IV. 



THE ASCIDIANS. 



The Ascidians, Tunicates, or sea-squirts, as they are 

 indifferently called, constitute one of the most clearly 

 defined and yet most heterogeneous groups of animals 

 which it is possible to imagine. There is a great variety 

 of families, genera, and species occurring all the world 

 over, and in all depths of the ocean from the tide-marks 

 to the profoundest depths. 



Most of them are sedentary animals, remaining fixed 

 all their lifetime on one spot, whether attached to rocks, 

 stones, shells, or sea-weeds, from which they are incapable 

 of moving. There are, however, several very extraordi- 

 nary genera of Ascidians which swim or float about per- 

 petually in the open ocean, and have become adapted in 

 the extremest manner to a purely pelagic environment. 

 These pelagic Ascidians have become so modified in adap- 

 tation to their oceanic existence, and their development 

 diverges, as a rule, so much from the normal, that they 

 will hardly enter at all into the present discussion, with 

 the exception of one family, the Appcndicnlarice. 



Just as there are two kinds of sessile Ascidians, simple 

 and compound ox colonial, so there are two analogous kinds 

 of pelagic Ascidians. In some of the latter, however, 

 where there is an alternation of trenerations, one o-enera- 

 tion, namely, the asexual generation, is a solitary form, 

 while the sexual generation is a colonial form, as, for 

 example, the solitary Salpa and the chain-Salpa. 



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