172 MOLLTTSCA. 



a circle of short tentacles. Hancock considered the sac to be 

 the rudiments of the lamellibranchiate gills. A peculiarity of 

 the Ascidians is the longitudinal fold in the pharynx, termed the 

 " endostyle ;" its use is unknown, but it may be a sensitive 

 organ. 



The outer integument, or sac, of the Ascidians " secretes upon 

 its surface a case, or ' test,' which may vary in consistence from 

 jelly to hard leather or horn :" the test is " rendered solid by 

 impregnation with a substance identical in all respects with the 

 ' cellulose ' which is the proximate principle of woody fibre and 

 forms the chief part of the skeleton of plants." An inner inte- 

 gument or tunic is composed of longitudinal and circular mus- 

 cular fibres. Of the two apertures, the atrial leads into a large 

 cavity lined by a membrane or third tunic. The anus opens 

 near the mouth. 



With a somewhat complex organization, the only vital action 

 seems to be an occasional ejection of water from the two openings 

 followed by a sudden contraction. 



^Reproduction, whether by buds or by ova, is of the most com- 

 plex description. Some larval forms develop zooids, and these 

 maybe " supplanted " by other zooids. The morphological con- 

 ditions are peculiar and deceptive, and varied or modified ac- 

 cording to the species. The young from the ova are like tad- 

 poles, swimming about by means of a long tail ; but in the com- 

 pound Ascidians the tail is lost before leaving the egg. As adults 

 they are mostly fixed to some foreign body. The compound 

 species may be seen coating the under surfaces of rocks on most 

 of our shores. Their food, according to Hancock, is " extracted 

 from sedimentary matters." 



The Ascidians are regarded by Kowalewsky and others as the 

 nearest relations of the Vertebrata, in consequence of the pre- 

 sence in the larval form of the rod-like body (notochord), which 

 disappears in the mature animal, and in the Vertebrata is re- 

 placed by the spinal column. One of many objections to any 

 identity of development in the Ascidians and Vertebrates in this 

 respect is that the rod-like body is ventral in the former and is 

 developed in the same cavity as the viscera. 



Huxley remarks that "In the Ascidians the central nervous 

 system is produced by the invagination of the epiblast, as in the 

 Vertebrata, and that, in most, the m'esoblast of a caudal prolon- 

 gation gives rise to an axial column flanked by paired myotomes, 

 which are comparable to the notochord and myotomes of the 

 vertebrate embryo." Other authorities, however, consider that 

 the Annelids stand nearer to the Vertebrata. 



