GEPHYREA. 625 



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 viEchiurus 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 provision- 

 ally 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 pro- 

 stomium; 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). 



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