344 PHYSIOLOGY OF THE INVERTEBRATA. 



system is composed of a cerebral ganglion, which has at first 

 the form of a cord containing a cavity. This ganglion is 

 constricted into three parts, and is connected with ganglia 

 in the tail. The first or anterior part of this ganglion gives 

 off nerves to the margin of the pharyngeal aperture. The 

 middle portion of this ganglion has on it the auditory vesicle, 

 the optic organ, and a stalked ciliated olfactory organ. The 

 optic and auditory organs degenerate just before the adult 

 condition is reached. The third or posterior portion of the 

 ganglion is continued into a long nerve, which at the base 

 of the tail forms a ganglionic enlargement. This ganglion 

 gives rise to a nervous cord, which passes into the tail, where 

 it forms a number of small ganglia. Just before the animal 

 i"eaches maturity, the tail aborts, the muscles and notochordal 

 sheath degenerate, and the notochordal axis contracts. The 

 nervous system and sense-organs also degenerate, and the 

 cavity in the nerve-cord and cerebral ganglion disappears. 



In concluding the chapter it may be stated that Prof. E. 

 Kay Laukester* states that " the structure and life-history 

 of the Ascidians may be best explained on the liypothesis 

 that they are instances of degeneration ; that they are the 

 modified descendents of animals of higher, that is, more 

 elaborate structure, and, in fact, are degenerate Vcrtebrcda, 

 standing in the same relation to fishes, frogs, and men, as do 

 the barnacles to shrimps, crabs and lobsters."t 



* Degeneration^ p. 41. 



t For further information relative to the above -subject see the papers of 

 Dr. A. Giard in the ArcJarcs de Zoolor/ie Exjierimevtalc, t. i (1872); Associa- 

 tion Framjaisc ijour VAvunccmcnt des fSciences, t. 3 (1874) : Jierve /Srientifque 

 du II juillet 1S74 ; Revue des Sciences KatureUes, septembre 1874; et 

 Cmiqjtes licndus, 1874-5 ! and also Dr. W. A. Herdman's papers in the 

 Challenger Jiejwrts. 



