404 ZOOLOGY. 



embryonic parts of the placenta arising, respectively, from 

 the wall of the ovarian sac and from certain large cells (blas- 

 tomeres) on the adjacent (haemal) face of the embryo. Thus 

 the asexual development of the Salpa is like that of the germ- 

 masses destined to form the Cercarim developed in the body 

 of the Redid of the Distoma ; and is also like that of the 

 jjlant lice (Huxley). Tliis is a reaffirmation and extension 

 of the original view of Chamisso. 



To recapitulate, tlie life-history of the Salpa is as follows : 

 There are two kinds of individuals : a, solitary, asexual ; h, 

 social, aggregated, and hermaphroditic. 



(1.) The solitary, asexual Salpa produces by budding a 

 chain of hermaphrodite Salpse ; the latter produce a fertil- 

 ized 



(3.) Egg, which passes througli a — 



(3.) Morula and— 



(4. ) Gastrula stage, contained and growing in a placenta- 

 like organ, whei-e the embryo is directly nourished by the 

 blood of the parent, the embryo finally becoming — 



(5.) A solitary asexual Salpa. 



We thus have a true alternation of generations, like the 

 sexless Hydroid and its sexual Medusa, the asexual Aphis 

 and its last brood of males and females ; the asexual Redia 

 and the sexual Diatoma ; in all these cases the ofEspring (V) 

 of the asexual individual («) is unlike the parent, but the off- 

 spring (f) of the second generation (V) is like (a) the grand- 

 jiarent. 



" In Doliolmn the reproductive processes are much more 

 complicated, for not only do the sexually produced young 

 undergo a metamorphosis, but a' new series of generations is 

 introduced into the life-history. The eggs are laid, and the 

 larvffi which issue from them are provided with tails and re- 

 semble Ascidian larvse. They develop into asexual forms, 

 which difEer from the sexual forms, and are provided with a 

 dorsal stolon; the ventral stolon (stolon of Salpa) is rudimen- 

 tary. Two different kinds of buds are formed on this dorsal 

 stolon, viz., median buds and lateral buds. The lateral buds 

 have a slipper-like form, and are without the cloacal cavity; 

 they do not reproduce themselves, but are concerned with the 

 nourishment of the asexual form. The latter as it increases 



