Vermes with a sub-cylindrical non-segmented body, usually brightly coloured. The fore-part of the body is either invaginable, and provided with tentacles, or prolonged into an extensile prostomium, which is readily cast off. Setae may be present as an anterior ventral pair, and one or two posterior circlets. The mouth is anterior, or at the base of the pros tomium; the anus dorsal and anterior, or terminal and posterior. The nervous system consists of a peripharyngeal band, with or without a supra-oesophageal swelling, and of a ventral non-ganglionated cord. There are no specialised organs of res-piration. A vascular system is usually present; the coelome large. Specialised corpuscles tinged with haemoglobin or haemerythrin are frequently found in the coelomic fluid, and sometimes in the vascular. Nephridia are usually present. The sexes are separate, the male sometimes degenerate; and the genital products are either shed into the coelome and taken up by the nephridia; or the glands are continuous with their ducts.
Exclusively marine and generally distributed.
Of the two sub-groups of Gephyrea, the G. achaeta and G. chaetifera, the former contains the families Sipunculidae and Priapulidae, the latter the Echiuridae. The Sipunculidae have the forepart of the body invaginable, and this 'introvert,' often termed proboscis, is of considerable length, and usually slender as compared with the rest of the body. The introvert is generally covered with hooks, except in the genus Siptmculus1. There
1 Billow found that the anterior part of the introvert was regenerated after removal in 3-5 weeks' time in Phascolosoma and Aspidosiphon (Biol. Centrablatt. iii. p. 14). Sipunculus has often are, with the exception of two genera, oral tentacles variable in form surrounding the mouth, either in a complete or incomplete ring, or aggregated in bundles, which may be entirely dorsal to the mouth. A pair of ciliated tubercles may lie above the supra-oesophageal ganglion. The hind-end of the body is either expanded or pointed and narrow. The Priapulidae are cylindrical in shape, the introvert is short and not slender, and there are no tentacles. The mouth is surrounded by chitinoid teeth, long and curved in Halicryptus. Priapulus has a caudal appendage (double in P. bicaudatus) which is ventral to the anus, hollow, communicating with the coelome, and beset with hollow papillae. The Echiuridae have a long prostomium developed from the larval prostomium, and often termed proboscis, capable of great extension and very mobile, at the base of which the mouth lies ventrally. It is bifid at its apex in Bonellia. The body is cylindrical and tapers posteriorly.
There is a pair of ventral chitinoid hooks placed anteriorly in Echiurus, Thalassemia, and Bonellia. Echiurus Pallasii is also provided with two posterior, nearly complete circles of setae; E. unicinctus with one; but in some species of the genus the circles in question are wanting altogether. The female Hamingia has no setae. The male in this genus as well as in Bonellia is minute, and Planarian in aspect.
The integument is composed of a cuticle, a hypodermis, and muscle layers. A layer of connective tissue, or cutis, may underlie the hypodermis, as in Sipunculus and Echiuridae. The cuticle is chitinoid, and sometimes cast off and renewed (Echiurus). The hypodermis consists of a single layer of cells usually close-set and columnar, sometimes stellate, the intervals being occupied by a clear material (Priapilus, Halicryptus). The colouring-matter of the animal is contained in them. The green hue of Bonellia is due to a peculiar pigment known as Bonellein, which is not identical with chlorophyl as usually stated. It may also be the pigment of the green-coloured Hamingia. Pigment cells are found in clumps in the cutis, etc. The cuticle and hypodermis form together the various processes, - papillae, hooks, plates bearing papillae, which occur in different genera. The spines of the Echiuridae are formed in sacs of invaginated hypodermis, those of the posterior circles in Echiurus from a single cell. Glands are generally present, usually unicellular, sometimes bi- or multicellular as in Sipunculus nudus.
The musculature of the body consists of an external circular layer followed in Sipunculidae by a layer of oblique and then of longitudinal fibres; in Priapulidae by a layer of longitudinal fibres alone; in Echiuridae by a layer of longitudinal and of circular (Greeff) or oblique (Spengel) fibres. The muscles of the different layers been said to possess a posterior terminal pore opening into the coelome. There is a depression, but no pore; and the larva, as figured by Hatschek, has a bundle of sensory hairs at the spot in question.
May be disposed in parallel bands, sometimes connected by anastomotic bundles, e. g. in Sipunculus, some species of Phascolosoma; or one layer . only may be so disposed, e. g. the circular muscles of Priapulus and Hali-cryptus; or all the layers may be continuous, e. g. in Echiurus. But species in the same genus may vary in this respect. The musculature is covered internally like all the organs of the body by a peritoneal epithelium which may be uniformly ciliated (Sipunculus, Phascolosoma).
The nervous system consists of a median ventral cord, and of a pharyngeal ring, which extends in the Echiuridae into the proboscis, coursing along its edges. In Priapulus and Halicryptus the ring and cord are in continuity with the hypodermis, in which the nerves also run; in other cases they lose this connection, and lie internally to the muscular layers. The cord is never ganglionated, and it is only in the Sipunculidae that a bilobed supra-oesophageal swelling is developed. In Echiurus and Thalasse7na it is traversed by a fine canal. The ganglion cells are evenly distributed either as two lateral bands, or more rarely as a single ventral band. The nerves originate as a rule from the cord opposite, or nearly opposite to one another, and in Sipunculus nudus, Echiurus Pallasii, Tha-lassema erythrogrammon, they unite dorsally, thus forming complete rings. The cord may bifurcate posteriorly, as in Sipunculus nudus 1. A plexus of nerve-fibres has been observed in the same animal in the cutis, with terminal branches in connection with small groups of elongated hypodermis cells. A fine pore may perforate the cuticle above these groups.