists of a contractile dorsal vessel and of a non-contractile ventral vessel extending along the whole length of the body, from each of which paired and segmentally-arranged {252}vessels pass to the intestinal wall and to the body-wall, and here form extensive capillary networks (Fig. 124, p. 247). This type of vascular system is pretty generally adhered to throughout the Order, but in the Terebelliformia, Scoleciformia, and Cryptocephala the dorsal vessel and capillary plexus on the intestine are replaced by a continuous blood sinus, situated in the substance of the gut-wall. This "perienteric sinus" has the same relation to the segmental vessels as the dorsal vessel has in the Nereidiformia, and from it a tubular dorsal vessel arises anteriorly. In _Arenicola_ the sinus is preceded in the young stage by a network the branches of which gradually enlarge, meet, and fuse to form the sinus.[302] Whether it is in all cases secondary is a moot point.

This system of vessels in the majority of Chaetopoda contains a respiratory fluid coloured red[303] by haemoglobin in solution; in it float a very few small oval nucleated non-amoeboid corpuscles. But the place of this red pigment is taken by a green one, named "chlorocruorin," in the Chlorhaemidae and many Sabelliformia;[304] whilst in _Magelona_[305] the blood is tinted madder-pink by a number of globules of "haemerythrin." The blood (or "haemal fluid") is driven forwards in the dorsal vessel, and passes backwards in the ventral vessel. Respiration in _Nereis_ is carried on by the whole surface of the body, but naturally with greater activity in the surface of the parapodia, the lobes of which, with their extensive vascular plexus, may be termed "gills"; but it must be borne in mind that these organs have other functions as well.

The _coelomic fluid_, which fills the general body-cavity, is colourless, and contains amoeboid corpuscles or "leucocytes." It corresponds to the lymph of Vertebrates, being nutritive in function, in that it conveys absorbed material from the wall of the intestine to the organs of the body, and at the same time removes any waste substances from these organs; these waste substances contain nitrogen, and are ultimately removed by the nephridia. In _Ophelia_ many of the corpuscles contain a curious dumb-bell-shaped rod of chitin, and it has been shown[306] that this substance {253}is a highly complex form of excretory material,—more complex than guanin, for instance, which exists in the corpuscles of the Capitelliformia.

In Glyceridae, Capitelliformia, and _Polycirrus haematodes_ (a Terebellid), the vascular system is absent, and the coelomic corpuscles become coloured by haemoglobin, and in order that the coelomic fluid may be distributed to the organs of the body, the peritoneum is ciliated along certain definite tracts. The fluid in these "anangian" worms thus combines originally separate functions, and behaves like the "blood" of Vertebrates.

The EXCRETORY SYSTEM is represented by a pair of nephridia in each segment, with the exception of a few anteriorly and a few posteriorly. The nephridium of _Nereis_ differs from that of most other Polychaetes hitherto examined carefully, and rather resembles that of the Oligochaetous Enchytraeids. It consists of a compact gland-like organ, containing a much coiled tube, ciliated for the greater part of its length, but deprived of cilia in its last coils; this latter part—or duct—leaves the "gland" and pierces the body-wall, opening to the exterior at the base of the parapodium. The ciliated canal passes forwards into the next segment, where it opens by a funnel into the coelom. The lip of the funnel is extremely curious, for the cells constituting it are drawn out into very long, delicate processes covered with cilia.[307]

In most Polychaetes the nephridium is a wide, sac-like tube as in _Arenicola_[308] (Fig. 129). Its walls are covered by a dense network of blood-vessels, and it not only acts as an excretory organ, but also as a genital duct (see p. 273).

Excretion, in the strict sense of the word, is carried out by {254}the cells forming the wall of the tube; they remove waste materials from the blood distributed over the surface of the organ. But, in addition, there is a removal from the coelom, by means of the funnel, of any dead or dying coelomic corpuscles which in their turn have eaten up or otherwise destroyed foreign bodies (such as Bacteria, etc.) that may have entered the animal.

In _Nereis_ there is in each segment, in addition to the pair of nephridia, a pair of "dorsal ciliated organs" (Goodrich) (_cil.org_ in Fig. 124). Each appears as a wide-mouthed funnel, greatly folded, and without any permanent outlet. But it is possible that these organs function as genital ducts, and that the external aperture will make its appearance temporarily at the period of maturity. This "dorsal ciliated organ" has not been met with in allied genera—such as _Eunice_, _Nephthys_, _Polynoë_, _Glycera_—where the nephridium is a wide tube, and serves as a genital duct.

