184 JOHNS HOPKINS UNIVEESITY MORPHOLOGICAL MONOGRAPHS. 



change itself is not a complicated one. It presents little difficulty, 

 although the resulting differentiation of appendicularia into two regions 

 or " segments," a body and a tail, has been made the basis of much 

 speculation. 



The great development of the pharynx and the reduction of the tail 

 to an organ of locomotion soon resulted in a pronounced change, of the 

 sort for which Dana long ago proposed the term cephalization. 



As the functions of the pharynx, and of its oral end in particular, 

 became more and more complicated and more and more exactly co-ordi- 

 nated, while those of the tail became simplified, the elongated nervous 

 system became differentiated in a corresponding way, and its caudal 

 portion became reduced to a caudal nerve, while its oral extremity 

 became evolved into a cerebral vesicle with sense-organs and nerves in 

 relation with the co-ordinated structures of the pharynx. 



All the characteristics of appendicularia, except the structure of the 

 heart and the structure and position of the reproductive organ, are thus 

 seen to be intelligible as direct adaptations to a pelagic life; for its 

 distinctive features, as compared with other primitive chordata, are the 

 U-shaped folds of the digestive cavity, the sharp separation of the tail 

 from the body, and the differentiation of the nervous system into a 

 caudal nerve and anterior vesicle. 



We have little basis for speculation as to the path by which the 

 reproductive organ acquired its present position, and it is by no means 

 certain whether the tunicate heart is homologous with that of the other 

 chordata. 



The conditions of pelagic life are so permanent that we may safely 

 make use of the structure and habits of the modern pelagic forms to 

 reconstruct this part of the ancestral history of the tunicates, for time 

 writes no wrinkles on the azure brow of the ocean. 



As regards the later history the case is different. Between appendi- 

 cularia and the ascidians there is a great gap which we can bridge only 

 in imagination. The transitional animals are totally unknown, and the 

 conditions of life on the bottom of the modern ocean may, possibly, be 

 very different from those which prevailed when the fixed ascidians were 

 first evolved. 



It is easy to imagine changes which might have gradually converted 

 an ancestor like appendicularia into a descendant like the fixed ascidians, 

 through successive adaptations to a sedentary life, but in the absence of 

 all evidence we cannot feel implicit confidence that the imaginary picture 

 bears any minute and detailed resemblance to the actual history. 



