472 THE ANATOMY OF INVERTEBRATED ANIMALS. 



gitudinal section of the anterior end of the body of an adult Synapta digitata. a, 

 perisoma with the longitudinal muscles and radial nerve-trunks ; 6, calcareous 

 plates which surround the gullet; c, tentacular canals ; d, oesophagus ; e, radiat- 

 ing muscles of the pharynx ; g, divided ends of the circum-oral nerve; h, circular 

 ambulacral vessel with Polian vesicle ; i, cavity of a longitudinally divided tenta- 

 cle, into which a tentacular canal opens ; 7c, generative CEeca ; 7, mesentery with 

 the dorsal blood-vessel; m, " auditory vesicle" on the radial nerve ; n, longitudi- 

 nal muscles; o, tentacular pedicels; p, oral disk. F, calcareous plate and anchor 

 of Synapta inhcerens. 



these are attached to the anal circlet, and five to the circum- 

 'cesophageal circlet. Until, however, it has been shown that 

 the circular ambulacral vessel incloses the cloaca as well as 

 the oesophagus — which is highly improbable — it is justifiable 

 to assume that the anus of Rhopalodina is really, as in the 

 Crmoidea, interradial in position. 



The development of the Holothuridea is extremely in- 

 structive. Yelk-division gives rise to a vesicular morula, 

 which undergoes invagination, and becomes converted into 

 an oval ciliated gastrula. The opening of invagination be- 

 comes the anus, while a mouth and gullet are produced by an 

 invagination of the ectoderm, near the anterior end of the 

 body, which unites with and opens into the blind end of the 

 endodermal sac, or archenteron. The completed alimentary 

 canal is thus composed of a gullet, a rounded stomach, and an 

 intestine ; and the cilia of the ectoderm usually become re- 

 stricted to a single band, bent upon itself, though its general 

 direction is transverse to the axis of the body (Fig. 135, B ; 

 Fig. 136, A). At a subsequent period, this single band may 

 be replaced by a series of hoops of cilia (Fig. 136, B). Ac- 

 cording to Kowalewsky, 1 the embryo of Pentacta doliolum 

 does not become ciliated at all, and that of Psoliiius passes 

 from the condition in which the cilia are dispersed over the 

 surface directly into one in which it is provided with five 

 zones of cilia, between two of which the mouth opens. In 

 this condition it singularly resembles the embryo of Coma- 

 tula. And, indeed, in the further advanced condition of the 

 JPsollnus, the oral end of the body, surrounded by triangular 

 calcareous plates, within which the tentacles take their origin, 

 has a striking resemblance to the oral end of the young Pen- 

 tacrinoid larva of Comatula. 



The peritoneal cavity and the ambulacral vessels take their 

 origin, 3 in a very remarkable manner, from the archenteron, 



1 " Mem. de 1' Acad, de St.-P6tersbourg," 1868. 



2 See Metschnikoff, " Studien iiber die Eniwickclung der Echinodermen und 

 Nemertineii" (" Mem. de l'Acad. de St.-P(-tersbourg," xiv., 1869) ; and espe- 

 cially the very satisfactory memoir of Salenka, " Zur Entwickelung der Holo- 

 thurien" {Zeitschrift fur wiss. Zoologie, 1876). 



