ENTEEOPNEUSTA 



381 



A 



B 



.'.liJSi'.C;'. 





ji'4!toi<^. 



fc 





0. 



the archenteron (Fig. 167 Cjj and Cm) ; tliese also become 

 detached, and are subsequently found next to the intestine 

 as flattened sacs. The mouth is formed at a somewhat later 

 stage on the ventral surface at the point where a transverse 

 furrow has made its appearance on the outside of the larva 

 (Fig. 168 A). The anus arises at the posterior end of the 

 larva near the place where the blastopore closed. Both 

 month and anus are formed by the fusion of the inner with 

 the outer germ-layer. During these processes the external 

 shape of the larva undergoes important changes. At first a 

 transverse furrow, which gradually deepens, and behind 

 which a second one 

 soon makes its appear- 

 ance, arises at about 

 the middle of the body 

 (Fig. 168 A and B). 

 While the first furrow, 

 even as early as this, 

 marks off the anterior 

 portion — namely, the 

 acorn — from the rest of 

 the body, the second 

 furrow, together with 

 the first, bounds the 

 future collar. Behind 

 this the gill region is 

 now also indicated by 

 the appearance of two 

 pores as evidence of 

 the first pair of gill- 

 pouches (Fig. 168 B 

 and (7, ^•). The part of the larva lying behind the ciliated 

 band gradually elongates (Fig. 168 C). Thus the principal 

 parts of the body of the adult animal are established in the 

 larva even at this stage. 



4r(l 





i 



■'■^^ftrf'rf 



,.•>' 



-< tir 



Fig. 168. — A to C, free-swimming larvas of 

 'BaXa.yiOOiXo&ius Kou-alevskii in different stages of 

 development (after Bateson). e, acorn ["pro- 

 boscis"]; fc, branchial openings ; fcv, collar. 



Development by means of the Tornaria Larva. — 



Not all species of Balanoglossus, however, develop from 

 the egg into the form of the adult animal in so simple a 



