238 Introduction to Animal Morphology. 



thus formed, and the ventral space, a double row of large 

 cells appears, which extends into the tail, and forms 

 an axis for that organ. These cells resemble those of 

 the chorda dorsalis of Vertebrates, and have a similar 

 relation to the neural and visceral cavities of the pri- 

 marily bicavitary body to that possessed by the 

 dorsal chord. Upon these phenomena, observed by 

 Ktfivalcwsky, KupJJcr, and others, is rested the theory 

 of relationship of Tunicates and Vertebrates, which is 

 strengthened by the setting apart here of a portion of 

 the digestive canal for respiratory purposes. A dif- 

 ferent, but much less satisfactory, interpretation of 

 these appearances is given by Donitz and Alccznikaw. 

 The embryo is usually tadpole-like, rarely without a 

 tail (Molgula tubulosa). 



In Appendicularia the branchial openings are 

 formed by a double imagination, one outside of the 

 skin and one inside of the pharynx. In all the en- 

 dostyle is developed from the original epithelium of the 

 branchial openings. 



In Salpee a curious life cycle exists. An egg de- 

 velops within a brood-sac of a parent Salpa, and dif- 

 ferentiates into two parts : one, the placenta, which is 

 always on the haemal side ; and the other, the embryo 

 itself. In this fcetal stage, digestive, nervous, and 

 circulatory organs are developed, and at the hinder 

 end of the foetus a curious cluster of fat-holding cells, 

 the elceoblast, forms, which soon afterwards disap- 

 pears. This solitary form soon becomes free-swim- 

 ming, and from the ab-oral side of the heart it develops 

 a thread-like organ, which elongates, thickens, and 

 segments into a chain of variously aggregated Salpse, 

 which have communicating circulatory organs. Each 



