SALPA IN RELATION TO EVOLUTION OF LIFE. 197 



In the chapter on the significance of the sal pa-embryo I have 

 given ray reasons for believing that this space is homologous with 

 the segmentation cavity of more normal tunicate embryos, and if 

 this view be correct the body cavity of salpa is not an enterocoel 

 but a primary body cavity or blastocoel. The mesoderm of salpa 

 consists of free migrating cells, and the chamber of the heart is part 

 of the body cavity, so that these cells pass through it ; and while 

 salpa is a peculiarly unfavorable subject, my observations are in 

 complete accord with those which Seeligerand Davidoff have made 

 under simpler and more favorable conditions. 



No student of the embryology of tunicates has ever described any 

 trace of a series of body cavities, and Kowalevsky, the discoverer 

 of the coelomic pouches of amphioxus, failed to find anything com- 

 parable to them in the tunicates, although the existence of a single 

 pair of enterocoels has been claimed by certain observers. Van 

 Beneden and Julin (Zool. Anzeiger, 4, 1881 ; Bull. Acad. Belg. (3) 

 7, 1884; Arch. Biol. 6, 1884) believe that the anterior portion of 

 the body cavity of ascidians arises as a pair of gut-pouches, and that 

 its mesoderm consists of a somatopleur and a splanchnopleur, but 

 Davidoif has shown by careful serial sections that this statement is 

 probably based upon erroneous observations. 



Salensky holds (17, 460) that the mesoderm of the blastoderm of 

 pyrosoma consists of two symmetrically placed coelomic pouches, 

 and that pyrosoma is, therefore, to be placed among the true entero- 

 coelomata. The space between the vertebrate blastoderm and the 

 yolk is undoubtedly homologous with the enteron, but it is by no 

 means certain that this is the case in pyrosoma, where the food-yolk 

 is an independent acquisition ; nor do Salensky's figures show, as 

 clearly as we might wish, that the two coelomic vesicles open into 

 this space, and even if this is the case, we must remember that the 

 pyrosoraa-embryo is very aberrant, and that the structure of its body 

 cavity may be a secondary adaptation to the presence of the yolk. 

 Taken alone it certainly is not enough to prove, without corrobora- 

 tion from other sources, that the body cavity of the tunicate is an 

 enterocoel. 



The ontogeny and homology of the tunicate mesoderm have been 

 recently discussed at very great length by Seeliger (11, pp. 85-104 

 and pp. 126-131), by Davidoff (16, pp. 592-628), and by Salensky 



