NERVOUS SYSTEM. 



205 



FIG. 88. 



they cease to embrace the alimentary canal. In the lowest 

 compound tunicated animals as in the botryllus and the 

 pyrosoma, there is a small round white coloured opaque 

 ganglion, within the muscular tunic of each of the com- 

 ponent animals, placed near the entrance of the respiratory 

 sac, and between that orifice and the anal. When we com- 

 pare the position of the tho- 

 racic cavity (Fig. 87- /.) andits 

 apertures (a. b.) in the tuni- 

 cata with those of the conch- 

 ifera, we perceive that this 

 single median ganglion (e,) is 

 situate on the ventral side of 

 the body, though at some 

 distance from the entrance 

 (g.) to the stomach, (h.) This 

 ganglion is seen in the same 

 position in the minute com- 

 ponent animals of the poly- 

 clinum, the aplidium, the di- 

 demnum, the euccelium, the 

 synoicum, the diazona, and 

 the distoma. In the larger 

 forms of simple ascidia as in 

 the boltenia, phallusia, and 

 cynthia (Fig. 87,) this last 

 ganglion (e.) has generally a 



more lengthened oval form, and I have sometimes found it bifid 

 both before and behind, where two nervous branches come off 

 from each of its extremities. The two anterior nervous branches 

 are larger than the two posterior which pass backwards 

 on each side of the anal opening (b.) of the muscular tunic. 

 The anterior pair encompasses the respiratory orifice (a), 

 sending off filaments to the fringed and highly sensitive 

 tentacula (c.) which guard this thoracic aperture ; these two 

 branches again meet behind the orifice (d.) and continue as 

 a broad chord along the dorsal part of the muscular coat 

 or mantle. Besides this ganglion, which is connected 

 with the muscular apparatus of the respiratory sac and 

 its openings, like the posterior pair of ganglia in the 

 conchifera which are also sometimes united into one, three 



