12 NATURAL SCIENCE. July. 



worms, and above all of many insects, are so modified in connection 

 with their own conditions of life, that only in the broadest way can 

 they be said to be more primitive than the adult forms to which they 

 give rise. They are more primitive chiefly in that they are simpler 

 animals, animals with less complexity of structure, and thus animals 

 which have a broad similarity to the less complicated more ancient 

 form of life. In the case of the larvae of those animals which are now 

 called Chordata because, though they may never attain to the 

 possession of a bony backbone, they possess the gelatinous notochord 

 which is accepted as the predecessor of the backbone, there is strong 

 reason to believe that the simple structure of the free-swimming stage 

 is misleading. The Tadpoles of Ascidians and Amphibia, for 

 instance, possess many simple characters which we believe that the 

 original ancestors of all the Vertebrata possessed. But this simplicity 

 is most probably a case of convergence. Tadpoles live in water. 

 They capture their food, escape from enemies, move about, live and 

 breathe much in the same way and in the same surroundings as the 

 simple pelagic ancestors of the Vertebrata. And thus in many ways 

 they resemble these ancestors. But they are not now taken, at least 

 by most zoologists, to be actual surviving ancestral stages. 



The curious Tadpoles of the African Frog, Xenopus la-vis {Dady- 

 lethra capensis), may be taken as a favourable instance of this change 

 of view. Among the striking peculiarities of these Tadpoles are a 

 development of pigment so slight that the nerves and blood vessels 

 can be seen through the transparent skin ; a pair of long whip-like 

 tentacles protruding in front from the angles of the mouth, and a fish- 

 like appearance, due to the absence of any constriction between the 

 broad head and the body. These points, and some others, were taken 

 to imply that the Tadpoles of Xenopus represented in a peculiarly 

 intimate fashion some fish-like ancestor of the frogs. This resemblance 

 was exaggerated in some figures published by the late W. K. Parker 

 in an otherwise most valuable account of this Tadpole. The figures 

 were copied in many text-books, and the fish-like appearance was still 

 more exaggerated, some innocent markings on the back of the animal 

 being so drawn as to resemble the dorsal plates of some extinct 

 armoured fish. 



Mr. F. E. Beddard, F.R.S., recently had an opportunity of 

 re-investigating these forms, and the results of his work appear in the 

 last issue of the Zoological Society's Proceedings (1894, P- ^"o)- 

 He shows how some of the mistaken impressions have arisen, and 

 describes how, in most respects, the Tadpoles of Xenopus are not 

 markedly different from the Tadpoles of the common Frog. In the 

 first two days of their life they possess, just like the frog, a suctorial 

 gland below the mouth, and the head is separated from the body by 

 a constriction. After the third day these disappear, and the pecu- 

 liarities of the more advanced form come into existence. The details 

 of structure mentioned are too technical to repeat here, but the 



