The larva of Bonellia viridis possesses two rings of cilia: it also becomes ciliated all over. The mouth probably appears behind the first ring, but is formed after its disappearance. The ventral cord shows no ganglionic rudiments. The anal pouches are said to develope from the rectum (Spengel).
Sipunculus nudus has an invaginate gastrula. During development the epiblast forms an amnion, which does not cover the apex of the prostomium, indicated by a tuft of long cilia. There are no traces of somites. A postoral band of cilia is present, and the anterior nephridia (uterine pouches) appear early. The anus and proctodaeum are dorsal and at first nearly posterior, but the real posterior apex of the body is denoted by a group of sense cells with hairs. The ventral cord is an epiblastic thickening. There is a glandular appendage opening at the posterior margin of the mouth, and a pharyngeal apparatus attached to the ventral wall of the oesophagus. Both structures are lost during the growth of the larva. Ciliated organs, like the Topfchen, but not free, appear very early in the parietal coelomic epithelium of the body. During the growth of the larva, the prostomial process becomes shorter and shorter, and the anus more and more dorsal. The tentacles appear on either side of the mouth, and have no connection with the postoral ring of cilia.
In Phascolosoma elongatum there is an invaginate gastrula. The apex of the prostomium bears sensory cells with hairs. A postoral ring of long cilia appears, and later on a second ring of finer cilia. The body has three pairs of setae, of which the posterior appears first, then the anterior, and lastly the middle. The first circle of hooks on the 'introvert' appears between the two bands of cilia, just behind the mouth. The zona radiata of the egg becomes the larval cuticle.
The development of Sipunculus is, according to Hatschek, a remarkably abbreviated one. That of Echiurus is Chaetopodan, and shows very distinct traces of a metameric condition, but the somites disappear very early. Hatschek therefore proposes to separate the Echiuridae from the other Gephyrea. It is, however, possible that if the development of Sipunculus is abbreviated, it is so to a much greater degree than is that of Echiurus when compared with a typical Chaetopodan. In other words, it has lost even traces of what is most clearly preserved in the life-history of Echiurus. The existence of an embryonic membrane shows great specialisation. It is therefore, perhaps, advisable to retain all the forms provisionally in the same group until something is known as to the ontogeny of other Sipun-culids. Moreover, it is by no means certain that the Sternaspidae, which are variously regarded as belonging to the Gephyrea or Chaetopoda, may not prove to be a connecting link between the two classes.
Sternaspis scutata has been investigated by Vejdovsky and Rietsch. Its chief features are the following. The body is elongated, its fore-part retractile by special muscles. It is segmented, and divisible into an anterior region with seven rings, and a posterior with eight or more rings. It is covered by a cuticle and delicate cirri. All the somites carry setae except 5, 6, 7, but those of somites 8-15 never pierce the cuticle. The cuticle is thickened posteriorly and ventrally into a bilobed shield, the edges of which are set round with bundles of setae. There is a circular and longitudinal muscle-layer, the latter internal. The nervous system consists of a pair of supra-oesophageal ganglia, fibrous commissures, and a ventral cord, which is not segmented. The digestive tract has a pharynx, oesophagus, crop, an intestine disposed in longitudinal coils, and a rectum. Mouth and anus are terminal. A dorsal blood-vessel corresponds to the fore-part of the digestive tract; it forms a peripharyngeal ring and a ventral vessel, from which numerous lateral branches pass off. The capillary system is well developed; the blood, a red plasma.
Excretory organs are present as two brown lobed sacs attached to the skin between somites 6-7, but devoid of external aperture (?). Two posterior and dorsal bundles of long contractile cirri, each traversed by a looped vessel connected both to the dorsal and ventral vessels, constitute special respiratory organs. The sexes are separate. The genital ducts open between somites 8-9 by long tubes, and receive the sexual products direct from the glands. The ova and sperm originate from cells covering the branches of vascular capillaries. The ovum undergoes complete segmentation; the gastrula is epibolic. The larva is ciliated, with an anterior tuft of long cilia, and a large prostomium. Sluiter has investigated an Indian species (St. spinosa). He found a bifid prostomium, which the animals throw off in about a minute; the dorsal vessel bifurcated, and entering the prostomium; the two returning vessels united round the pharynx to form a ventral vessel. There were minute external apertures to the excretory organs.
Vejdovsky, Dk. Akad. Wien, xliii. 1882; Rietsch, A. Sc. N. (6), xiii. 1882; Sluiter, Tijdsch. fur Nederl. Indie, xli. 1882 (cf. Naples Zool. Jahresbericht, 'Vermes,' p. 287).
