284 TADPOLES OF THE SEA 



as a coincidence in form of no special significance, and ex- 

 plained on the principle that " like mechanical needs must 

 produce like forms," even in kinds of animals so remote from 

 one another as Ascidians and Vertebrates. The Ascidian 

 tadpole was considered merely as an exceptional "swim- 

 ming larva," or young form, occurring in a group the young 

 of which do not usually swim. Then came (in 1866) the 

 study of the minute structure of the Ascidian tadpole by 

 the great Russian discoverer, Kowalewsky, followed by 

 many other investigators. The result was a very impor- 

 tant one for all future conceptions as to the genetic rela- 

 tionships of animals — the making out of the pedigree of 

 animals. The chief facts established were that the 

 Ascidian tadpole has the same structure in several impor- 

 tant points as we find in the frog's tadpole, and must be 

 identified as a vertebrate animal — a member of that great 

 line of descent or branch of the animal pedigree which 

 includes fishes, amphibians, reptiles, birds, and mammals ! 

 All the vertebrate animals have an elastic rod running 

 from head to tail, a sort of supporting axis, to which the 

 muscles are attached. It is called the " notochord," and 

 whilst in some fishes (for instance, lampreys and sturgeons) 

 it remains rod-like and unchanged throughout life, in most 

 vertebrates it becomes enclosed in rings of bone, which 

 are the bony vertebrae or jointed " back-bone " familiar to 

 us, not only in fishes, snakes, birds, and joints of butchers' 

 meat, but also in our own backs. The frog's tadpole has 

 this notochord and so has the Ascidian tadpole (Fig. 35), 

 and they are formed in the same way in the very 

 young condition, namely, by a nipping-off of a long 

 fold or " pleat " of the wall of the young creature's 

 simple gut or digestive cavity ! The fold becomes 

 closed, solid, and gelatinous, and converted into a firm, 

 flexible rod ! That is one great point of resemblance 

 be.tween the two tadpoles, and it is of significance, 