The NERVOUS SYSTEM, as in all Chaetopods, consists of a dorsal cerebral ganglion or "brain" (Fig. 127, _br_), connected by circum-buccal commissures with the anterior end of a ventral chain of ganglia. The brain occupies the prostomium,[309] and from it nerves pass away to the prostomial tentacles and palps. The circum-buccal commissures spring from the outer corner of the brain, and from each arises a nerve to the first pair of peristomial cirri. The first ventral ganglion lies in the third segment, and represents at least two ganglion-pairs fused together, for from it arise (1) a pair of nerves to the second pair of peristomial cirri and (2) a {255}pair to the first parapodium. In the remainder of the body there is a ganglion in each segment, whence nerves pass outwards to the parapodium and muscles of the segment (Fig. 124).

In _Nereis_ the apparently single ganglion in each segment really consists of two halves, and the apparently single cord which traverses the whole length of the body consists of two closely apposed cords. In some worms, such as Serpulidae, the two cords are more or less widely separated, and the two ganglia of each segment are thus distinct, and connected by a transverse commissure. In _Nereis_, as well as in many other Polychaeta, the nerve-cords lie within the body-wall, but in other cases they lie in the epidermis, as they do in Archiannelida.

The visceral nervous system, supplying the muscles of the pharynx, is frequently highly developed. In _Nereis_ it arises on each side by two roots, one from the brain, the second from the circum-buccal commissure.

The ORGANS OF SENSE in _Nereis_ are eyes, tentacles, palps, and cirri. The four eyes, which rest upon the brain, have the structure represented in Fig. 130. The retina consists of a single layer of cells containing pigment; each cell is drawn out peripherally into a nerve-fibre, whilst centrally it forms a cuticular product—the "rod" (_h_). The edges of the retina are continuous with the surrounding epidermis, and the cup thus formed remains widely opened to the cuticle in a few Polychaetes, _e.g._ _Autolytus_, and in the young of _Nereis_, but more usually it has the relations represented in the figure. The lens is produced by the retinal cells (according to Andrews[310]), and is in some cases (_Eunice_, _Amphinome_) continuous with the cuticle. It appears to be composed in other cases (_Lepidonotus_) of continuations of the retinal rods. The structure of the other sense organs {256}indicates their adaptation to a tactile function; in each case a nerve traverses the axis of the organ, and the nerve-fibrils terminate in sensory cells. Very probably the palps have a certain power of testing the food—a combination of the senses of taste and smell.

The GENERATIVE SYSTEM.—In all the Polychaeta, with very few exceptions, the sexes are separate; and the reproductive cells—ova and spermatozoa—are produced at certain seasons of the year by the rapid proliferation and modification of coelomic epithelial cells surrounding the blood-vessels in the parapodium and its immediate neighbourhood. The sexual cells remain in the coelom till they are ripe.

The egg-cells become filled with yolk globules; a vitelline membrane is present, and an outer coat of albuminous material. It is doubtful by what means these sexual cells are discharged in _Nereis_. There is some evidence that the "dorsal ciliated organ" may act as a genital duct. In some other worms the nephridia serve this purpose, whilst in others a rupture of the body-wall allows the products to escape into the sea. According to Wistinghausen,[311] at the time of discharge the females of _Nereis dumerilii_ become surrounded by a kind of gelatinous tube formed from a secretion of the parapodial glands, and into this tube the ova are discharged, and arranged in a single layer round its wall.

The common species _Nereis diversicolor_ is viviparous. In a large number of species of _Nereis_ the sexually-mature individuals undergo very marked changes in various parts of their body, so that they differ very greatly from the immature individuals.

These changes resulting in the "heteronereid" condition will be dealt with at some length in Chap. X. p. 276. The larvae of Polychaetes and other facts connected with reproduction are described in the same chapter.

{257}CHAPTER X

CLASSIFICATION OF THE POLYCHAETA—SHAPE—HEAD—PARAPODIA—CHAETAE— GILLS—INTERNAL ORGANS—JAWS—SENSE ORGANS—REPRODUCTION—LARVAL FORMS— BUDDING—FISSION—BRANCHING—REGENERATION.

The POLYCHAETA are marine worms whose bodies are usually elongated and cylindrical; they either lead a free life, swimming in the open sea, or crawling along the bottom; or they pass their life in burrows or definite tubes of various kinds.

Each segment is normally provided on each side with a single or a couple of bundles of chaetae, by means of which locomotion is effected. These, in the free-living forms, are carried at the ends of lateral muscular outgrowths of the body, known as "parapodia," which are practically limbs.

The "head" of the worm generally carries eyes, and frequently more or less elongated tactile organs, the "tentacles" dorsally and "palps" ventrally. The foregut is frequently provided with a masticating apparatus in its anterior region, which is capable of protrusion; but this apparatus is absent in many burrowing and tubicolous forms. The sexes are separate, so that there is no such complicated system of generative organs as occurs in the Oligochaeta. The nephridia usually act as genital ducts. In the majority of cases the egg develops into a larva, the "Trochosphere," which leads a free life and undergoes a greater or less metamorphosis into the adult condition.

