CHAPTER IV, 
The Sea-Squirts or Ascidians,— Class Tunicata. 
A LEATHERY FIXED SEA-SQUIRT, MicrOCOSVIUS (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 
