The nervous system, which is in contact with the epidermis, consists of a brain in the prostomium, and, on each side of the body, a ventral cord with five ganglia, connected by transverse commissures in as many segments.
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.
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 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 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."