50 



BRITISH TITNICATA. 



of some importance in the economy of the animal; for it is 

 constantly pi*esent, and is usually closely associated with the 

 ganglion. In some species the tubercle rests upon the nervous 

 centre ; and when placed at a little distance from it, a nerve 

 may generally be traced running towards (and in some in- 

 stances having all the appearances of supplying) it. 



The nervous system is in a very rudimentary condition in 

 the Tunicata. There is but one ganglion; and it is invariably 

 placed between the two respiratory tubes, in a blood-sinus 

 situated between the inner tunic and linine" membrane, which 



O * 





FIG. 10. FIG. 11. 



FIGS. 10 and 11. The nerve-ganglion in Ascidia mentiila. Fig. 10, as seen 

 from the outside of the mantle. Fig. 11, as seen from the inside of the 

 mantle, a and 6 lead to incurrent tube, b sxibsequently dividing, and 

 one branch going to ventral side of txibe. c and d lead to mantle. 

 e, gland-like organ. Highly magnified. 



sinus, communicating directly with the great ventral channel 

 of the branchial sac, will be well supplied with aerated blood. 

 The ganglion is fusiform, more or less elongated in the antero- 

 posterior direction, and usually a little constricted in the 

 middle, as if composed of two centres (figs. 10 and 11). In 

 A. mentula, and in several other species that have been ex- 

 amined, it is partially folded in a much folliculated gland-like 

 substance, and gives off from each extremity three or four 

 nerves, all of which go to the respiratory tubes and to the 