The CLASSIFICATION OF POLYCHAETA adopted in this work is as follows:[312]—

{258}BRANCH A. PHANEROCEPHALA.

SUB-ORDER 1. _Nereidiformia_ [= _Errantia_, _auctt._ + _Ariciidae_].

Family 1. Syllidae see p. 306
" 2. Hesionidae " 308
" 3. Aphroditidae " 309
" 4. Phyllodocidae " 313
" 5. Tomopteridae " 315
" 6. Nereidae " 315
" 7. Nephthydidae " 317
" 8. Amphinomidae " 318
" 9. Eunicidae " 318
" 10. Glyceridae " 320
" 11. Sphaerodoridae " 320
" 12. Ariciidae " 321
" 13. Typhloscolecidae " 321

SUB-ORDER 2. _Spioniformia._

Family 1. Spionidae see p. 321
" 2. Polydoridae " 323
" 3. Chaetopteridae " 323
" 4. Magelonidae " 325
" 5. Ammocharidae " 325

SUB-ORDER 3. _Terebelliformia._

Family 1. Cirratulidae see p. 325
" 2. Terebellidae " 327
" 3. Ampharetidae " 330
" 4. Amphictenidae " 330

SUB-ORDER 4. _Capitelliformia._

Family. Capitellidae, see p. 331.

SUB-ORDER 5. _Scoleciformia._

Family 1. Opheliidae see p. 331
" 2. Maldanidae " 332
" 3. Arenicolidae " 333
" 4. Scalibregmidae " 334
" 5. Chlorhaemidae " 334
" 6. Sternaspidae " 335

BRANCH B. CRYPTOCEPHALA.

SUB-ORDER 1. _Sabelliformia._

Family 1. Sabellidae see p. 336
" 2. Eriographidae " 338
" 3. Amphicorinidae " 339
" 4. Serpulidae " 339

SUB-ORDER 2. _Hermelliformia._

Family. Hermellidae, see p. 341.

COMPARATIVE ANATOMY OF THE POLYCHAETA.

GENERAL SHAPE OF THE BODY.—The majority of the Polychaeta have an elongated and very mobile body, like that of _Nereis_, consisting of an indefinite and usually of a considerable number of segments; a few, however, have a shorter body, with fewer segments, definite in number, for instance _Aphrodite_ and _Polynoë_, which have thirty to forty segments; and some Hesionids, with only some seventeen to twenty segments.

{259}In Aphroditidae and certain Amphinomidae the body is more or less oval in shape. In _Lipobranchius_ and _Sternaspis_ it is grub-like, short, and cylindrical, with rounded ends; in the former it is difficult to distinguish head and tail, or dorsal and ventral surfaces.

The segments composing the trunk may be all alike, or may constitute two more or less sharply marked regions, the _thorax_ and _abdomen_, differing in the character of the chaetae, or in their arrangement, or in some other way, as in the Sabelliformia and the Capitelliformia.

As peculiar cuticular structures, the curious shields of _Sternaspis_, and of certain of the Maldanidae may be mentioned.

The posterior extremity is generally more or less narrowed, and most of the Nereidiformia are provided with special elongated cirri, borne by the anal segment. In the Maldanidae and others the body terminates in a funnel, at the bottom of which is placed the anus. Only in a few cases is the anus not terminal; in _Notopygos_ and other Amphinomidae, as well as in some species of _Polynoë_, it is dorsal.[313] In _Sabellaria_ and _Pectinaria_ the hinder end of the body undergoes great degeneration; in the former it is achaetous, but cylindrical and bent forwards alongside the body (Fig. 131). In _Pectinaria_ (Fig. 177), this region, which is called the "scapha," is leaf-like, and serves to close the narrower end of the tube in which the worm lives. _Arenicola marina_, and some Terebellids have no chaetae in the hinder, narrower part of the body.

The HEAD.—The prostomium is, in the majority of cases, rounded or conical, though it may be square (_Nephthys_) or elongated and jointed (_Glycera_), or even hammer-shaped (_Tomopteris_); or it may be fused with the peristomium, and apparently absent (_Arenicola_). In the great group Cryptocephala, the peristomium grows forwards so as to hide the prostomium entirely.

In a few of the Nereidiformia the prostomium is compressed, {260}and in the Amphinomidae it is provided with a dorsal ridge or "caruncle," which is a leaf-like process overlapping three or more segments. In many Aphroditidae (as well as in _Polydora_) there is a peculiar "frontal" ridge passing forwards from the prostomial tentacle, and downwards into the mouth (Figs. 132, _c_, and 133, A, _x_).

