XIII 



BACKBONED ANIMALS 



269 



INH 



There are only two or three genera of Tunicates, especi- 

 ally one called Appendicularia, in which these Vertebrate 

 characteristics are retained throughout life. The others 

 lose them more or less completely. The young Tunicates 

 are active, perhaps too active, "for a short time ; then 

 they settle down as if 

 fatigued, fix themselves 

 by their heads, absorb 

 their tails, and become 

 deformed. The nervous 

 system is reduced to a 

 single ganglion between 

 the two apertures ; the 

 original gill- slits are re- 

 placed by numerous 

 secondary slits making 

 the pharynx like a 

 basket work ; t h e e y e 

 is lost. From the skin 

 of the degenerate animal 

 the external tunic is se- 

 creted. Though cells 

 may eventually migrate 

 into it, it is, to begin 

 with, a mere cuticle 

 (i.e. a non-cellular, non- 

 living layer), and con- 

 sists, in part at least, of 

 cellulose, the substance 

 which forms the cell- 

 walls of plants. Thus 

 this characteristically 

 vegetable substance oc- 

 curs almost uniquely in 

 the most passive part of 

 a very passive animal. The sea-squirt's metamorphosis 

 is one of the most signal instances of individual retro- 

 gression ; the larva is in general architectural plan on a 

 higher grade than the adult, which is not inconsistent 

 with the fact that some structures in the adult, such as 



FIG. 85. THE EXTERNAL APPEARANCE 

 OF AN ASCIDIAN, SHOWING THE 

 INHALANT APERTURE (INH) AND 



THE EXHALANT APERTURE (EXH). 



The body is covered by a thick tunic, 

 mainly composed of cellulose. It is 

 fastened below to a stone. The figure 

 is about half the size of the specimen 

 drawn. 



