28a TADPOLES OF THE SEA 



quarter of an inch thick, in others soft and flexible, on the 

 surface of the delicate living layer of the Ascidian's 

 body. In some respects it may be compared to the 

 hard horn-like layer (called " crust " or " shell ") on the 

 surface of the body in large insects and in lobsters and 

 crabs. But it differs from that layer in its chemical 

 nature, which is similar to the wood of plants rather than 

 to the horn and shell of other animals, and also in the 

 fact that living protoplasmic cells from the epidermic 

 layer, which underlies it and secretes it, pass into it and 

 continue to live in it as detached floating or embedded 

 cells. 



The sac-like creature which I have just described lives 

 a perfectly quiescent existence, not unlike that of an 

 oyster. It is fixed by one end of its sac-like body, and 

 simply keeps up by means of the lashing hairs with which 

 its gullet is beset a constant stream of sea-water entering 

 its mouth and filtering through its sieve-like pharynx, and 

 so through its branchial chamber and orifice to the 

 exterior. One cannot but be struck with the fact that the 

 mechanism with which the Ascidian is provided — heart, 

 blood-vessels, brain and nerves, and muscles, besides the 

 elaborate perforated pharynx — is an extremely complex 

 one for a creature which leads a sort of vegetating exis- 

 tence, motionless within a tough protecting sac. What is 

 the origin and history of the Ascidians or Tunicata ? To 

 what other animals are they related by descent and 

 cousinship ? The answer given to these questions fifty 

 years ago was that they were a sort of soft-shelled oysters 

 — in fact, they were classed with the mollusca (the snails 

 and whelks, clams, oysters, and cuttle-fish). Then when 

 their structure was more fully ascertained, they were con- 

 sidered as a group apart — a puzzle. It was only when 

 their growth from the egg was studied by the most refined 

 microscopical methods that their real nature became 