In all the Nereidiformia, as well as in Sabelliformia and Chlorhaemidae, the prostomium bears sensory processes of two kinds, viz. dorsal tentacles and ventral palps. The latter are invariably two in number, and are particularly well developed in Aphroditidae, Nereidae, Syllidae, some of the Eunicidae, and in Chlorhaemidae. Even when they are apparently absent, as in _Nephthys_, it is possible that they are represented by certain lobes at the sides of the mouth, for in many Syllidae they are so fused with the prostomium as to be scarcely distinguishable. In the Chlorhaemids the palps[314] are grooved, and in the Sabelliformia they become considerably branched, and extend round the prostomium so as to nearly meet dorsally and ventrally. Each palp is, in this sub-Order, represented by a greater or smaller number of long, mobile filaments, arising from a common base; they are grooved along the inner side, ciliated, and provided with secondary processes. The crown of "gills," in fact, is nothing {261}more than the greatly subdivided and enormously elongated palps, as both Pruvot[315] and Meyer[316] have shown. In such forms as _Haplobranchus_ and _Amphicorine_ the process of subdivision (branching) has only gone a short way. In all the Sabelliformia each filament, in addition to its sensory function, aids in conveying food to the mouth by the action of the cilia, and has a blood-vessel within, thus acting as a respiratory organ. The filament may carry compound eyes (Fig. 143) either at its apex (_Branchiomma_) or at intervals along its course (_Dasychone_).

In the family Serpulidae one (rarely two) of the most dorsally placed gill filaments is enlarged terminally, and acts as a stopper or "operculum," which closes the mouth of the tube when the animal withdraws into it. Further, in _Spirorbis_ this operculum is grooved on one side, and serves as a brood pouch in which the eggs undergo development (Fig. 184, p. 341). It will be seen, therefore, that the palps may be very important organs for the life of the worm, and they are no less interesting to the comparative anatomist, serving as they do as an excellent illustration of the various uses which Nature finds for one and the same organ.

In the other sub-Orders the prostomium carries neither palps nor tentacles.

{262}

The _tentacles_ in the Nereidiformia present a wide variation in number; probably the typical number is three, one of which is median and two lateral—as in Polynoids, Syllidae, and some Eunicidae. Further, there is a certain amount of evidence in the nerve supply of the median tentacle to show that it was originally double. The presence of four tentacles, then, as in _Nephthys_, _Phyllodoce_, and _Glycera_, may be a primitive condition. By the disappearance of the paired lateral tentacles the worm possesses a single median one, as in _Aphrodite_ and Amphinomids;[317] whilst a duplication of these lateral ones leads to the condition of _Eunice_ and _Hyalinoecia_, which have five tentacles. In the Chlorhaemidae the number is further increased to five or more on each {263}side,[318] and in the Terebellidae these prostomial processes become very numerous.

In the Cryptocephala there is never more than a single pair of tentacles, and these are generally reduced to a group of sensory cells, though in _Sabellaria_ they retain a considerable size.

In a few genera, such as _Aphrodite_, _Nephthys_, _Capitella_, the first postoral segment is distinguished from the succeeding segments only by its position with regard to the mouth (Fig. 132) and by its smaller size. But in the remainder of the Polychaeta, with here and there an exception, the peristomium is achaetous in the adult.[319]

Except in the Nereidiformia, peristomial or tentacular cirri are rare, being represented in the Spioniformia by the very long "tentacles." In the Nereidiformia one or more of the following segments may be added to the peristomium, and share in the "cephalisation," which is so characteristic a feature in this group. In Amphinomids the first three or four chaetigerous segments are incomplete ventrally, owing to the shifting of the mouth backwards; these segments form lateral lips, but they are not otherwise modified. In _Phyllodoce_, however, there are four cirri on each side of the mouth, and from the arrangement in the Alciopids we are justified in concluding that the segment which carries the four pairs of cirri is really made up of three segments (Fig. 134, E). Among the Hesionids there are four such "cephalised" achaetous segments with long cirri.

{264}In a few cases, such as the Chlorhaemids and _Sternaspis_, and to a slight degree in _Arenicola_, the "head" and even the anterior part of the worm is capable of being withdrawn into the body.

THE PARAPODIA AND CHAETAE.—The typical parts of a parapodium have been described in the preceding chapter; here it is only necessary to refer to the series of diagrams (Figs. 136, 137) representing the parapodia of the more common Polychaetes, and to add a few remarks about them.

