CHAPTER IV. 



The Sea-Squirts or Ascidians, — Class Tunicata. 



A leathery fixed sea-squirt, Microcosmus (nat. 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 

 relationship 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 

 structures 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 perpetually in the open ocean, and have become adapted in the extremest 

 vol. v. — 36 



