214 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 (hsemal) face of the embryo. Thus 

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

 masses destined to form the Cercarice developed in the body 

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

 plant lice (Huxley). This is a reaffirmation and extension 

 of the original view of Chamisso. 



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

 There are two kinds of individuals : , solitary, asexual ; I, 

 social, aggregated, and hermaphroditic. 



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

 chain of hermaphrodite Salpaa ; the latter produce a fertil- 

 ized 



(2.) Egg, which passes through a 



(3. ) Morula and 



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

 like organ, where 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 Disloma ; in all these cases the offspring (t>) 

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

 spring (c) of the second generation (#) is like (a) the grand- 

 parent. 



Lastly, while some true worms (e.g., Balanoglossus), and the 

 mollusks through the cuttle-fish, present strong features an- 

 ticipating the vertebrate structure, the Tunicata seem more 

 closely allied to the vertebrates, particularly Ampliioxus, than 

 any other invertebrate type. To sum up these resemblances : 

 the larval Ascidian has a notochord, a spinal nerve-cord, 

 and sets of spinal nerves, like those of lowly organized or em- 

 bryonic vertebrates, though these are morphologically on the 

 ventral side of the body, as in other worms ; the nervous 

 cavity and its mode of closure in the embryo is as in the em- 

 bryo vertebrate ; the respiratory sac of the adult Tunicate 

 resembles morphologically that of the Aviphioxus. Conse- 



