SALPA IN RELATION TO EVOLUTION OF LIFE. 195 



the history of the tunicates, does not require us to enter into the 

 discussion of these complicated details of vertebrate morphology. 



The considerations which I have presented will undoubtedly be 

 met by the assertion that while the simple and direct origin of the 

 tunicates seems plausible so long as we confine ourselves to these 

 animals alone, such a restricted view is unscientific. I shall no 

 doubt be told tliat we are forced by more fundamental evidence 

 to believe that the body cavity of the chordata is, in ultimate 

 analysis, a segmented enterocoel formed from a series of pairs of 

 gut-pouches, and that the simplicity of appendicularia cannot be 

 primitive, inasmuch as the ancestors of the tunicates once possessed 

 these complicated structures. 



The first step to take in discussing this objection is to learn 

 whether there are any traces of gut-pouches in the tunicates. 



Seeliger (p. 9) has given us a very minute and detailed account 

 of the history of the mesoderm in the clavelina embryo, and has 

 shown that it arises from two rows of endoderm cells which give 

 origin, in the tail, to the caudal muscles and, in the body, to free 

 mesoderm cells which multiply with great rapidity and wander 

 everywhere through the body cavity, which is bounded on one 

 side by the endodermal wall of the gut, and on the other by the 

 ectoderm. 



He says emphatically (p. 128) that the mesoderm arises as two 

 totally im segmented rows of cells, each forming a single layer ; that 

 the body cavity is not an enterocoel, but a jprimary body cavity ; 

 and that the ontogeny of the tunicate mesoderm gives no evidence 

 of derivation from paired pouches comparable to the coelomic pouches 

 of amphioxus. 



It is a rare thing for students of tunicate morphology to agree, 

 but in this case the phenomena are simple, and DavidoflP (p. 16) 

 completely confirms Seeliger's observations, so far as they bear upon 

 the question, by his own studies of clavelina and distaplia. 



His account of the origin of the mesoderm differs from Seeliger's 

 in only one minor point, which has no bearing upon the question 

 under consideration. Like Seeliger, he derives the mesoderm from 

 two rows of endoderm cells, but he says that these cells remain as 

 endoderm cells after they have given rise to the mesoderm, while 

 Seeliger states that they become converted into the mesoderm. 