In most Annelids the chaetae are in two bundles on each side, but there are certain families in which the dorsal bundle, and even the notopodium itself, is absent, as in the Eunicidae, Syllidae, and Phyllodocidae; or the dorsal bundle may be absent only in certain regions of the body, as in the hind-body of Terebellids. In some Amphinomidae and Aphroditidae the notopodium is scarcely distinct as a separate lobe, being a slight tubercle on the upper surface of the neuropodium; but the notopodial chaetae are present, and indeed particularly well developed in many cases.

But whilst, in the Nereidiformia, the parapodia, whether {265}consisting of two lobes or only one, are always well developed, and project to a more or less pronounced degree from the sides of the body, it is otherwise in the rest of the group, where the chaetigerous lobes are usually reduced to mere tubercles or ridges, no doubt in relation to their burrowing or tubicolous habits. In _Sternaspis_ the chaetae issue directly from the body-wall.

Amongst the Nereidiformia we find examples in which the parapodia, instead of being more or less conical "legs," are flattened fore and aft so as to serve as efficient "fins," as in the active swimmers, _Nereis virens_ and _Nephthys caeca_, and in the pelagic Phyllodocids, Alciopids, Typhloscolecids, and _Tomopteris_.

Of the typical dorsal and ventral cirri, the ventral is only absent in some Amphinomids amongst the Nereidiformia; the dorsal is absent in _Nephthys_ and degenerate in _Glycera_, whilst in a very large number of families of the other sub-Orders neither cirrus is present. These cirri, though originally filamentous and sensory, may, by virtue of special blood supply, become "gills," and this occurs in several families of different sub-Orders. Thus in _Eunice_ this gill is comb-like; in _Amphinome_ and in _Arenicola_ (on certain segments) it is arborescent, as it is also in one to three segments in Terebellids; whilst in Ariciidae, Spioniformia, Cirratulidae, Opheliidae, and _Sabellaria_ it remains more or less finger-shaped or filamentous. In the family Serpulidae the thoracic cirri, both dorsal and ventral, {266}become flattened and extended antero-posteriorly, and unite with one another to form the "thoracic membrane."[320] In Phyllodocidae the cirri are foliaceous and natatory, and they contain a great quantity of glands of a peculiar character. The Aphroditidae are distinguished from other Annelids by the possession of "elytra" or dorsal scales, which appear to be the dorso-ventrally flattened cirri, retaining their sensory nature, but adding to this function several others.[321]

The CHAETAE or bristles are mainly used in locomotion, but it is not unreasonable to believe that some of the stronger, serrated kinds may be used as weapons of offence and defence; certainly the Polynoids, bristling as they do with stiff chaetae along each side, must be rather unpleasant to their smaller enemies.

The various bristles may be placed in three chief groups, viz. (1) simple; (2) jointed; (3) uncini (see Fig. 138).

(1) The simple chaetae may be smooth and hair-shaped, _i.e._ "capillary," such as are present in nearly all families: or they may be forked (Amphinomidae), comb-shaped (_Eunice_), notched or serrated, or provided with a series of frills at right angles to their length, as in Aphroditidae; or fringed along one or both sides with a membranous expansion, as in Terebellids and Sabellids. The simple chaetae may also be short and spine-like, as in the ventral bundles of _Arenicola_; or they may be slightly curved at the end and notched, forming what are generally termed "crotchets," such as are common amongst Oligochaeta. These "crotchets" may be simple, or have numerous denticulations at the end (Maldanidae), or be provided with a membranous hood (Spioniformia, Capitelliformia). In _Hermione_ peculiar sheathed, spear-like bristles occur (Fig. 138, N).

(2) Jointed chaetae have already been described (p. 246); they are confined to the sub-Order Nereidiformia, and occur only in certain families.

(3) The uncini are very short chaetae, which are simply embedded in the skin, and do not extend beyond the body-wall into the body-cavity. An uncinus is a sharply curved hook, which may have more or less numerous secondary teeth on it. They are characteristic of the Sabelliformia and the Terebelliformia.

The chaetae appear as solid, usually fibrillated structures, of a yellow or golden tint, transparent and refringent. Chemically {267}they consist of _chitin_, and each chaeta is the product of a single cell. The chaetae of _Euphrosyne_ are hollow and calcareous, being peculiar in both characters.

