POSITION OF TUB ASCIDIAKS. 



387 



While all Tunicates, except Appeiidicularia, are more or 

 less degenerate, losing their vertebrate characters, in Appeu- 

 dicularia these are retained. The heart is situated ventrally, 

 occupying nearly the same relation as in Fig. 386". Accord- 

 ing to Claus,* "the elongated cerebral ganglion is divided 

 by constrictions into three parts; it is connected with a cili- 

 ated pit and an otolithic vesicle, and is prolonged into a 

 nerve-cord of considerable size. The latter is continued 

 into the tail, at the base of which it swells out into a gan- 

 glion; in its further course it forms several small ganglia. 



e^c! i'yfy 



Fig. 3861.— Diagram of embiyo Lamprey. 



•n? br: f- 



Fig, 386'.— Diagram of larval Ascidian. Letterineap in Fig. A. m, mouth; i. 

 digestive tract; s/>, spiracles in the pharyngeal portion: ftt heart; e,eye; er, ear; 

 6r, brain; nc, nervous cord; 6', h", mid-brain; cl, cerebellum; sjtn, spinal nerves; 

 n, notocord; ol, nasal cavity; s, suckers (their homologues also occur in young 

 garpikes and tadpoles). 



vfhence lateral n-rves pass out. In consequence of a torsion 

 of the axis of the tail, the originally dorsally-placed caudal 

 nerve comes to have a lateral position. The segmentation 

 of the nerve-cord in the tail (as shown by the ganglionic 

 swellings) corresponds to the segmental divisions of the 

 muscles, which recall the myotomes of Amphioxus. The 

 large chorda (nrochord), which extends along the whole 

 length of the tail, constitutes another point of resemblance 

 to Amphioxus." 



* Text-book of Zoology, English translation, ii., p. 100. 



