3^4 



EMBRYOLOGY 



Further Developmental Processes of Both Types. 



— Thus far we have cliietiy considered the external shape of 

 the Tornaria. As regards the internal development, we find 

 confirmed the processes which, following Batesox, we have 

 already described. In the youngest stages of Tornnria yet 

 observed (Fig. 170), the archenteron, which in this case 

 never loses its connection with the ectoderm, develops an 

 unpaired evagination. This is said to be the fundament of 

 the so-called water-vascular vesicle, which, like the corre- 

 sponding organ of the Echinoderm larva, opens out by means 

 of a pore on the dorsal surface (Fig. 169 A). This in 

 particular has given rise to a comparison with the Echino- 

 derm larvse. In addition to this diverticulum, two pairs of 

 evaginations arise farther back on the intestine (Agassiz). 



They are the ccelomic sacs, which soun 

 become detached from the intestine, 

 and lie close to it on either side as 

 two pairs of flattened, vesicles. They 

 soon become considerably enlarged, 

 and then their walls are applied to the 

 wall of the body and that of the in- 

 testine as the somatic and splanchnic 

 layers.^ A mesentery is developed 

 dorsally and ventrally, separating the 

 sacs of the body cavity of the two 

 sides ; but, according to Spengel, the 

 dorsal mesentery may afterwards de- 

 o-enerate. The hindermost pair of 

 ccelomic sacs supplies the greater 

 part of the body cavity, — namely, that of the entire trunk, 

 — whereas the body cavity of the collar arises from the 

 anterior pair, and the cavity of the acorn is developed from 

 the so-called water-vascular vesicle (Si'ENGEI,)- The re- 

 semblance of the latter to the paired structures points to 

 the fact that they originally hail the same significance, and 

 the appearance of two acorn pores in Balanoglnssus Kupferi 



1 As is to be seen in Fij,'. 172 (p. 387), entirely similar conditions 

 appear also in Bolunoijloiisus Kowalei skii, which does not develop by 

 means of a Tornaria. 



Fig. 170.— Kiu-ly 8taj;e of 

 a Toriiavia (after Goettk, 

 from Bai.foub'8 Co/ujiaivi- 

 tim Emhryologij). in, mouth ; 

 oil, anus; W, so-called 

 water-vascular vesicle. 