Certain modifications of the chaetae presented by various worms deserve mention. In _Polydora_ (Fig. 133, A) and in _Chaetopterus_ (Fig. 173, p. 324) those of one segment are especially strong, but their significance is uncertain. In _Capitella_ those {268}of the notopodium of the eighth and ninth segments are specially modified; they are analogous to the copulatory chaetae of Oligochaeta. In _Aphrodite_, in addition to the ordinary locomotor chaetae, there are brilliant, iridescent bristles and peculiar felting threads arising from the indistinct notopodium; these latter, however, are not true "chaetae," but are separate chitinous filaments similar to the constituent fibres of an ordinary chaeta.[322]

While the chaetae in the Nereidiformia and others are grouped in bundles, those of many other families are in vertical, transverse rows, as in Maldanidae and in _Arenicola_. The uncini are always embedded in such rows, usually slightly raised from the general level of the body surface, each being termed a "torus uncinigerus." These tori are usually limited to the sides of the body, but in _Myxicola_ and in _Notomastus_ they encroach upon the dorsal surface, and in _Chaetozone_, also upon the ventral, so as nearly to encircle the body, recalling the "perichaetous" condition of some earth-worms.

GILLS.—We have already seen that several different organs, _e.g._ the palps in Sabelliformia, the prostomial tentacles of Chlorhaemidae, and the notopodial cirri of sundry other Polychaetes, may take on a respiratory function. There are, however, certain "gills" developed either on the parapodium itself or elsewhere on the body which it is difficult to homologise. Such are the _retractile gills_ on the parapodia of the Glyceridae (Fig. 136, C); those of _Dasybranchus_, near the abdominal neuropodia; those of _Mastobranchus_, near the notopodia. _Nephthys_ has a sickle-shaped gill on the under surface of the notopodium. The long gill filaments at the posterior end of _Sternaspis_, again, are only doubtfully interpreted as the dorsal cirri of some of the posterior segments.

{269}Since primitively the whole skin of the worm is respiratory, any part of the skin may become more or less specialised for this function, and chiefly, of course, on the more actively moving parapodia. The blood-vessels constituting the essential part of the "gill" may make use of any already existing outgrowth (such as a cirrus or a tentacle), or may push the body-wall out on their own account.

INTERNAL ANATOMY.

Probably those organs which have the greatest effect in modifying the shape of the body are the SEPTA, for we find in the long, free-swimming worms that these are regularly present throughout the body, and external "segmentation" of the body is well marked. In burrowing and tubicolous forms the septa are frequently incompletely developed, or more or fewer may be absent; and the body becomes less distinctly segmented externally, tends to vary greatly in diameter during movement, or becomes plumper. With the disappearance of the septa there is also a diminution in the number of NEPHRIDIA, as in _Arenicola_, with only six pairs. Further, there is frequently a dimorphism of these organs; instead of all of them serving equally as excretory organs and as genital ducts, some of the most anterior in the Sabelliformia and Terebelliformia become greatly enlarged, and take on practically the whole of the former function; whilst more or fewer of the posterior nephridia dwindle in size, and become genital ducts. The absence of septa allows a free communication between the successive segments, and thus a freer flow of coelomic fluid for the distension of the anterior end of the worm during burrowing.

The ALIMENTARY SYSTEM presents certain modifications of a systematic value. In the Nereidiformia the muscular pharynx, which is always protrusible and is preceded by an eversible buccal region, frequently encloses thickened cuticular plates which serve as crushing and grasping organs. The form, number, and arrangement of these "jaws" vary in the different families. They form valuable fossil records of extinct worms.

In the Scoleciformia and Capitelliformia the buccal region exists, but there are no jaws. In the Sabelliformia and Terebelliformia eversion does not take place and jaws are absent.

{270}Amongst the Nereidiformia the jaws are absent in the Phyllodocidae and Hesionidae; when present they are usually set in the direct course of the food. There may be one small tooth used for stabbing, as in some Syllids (Fig. 141, A); or a circle of such denticles (_Autolytus_, Fig. 140, D). To these are added powerful grasping jaws in _Nereis_ (E); or the latter may alone be present, as in _Glycera_ (F). In _Polynoë_ the four jaws are carried by hard pieces, to which the muscles are attached (C and G). In _Nephthys_ there is a dorsal and a ventral jaw.

In the Eunicidae, however, the numerous denticles are carried in a special pouch below the food tract, with which it communicates anteriorly.[323] They are arranged in an upper and lower series. The lower series (L) consists of a pair of flat plates (_k_) on each side partially embedded in and acted upon by muscles, with a harder enamelled piece—the actual lower "tooth" (_j_)—at its anterior end. The upper series (U) consists of several pieces, varying in shape and size in the various genera of this {271}family; but developmentally they result from modifications of two rows of small, similar pieces.[324]

The _intestine_ is generally straight and cylindrical, and is usually constricted by the septa, if these are present. In the Polynoids the intervening sacculations become so long as to receive the name of "caeca," which, in _Aphrodite_, become enormously elongated (Fig. 142); there are eighteen pairs of them (_c_), each being a slender tube bent upon itself, giving off short branches and dilated distally, where it lies in the base of the parapodium.

