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 
Fic. 259.—Diagrammatie section through the an- 
terior dorsal part of Ascidia mentula, showing 
the relations of the nerve ganglion, subneural 
gland, ete. After Herdman. 
ti. Tentacle. 
t. Test. 
m. Mantle. 
dt. Opening of subneural gland. 
pp. Peribranchial band, 
t’. Test turned in at mouth. 
dl. Dorsal lamina. 
sgd. Duct of subneural gland. 
sgl. Subneural gland, 
ng. Brain, 
n. Anterior nerve. 
n’. Posterior nerve. 
brs. Branchial sac. 
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 
