The division Anarthropoda includes the three classes of the Spoon-worms (Gephyred), the Ringed Worms (Annelida), and the Arrow-worms (Chaetognatha), and constitutes the highest section of the "Vermes" of modern zoologists. The members of this division are characterised by the possession of an elongated worm-like body, which usually shows a conspicuous composition out of similar, or nearly similar, segments, which, however, are not numerically definite. The nervous system, in the typical members of the division, consists of a ventrally-placed double chain of ganglia, one pair of ganglia corresponding with each segment, the anterior pair being placed above the gullet, and all being united by longitudinal commissures. Lateral locomotive appendages are usually present, but are not composed of successive joints, nor are articulated to the body. A true blood-vascular system is not developed; but there is usually a closed system of "pseudohaemal vessels."

Class I. Gephyrea ( = Sipunculoidea)

Vermiform marine animals, with usually elongated bodies which may be indistinctly annulated, but are not segmented, and carry no locomotive appendages (beyond, occasionally, bristles). Ventral nerve-cords not gang-lionated; sexes generally distinct; a distinct metamorphosis in development.

The members of this class are generally known as "Spoon-worms," and they form a connecting link between the Echino-dermata and the Annelides. The body is worm-like, the integument sometimes ringed (fig. 124), but never divided into distinct segments. There are no ambulacral tube-feet, nor foot-tubercles; but there may be bristles which act as locomotive organs (as in Echiurus), while in Chaetoderma calcareous spines occur in the integument. The outer layer of the integument is chitinous, and beneath the skin is a strong muscular coat of longitudinal and circular fibres. The mouth is placed at the front end of the body, and the ciliated alimentary canal terminates in an anal aperture which may occupy the hinder end of the body, or may be placed far forward on the dorsal surface. A retractile or contractile proboscis is present, and may be provided with bristles or tentacles. The intestine is convoluted, and is suspended freely in a body-cavity, which is filled with a corpusculated fluid, kept in circulation by ciliary action. A vascular ("pseudohaemal") system may be present, or in other cases is not developed. Respiration is carried on by the general surface of the body, by hollow tentacles placed round the mouth (Sipunculus), or by abdominal organs resembling the "respiratory tree" of the Holothurians (as in Echiurus), the latter, however, being rather excretory than respiratory in function. The nervous system has the form of a gangliated oesophageal ring, giving off a ventral cord, which is peculiar in not having distinct ganglia developed upon it. The sexes are in different individuals (united in some species of Sipunculus); and the males may differ greatly from the females in form. The reproductive elements often reach the exterior by the ducts of certain ciliated sacs, which open by pores on the ventral surface of the body, and which correspond with the "segmental organs" of the Annelides.

Fig. 124.   Gephyrea. Syrinx nudus. (After Forbes.)

Fig. 124. - Gephyrea. Syrinx nudus. (After Forbes.)

In their development, the Gephyrea pass through a metamorphosis, generally of a very striking character. The larva (fig. 125) is locomotive, and swims about actively, partly by means of a ciliary ring placed round the neck, and partly by means of ciliated lobes developed on the head. In this "actino-trocha" stage the larva resembles that of the Echinoderms in many respects; whilst the ciliated cephalic lobes call to mind the trochal discs of the Rotifers. The further process of development is also strikingly similar to that characteristic of the Echinoderms, a considerable portion of the larva, including the ciliated cephalic expansion, being ultimately absorbed.

The Gephyrea are exclusively inhabitants of the sea, mostly burrowing in the sand, or hiding in crevices in the rocks. The principal and most widely distributed genus is Sipunculus, the species of which often inhabit the cast-away shells of Univalves, and which ranges from the littoral zone down to a depth of 2500 fathoms.

As to their affinities, the Gephyrea are related, on the one hand, to the Holothurians, from which they differ in the absence of an ambulacral system,* in the fact that the integument is almost universally incapable of secreting calcareous matter, and in the absence of any traces of a radiate arrangement of the nervous system. On the other hand, they show their close relation to the true Annelides, by their general possession of a pseudohaemal system, and of "segmental organs," by the form of the nervous system, and by the resemblance of their larvae to those of many of the Errant Ringed Worms.

Fig. 125.   Larva of Phas colosoma elongation, after development has proceeded to some extent.

Fig. 125. - Larva of Phas-colosoma elongation, after development has proceeded to some extent.

* Sipunculus has a system of peculiar water-vessels developed in connection with the oral tentacles, which may, perhaps, be homologous with the ambulacral vessels of the Echinoderms.

Annelida.

Class II. Annelida (= Annulata). The Annelida are vermiform animals, distinguished from the preceding by the possession of distinct external segmentation ; the nervous system is composed of a ventral, double, gangliated cord, with an oesophageal collar and prae-oeesophageal ganglion.

This class comprises elongated worm-like animals, in which the integument is always soft, and the body is more or less distinctly segmented, each segment usually corresponding with a single pair of ganglia in the ventral cord. All the segments are similar to one another except those at the anterior and posterior extremities of the body. Each segment may also be provided with a pair of lateral appendages, but these are never articulated to the body, and are never so modified in the region of the head as to be converted into masticatory organs.