1938] Michael: BeJiavior of Salpa dernocratica 245 



The position of the cars will then illustrate the final positions assumed 

 by the salpae in a chain of S. democratica. 



The embryo which is to become an individual of the solitary gen- 

 eration is carried and developed within the body of the aggregate 

 salpa. Before fertilization the egg is suspended hy means of the 

 fertilizing duct, which opens into the cloaca, into one of the blood- 

 channels of the salpa. Quoting from Brooks (1893, p. 21) : "The 

 spermatozoa, which are draw^n into the pharynx . . . with the sea 

 water, are swept past this opening by the contractions of the muscles 

 in swimming, and some of them enter it and one^ penetrating to the 

 egg, fertilizes it." 



Without going into detail, suffice it to say that the embryo at an 

 early stage pushes into the cloaca, carrying its wall before it, and thus 

 becomes inclosed in an epithelial capsule. Changes soon take place 

 by which this capsule becomes cast off and in its place, a placenta 

 forms which, communicating with the blood-channels of the salpa, 

 nourishes the growing embryo until birth. While the embryo projects 

 into the cloaca, it is not at first exposed to the water but is inclosed by 

 the epithelial capsule and after its disappearance, by an embryo sac 

 resembling somewhat the amnion of vertebrates. Later, but still 

 while the embryo is very young, this sac is distended and finally 

 broken by the growing embryo, and from then until birth the embryo 

 is directly exposed to the water in the cloaca, being fastened to the 

 salpa only by the placenta and a narrow band of ectoderm which 

 connects the neck of the placenta to the walls of the cloaca. 



As the embryo begins its growth shortly after the aggregate salpa 

 emerges from the mantle cavity of the solitary salpa, and as its 

 growth is rapid, a fully grown embryo is enormous in comparison to 

 the salpa which carries it (pi. 9, fig. 3). Leuckart (1854, p. 52) 

 says that, at birth the embryo of ;S^. democratica is fully two-fifths the 

 size of the aggregate salpa. Before birth the stolon is completely 

 developed in the embryo and has begun to be converted into aggre- 

 gate salpae of the succeeding generation. After the embryo has made 

 its escape a testis develops in the aggregate salpae and they become 

 mature males. 



In closing this description attention must be called to several 

 inaccurate and misleading statements permeating the literature. One 

 frequently reads that the solitary form is asexual, that the aggregate 

 form is sexual (hermaphroditic), and that the developing embryo is 

 carried within the body of its mother. Such is not the case, for 

 Brooks has clearly demonstrated ,that the solitary form is, in the 



