444 



ZOOLOGY 



The free -swimming larvae of the Tunicata are provided 

 with a tubular nervous system derived from the epiblast. 

 This has an enlarged anterior end into which two unpaired 

 sense organs — a dorsal eye and a ventral auditory sac — project. 

 The rest of the nervous system is traversed by a canal, and in 

 its course along the tail of the larva it gives off paired nerves 

 to the segmentally arranged muscles. In the larva a noto- 

 chord also is present underlying the nervous system. When 



Fig. 259. — Diagrammatic section through the an- 

 terior dorsal part of Ascidia mentula, showing 

 the relations of the nerve ganglion, suhnenral 

 gland, etc. After Herdman. 



the larva fixes itself, the notochord and the greater part of the 

 nervous system atrophy. In Ciona the latter is reduced to an 

 oblong ganglion, situated just below the ectoderm and above 

 the subneural gland, in the angle between the oral and atrial 

 cones. The ganglion is solid, and gives off numerous nerves, 

 chiefly from its angles ; these exhibit a rather marked asym- 

 metry, the nerves of the left side dividing close to their origin, 

 whilst those of the right remain for some distance un- 

 branched. The specialised sense organs of the larva have 

 atrophied. 



Ascidians have no nephridia, and their nitrogenous waste 