The intestine is looped in a few genera only, as in _Trophonia_, or coiled, as in _Sternaspis_, _Petta_ (Amphictenid, Fig. 141, C), and _Ammotrypane_. In the course of the tube there may be a thick-walled muscular _gizzard_, with hard chitinous lining, as in certain Terebellids, where it appears to replace, in function, the pharynx of the Nereidiformia; in the Syllidae the gizzard is present in addition to the pharynx (Fig. 141, A).

Glandular appendages of the oesophagus are present in many {272}worms. Amongst the Nereidiformia, the Syllidae and Hesionidae possess oesophageal diverticula (Fig. 141, A, _d_), which are used, not for secreting a digestive fluid, but as reservoirs for water and air swallowed by the worms; and are provided with muscular walls, by which their contents can be driven out. They appear, in fact, to be used like the swim-bladder of fishes.[325] Many Chaetopods take in water by the anus—no doubt for respiratory purposes—and pass it forwards along the intestine. In the Capitelliformia a special groove conducts the water for some distance, then the groove becomes closed to form a canal, which, after a course forwards as a free tube below the intestine, again enters the latter, constituting a "siphonal apparatus," similar to that of the Echiuroids and the sea urchins.

SENSE ORGANS.—In addition to the prostomial _eyes_, which are present in nearly all the Nereidiformia and Spioniformia, eyes may exist elsewhere on the body: thus _Myxicola infundibulum_ and _Fabricia_ possess a pair on the anal segment; in _M. aesthetica_ Clap. there is a pair to every segment; in _Branchiomma_ there is a compound eye near the tip of each gill filament (_i.e._ palp); whilst in _Dasychone_ a series occurs along each gill filament. All these examples belong to the Cryptocephala, in which, owing to certain peculiar modes of life, these sense organs are required in correspondingly peculiar positions. It is usually stated that _Polyophthalmus_ possesses, in addition to the usual prostomial eyes, twelve pairs on as many successive segments; but the minute structure of these organs points rather to their function as light-producing organs.

The Capitelliformia and Opheliidae possess a pair of peculiar "ciliated pits" or "nuchal organs" at the upper side of the head, {273}between the prostomium and peristomium, and capable of eversion (Fig. 144). They are most characteristically developed in the Capitelliformia, where each organ abuts upon a special lobe of the brain. The function of these "ciliated organs," which bear a great resemblance to those of the Nemertines, is a matter of speculation. Similar organs, in the form of simple pits or grooves, occur in many of the Nereidiformia, Terebelliformia, and others.[326]

Otocysts are rare. _Arenicola_ possesses a pair at the base of the prostomium, each of which in some species retains an opening to the exterior.[327] They probably serve as "organs of direction" rather than of "hearing." _Aricia_ and _Polyophthalmus_ likewise have such organs on the prostomium; whilst _Fabricia_, _Myxicola_, _Terebella_, and a few others possess them in the peristomium, or in some other segment of the body.

REPRODUCTIVE PHENOMENA.—With a few exceptions mentioned below, the Polychaeta are unisexual. The sexual cells are developed in all cases from the lining epithelium of the body-cavity. The exact spot at which this occurs varies in different cases; it may be, though rarely, on the floor of the body-cavity; it is more usually on the wall of some blood-vessel, either the ventral vessel or on branches of it; or on the many blind blood-vessels of _Aphrodite_. The number of such genital organs is very great in most worms, but in those presenting two regions of the body they are confined to the posterior segments (Sabelliformia, Terebelliformia, Capitelliformia). The number is very limited in _Arenicola_ and other worms presenting but few nephridia: in the former genus there being six pairs, in _Trophonia_ only one pair.

The following genera are hermaphrodite:—_Amphiglena_, _Salmacina_, _Protula_, _Spirorbis_, belonging to the Sabelliformia, to {274}which must be added some Hesionidae. In this family ova and spermatozoa are developed around the same blood-vessel. But in the former group of worms (as also in _Ophryotrocha_) the two kinds of cells are produced in different regions of the body. Thus in _Protula_ the anterior abdominal segments are male, the posterior ones female, while in _Spirorbis_ the reverse arrangement holds; and in _Syllis corruscans_ the anterior segments of the body contain eggs, whilst the posterior region contains spermatozoa, and this region separates and becomes a male worm.

The eggs and spermatozoa in the Polychaeta are discharged into the sea either by rupture of the body-wall or through the nephridia; the male and female elements unite, and the resulting fertilised eggs undergo development, either floating separately in the water, or embedded in jelly, or attached to the body or to the tube of the worm.

