CHAPTER IV 

 THE SEA SQUIRTS OR ASCIDIANS CLASS Tunkata 



A LEATHERY FIXED SEA SQUIRT, MicroCOSIHUS. 

 (Natural size.) 



EXTERNALLY, scarcely any creatures are more unlike the lancelet than those 

 fixed marine animals commonly known as sea squirts, and technically as ascidians, 

 or tunicates. Nevertheless, in the opinion of those best qualified to judge, the rela- 

 tionship is probably closer than that existing between the former animal and the 

 larva of a lamprey, in spite of the much greater external resemblance between the 

 two latter. It is, however, when we dissect a sea squirt that we meet with struc- 

 tures recalling certain features in the anatomy of the lancelet; while to find evidence 

 of the chordate affinities of the former, we have to go back to its larval condition. 

 In the adult condition, writes Mr. Willey, most of the sea squirts "are sedentary 

 animals, remaining fixed for their lifetime on one spot, whether attached to rocks, 

 stones, shells, or seaweeds, from which they are incapable of moving. There are, 

 however, several very extraordinary genera of ascidians, which swim or float about 

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