430 



MOLLUSCA: TUNICATA. 



They are hollow, and have two orifices, one for the in- 

 gress of water and food, and another for the egress 



of water and the waste of the 

 body. 



In most genera of the Tnni- 

 cata the sexes are united, and 

 reproduction is carried on by 

 means of eggs, and by a bud- 

 ding process or gemination. 



The larvae of Ascidians are tadpole- 

 like in shape, and move freely about in 

 the water. It is claimed that they are 

 related to the Vertebrates in their mode 

 of development, in the relative posi- 

 tion of their nervous system, and in 

 possessing a structure closely like the 

 cJiorda dorxalis of vertebrated animals. 

 And according to Darwin, the Verte- 

 brates may have been derived from 

 animals resembling the larvae of our 

 present Ascidians ! 



The tunicates, which are called 

 Salps or Salpidse, are remarka- 

 ble from the fact that the gen- 

 erations are alternately solitary 

 and aggregated. 



Tunicate, Ascidium monachus. 

 in, incurrent orifice ; ex, excur- 

 rent orifice ; t', outer tunic; t, in- 

 ner tunic, composed of crossing 

 muscular fibers; b, branchial sac; 

 o, tentacular fringe; g, nervous 

 ganglion; vv', dorsal and ventral 

 sinuses; jn, mouth at the bottom 



termination of the intestine ; r, re- long chains of eillbryOS wholly 



unlike the parent. Each indi- 

 vidual of these compound specimens produces solitary 

 young wholly unlike the parent, but like the grand- 

 parents. And these solitary salps produce, in turn, long- 

 chains of embryos and so on ; only the alternate genera- 

 tions being alike. 



