ECHINODERMATA 405 



The statements of authors differ regarding the position of the blasto- 

 pore ; according to Barbois, it is found at one pole of the larva, whereas 

 Bury and Öoette locate it more toward the future ventral side (Fig. 189). 



II. THE ORIGIN OF THE ENTEROCCELE AND 

 HYDROCCZLE. 



Ill the first part of this chapter we traced the develop- 

 ment of the Echinoderm larva to the point at which the 

 mesenchyma was formed and the larval mouth had broken 

 through on the ventral side. Even before this occurs, im- 

 portant developmental processes, wliicli result in the abstric- 

 tion of the fundaments of the body cavity and water-vascular 

 system, take place in the arclienteron. Both of these arise 

 in all Echinoderms as diverticula of the arclienteron. We 

 apply to them the names employed by Ludwig, enterocoele 

 and hydrociele, without, however, abandoning for the common 

 fundament of both the older, but very significant, name of 

 vaso-perito ileal vesicle (Selenka). 



The formation of the body cavity and the water-vascular 

 system, although taking place in various ways in the differ- 

 ent divisions of the Echinodermata, nevertheless exhibits 

 close relationships between the different groups. We shall 

 accordingly treat them here in the order in which they 

 follow each other most naturally. 



Asteroidea [Agassiz (No. 1), Metschxikoff (No. 37), 

 Gkeeff (Nos. 18 and 19)]. — Before the mouth-opening of 

 the larva is formed, two bilaterally symmetrical outfoldings 

 arise at the blind end of the arclienteron. These soon 

 become considerably enlarged by growing out toward the 

 posterior end of the larva (Fig. 190 A and B). Then 

 each of the two vesicles separates from the intestine, and 

 the left-hand one unites by means of a tube with the dorsal 

 surface of the larva. This pair of vesicles constitutes the 

 fundaments of the body cavity and water- vascular system. 

 Their further development takes place in such a way that the 

 left vesicle is constricted in its posterior portion, whereby 

 a part finally becomes separated off. The abstricted 

 anterior part constitutes the earliest fundament of the 

 water-vascular system [hydrocoele]. It is soon transformed 



