2/6 



THE PRO TO CHORDA TA . 



^y 



^-^' 



Fig. 132. — Saj^iita hexaptera from the 

 ventral surface ; nearly thi'ee times natural 

 size. (After O. Hertwig.) 



a. Anus. bc^. Head-cavities. bc^. 

 Trunk-ccelom. b3. Caudal coslom. c.L 

 Caudal septum, com. Commissure, from 

 the cerebral ganglion to the single ventral 

 ganglion. f^^P', f^- Fins. vi. Mouth. 

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

 sile bristles. J. z*. Seminal vesicle. ^.Tes- 

 tis, v.g. Ventral ganglion. 



neural tube into the extreme 

 posterior end of tiie 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 



