276 



THE PROTOCHOKDATA. 



r/' 



-0. < 



Fig. 132. Sagitta hexaptera from the 

 ventral surface ; nearly three times natural 

 size. (After O. HERTWIG.) 



a. Anus. be 1 . Head-cavities. be 1 -. 

 Trunk-coelom. 6c 3 . Caudal coelom. c.l. 

 Caudal septum, corn. Commissure, from 

 the cerebral ganglion to the single ventral 

 ganglion. f l ,f 2 ,f z - Fins. m. Mouth. 

 o.d. Oviduct. ov. Ovary, sp. Prehen- 

 sile bristles, s.v. Seminal vesicle, t. Tes- 

 tis. v.g. Ventral ganglion. 



neural tube into the extreme 

 posterior end of the aliment- 

 ary canal ; in fact, into that 

 portion of it which, in the 

 embryos of the higher forms, 

 is known as the post-anal 

 gut. The anterior neuren- 

 teric canal, in its most primi- 

 tive condition, opens into the 

 base of the buccal tube 



(Fig. 131). 



On this account we find 

 in the Ascidian tadpole that 

 the mouth is no longer ven- 

 tral, as it is in Balanoglossus, 

 but is placed dorsally, im- 

 mediately in front of the 

 anterior extremity of the 

 medullary tube. This in- 

 timate relation between the 

 mouth and the central ner- 

 vous system gives a reason 

 for the contrast between the 

 dorsal position of the mouth 

 in the Ascidian tadpole and 

 its ventral position in Bala- 

 noglossus. 



In Amphioxus we have 

 seen that the mouth has been 

 forced aside from its more 

 primitive dorsal position by 

 the forward extension of the 

 notochord to the tip of the 