The Gephyrea are classified as follows
1. Gephyrea Achaeta: unisegmental (?); fore-part of the body invaginable by means of special retractor muscles; no setae, at least in the adult; mouth anterior and terminal.
(a) Sipunculidae: tentacles usually present at the oral extremity. Nervous system with supra-oesophageal swellings. Alimentary canal spirally coiled as a rule, rarely disposed in loops; anus dorsal and anterior. Vascular system rarely absent; closed; enters tentacles; blood moved by cilia as well as by one or two contractile vessels. An anterior pair of nephridial sacs, which serve as genital ducts. Genital products set free into coelome, where they mature. Sexes separate (? always). Ova with a zona radiata. Prostomium of larva aborted; larval setae in Phascolo-soma. Free, or living in shells or tubes, e.g. Phascolosoma, Sipunculus, etc.
(b) Priapulidae: introvert short. Nervous system in continuity with hypo-dermis. Pharynx armed with chitinoid teeth. Alimentary canal either straight or slightly looped; anus terminal. No vascular system nor anterior nephridia. Sexes separate; genital apertures posterior on either side of the anus; ducts continuous with the glands. Priapulus with ventral caudal appendage, single or double, beset with papillae; Halicrypius.
2. Gephyrea Chaetifera: multisegmental, but segmentation lost in adult. A long extensile prostomium, with the mouth at its base. A pair of ventral anterior setae. Nervous and vascular systems extend into the prostomium. Alimentary canal convoluted; anus terminal; a pair of posterior nephridia open into the rectum. One to four pairs of anterior nephridia present, and act as genital ducts. Sexes separate; genital products set free into the coelome. A minute Planarian-like male in the genera Bonellia and Hamingia. Live in mud or sand, or holes in rock, or shells.
Echiuridae: Echiurus, Tha/assema, Bonellia with bifid prostomium; Hamingia.
Gephyrea in general, Danielssen and Koren, Den Norske Nordhavs Expedition, iii. 1881 (cf. A. N. H. (5), vi. 1881, p. 462); Iid. Fauna Littoralis Norwegiae, pt. 3, 1877.
Sipunculidae, Die Sipunculiden, De Mace, Bulow, and Selenka, 1884 (with lit.) in Semper's Reisen im Archipel der Philippinen, Part II, iv. pt. I. Golfingia, Ray Lankester, Tr. L. S. (2), ii. pt. 16, 1884 (key to genera). Sipunculus nudus, Andreae, Z. W. Z. xxxvi. 1882 (with lit.); Keferstein, Z. W. Z. xv. 1865; development of, Hatschek, Arb. Zool. Inst. Wien, v. 1884. Phascolosoma, Keferstein, Z. W. Z. xii. 1862-63, p. 35; development of, Selenka, Z. W. Z. xxv. 1875. Aspi-dosiphon, etc, Sluiter, Beitrage, etc, Nat. Tijdschrift. Nederl. Indie, 43, 1883. Segmental and sex organs, Sluiter, Tijdschrift Nederl. Dierk. Vereen. vi. 1882-85. Blood corpuscles and 'Topfchen) Brandt, Mdmoires de l'Acad. Imp. St. Peters-burgh (7), xvi. 1871. Blood-colouring matter, Ray Lankester, P. R. S. xxi. 1873, p. 80; Krukenberg, Vergleich. Physiol. Studien, i. pt. 3, 1880, p. 82.
Priapulidae, Apel, Z. W. Z. xlii. 1885; skin and nervous system, Scharff, Q. J. M. xxv. 1885; Ehlers, Z. W. Z. xi. 1861-62; Horst, Arch. f. Nederl. Zoologie, Suppl. i. 1881.
Echiuridae: monograph, Rietsch, Recueil Zool. Suisse, iii. 1886; Greeff, Nova Acta, 41, 1879. Echiurus, Spengel, Z. W. Z. xxxiv. 1880; development of, Hatschek, Arb. Zool. Inst. Wien, iii. 1881. Thalassema, Ray Lankester, Z. A. iv. 1881;
Lampert, Z. W. Z. xxxix. 1883; Sluiter, Tijdschrift Nederl. Indie, 43, 1883; development of, Conn, Studies Biol. Lab. Johns Hopkins Univ. iii. pt. 7, 1886. Bonellia, Rietsch (supra); de Lacaze Duthiers, A. Sc. N. (4), x. 1858; development of, and male, Spengel, Mitth. Zool. Stat. Naples, i. 1879; figure of male, Selenka, 'Gephyrea,' Challenger Reports, xiii. 1885, Pl. ii., figs. 7-10. Bonellein, Krukenberg, Vergleich. Physiol. Studien (2), ii. 1882. Hamingia, Ray Lankester, A. N. H. (5), xi. 1883.