Nervous system and sense organs. 



401 



nervous system consists merely of a ganglionic mass lying 

 between the two apertures, giving off a few nerves forwards 

 and backwards. 



A structure of doubtful sensory, but of considerable morphological, 



Ph. 



Fig. 174. — Diagram of Ascidian. — After Herdman. 



The arrows indicate the two openings ; the dark border the test. 

 Ph., pharynx, with gill-slits; G., reproductive organs; H., 

 heart, with blood vessels; G.D., genital ducts; R., rectum, 

 ending in cloacal chamber. Surrounding the pharynx the 

 peribranchial cavity is shown. 



importance, is the small sub-neural gland which lies beneath the 

 ganglion, and communicates by a ciliated duct with the pharynx. The 

 opening is usually complex, and forms the so-called dorsal tubercle, 

 which is very distinct on the wall of the pharynx. It lies at the point 

 where the two halves of the ciliated groove, or peripharyngeal band, 

 26 