The result of the segmentation of the egg is a free-swimming larva known as a "Trochosphere," similar to that of _Polygordius_. The larvae of different species present various more or less marked departures from this type, for instead of the two girdles of cilia there may be only the anterior girdle, or there may be several complete or incomplete girdles between the two typical ones, or there may be (Chaetopterids) only a single girdle of cilia about the middle of the body, the two typical girdles being absent.[328] The postoral region, after elongation, generally becomes marked out into three segments, and these segments develop chaetae, which are usually temporary and specially long.

The little animal is thus equipped for an independent life: the provisional chaetae help in keeping it balanced; and in some cases (Spionidae) serve to protect the little soft creature, for when it is touched it curls up, and its chaetae stick out at the sides, so that it looks like a hairy caterpillar. But the larva is quite at the mercy of the sea, for it is carried hither and thither by currents, and in this way the species is disseminated. The larvae of the Polychaetes, like those of other animals, occur at certain periods of the year in large quantities at the surface of the sea, and serve as food for various larger animals.

{275}

These larvae are at first very different from the adult animal, and the necessary changes to be passed through are more or less great according to the species. It is not our intention to describe these changes in detail.[329] The larva increases in size, the permanent chaetae make their appearance in regular order, and the body exhibits segmentation, the new segments always appearing just in front of the anal segment. The internal organs gradually develop, and the prostomial and parapodial appendages grow out in their turn. In the Sabelliformia the multifilamentous "gills" arise by the continued branching of an at first simple process (the palp) arising from the latero-ventral surface of each side of the preoral lobe.[330] These gradually encroach dorsally and ventrally till the prostomium is more or less encircled; meanwhile the peristomium grows forwards so as to conceal the prostomium, which no longer increases at the same rate as does the rest of the body.

Although most worms appear to discharge their ova directly into the sea and take no further care of them, some make provision for their offspring either by laying the eggs in a jelly, which will serve as food for the young larvae—_Aricia_, _Ophelia_, _Protula_, _Phyllodoce_—or by attaching them to their body. In certain Polynoids the eggs are attached by means of a secretion to the back, under the elytra, where they undergo development up to a certain stage. In _Exogone_ and some other Syllids they are attached to the ventral cirri, or in _Grubea limbata_, all over the back. In the female _Autolytus_ (_Sacconereis_) a ventrally-placed brood sac is formed by the hardening of a {276}secretion; the eggs develop into embryos inside the brood sac, and then become free, with head appendages and three pairs of parapodia. Enormous numbers of such embryos may occur; for instance, some 300 were counted in a brood sac of _Autolytus ebiensis_. In the case of tubicolous worms, the eggs are frequently attached to the tube, either inside or outside. In _Spirorbis_ and _Salmacina_ the operculum serves as a brood pouch.

Only a very few species are known to be viviparous, viz. _Syllis vivipara_ Kr., _Cirratulus chrysoderma_ Clap., _Marphysa sanguinea_ Mont., and _Nereis diversicolor_ Müll.

In most genera there is no external difference between a mature worm filled with generative products and an immature one, except, it may be, in the colour; for the yolk of the eggs is frequently tinted yellow, or pink, or bluish, while the spermatozoa in mass are white; so that the normal colouring of the worm may be modified when filled with these elements. But in a few instances striking anatomical peculiarities are exhibited by the mature worm.[331] In many species of _Nereis_, for instance, those segments containing the generative products undergo more or less extensive changes, while the anterior ones remain unaltered. The body of the ripe _Nereis_ is then distinguishable into an anterior non-sexual region and a posterior sexual region; and so great are these changes in certain species that the mature worms were for a long time believed to belong to a different genus, and received the name _Heteronereis_. But we now know their true relations, thanks to the work of Claparède and others. The males in the Heteronereid phase have fewer unaltered anterior segments than the females, so that there is a sexual dimorphism.

The changes which _Nereis_ undergoes in its transformation affect chiefly (_a_) the shape of the parapodia, and (_b_) the form of the chaetae of these parapodia. Other organs may also be affected, though less noticeably; thus the eyes become enlarged, the intestine may become so compressed by the generative {277}products as to be functionless, and the tail develops special sensory papillae.[332]

In the parapodia an increase in size and a sharper delineation of the various parts take place; then flattened foliaceous outgrowths (Fig. 147, _x_, _y_) arise from certain lobes of the feet, in which, too, the blood supply becomes greatly increased. The old chaetae are pushed out by the development of new ones of quite a different shape; these are jointed like the old ones, but the appendix is, in many species at least, flattened and oar-shaped (Fig. 123, C, p. 246); and the chaetae are arranged in a fan-like manner. Both these modifications are in evident relation to the free-swimming habit which the Heteronereid now adopts. The new foot serves as a swimming organ, the old one was a walking appendage.