OVUM. 



23 



on leaving the parent body they become deve- 

 loped into a ciliated embryo, which, for a time, 

 moves freely about, then becomes fixed, un. 

 dergoes farther changes in being developed, 



Fig. 19. 



A series illustrating the development by ova of Pedi- 

 cellaria. (After Van Beneden.) 



and now from its own body in some, and in 

 others only from the spreading part of the 

 stem or base which supports it, proceeds the 

 gemmation of other individuals of the colony, 

 all of which apparently are capable of sexual 

 generation when they arrive at maturity.;}: 



The Ascidian Tunicata present another mo- 

 dification of the reproductive process now 

 under consideration. Two forms of these 

 animals exist, both perfect, viz. the simple and 

 the compound; but these are not related to 

 each other in the same manner as the two 

 kinds of Salpians; for each kind is capable of 

 propagating its like by generation. The soli- 

 tary ones rarely multiply by gemmation, and 

 when the}' do so the individuals separate from 

 the stock; but the compound animals always 

 undergo this mode of multiplication, and the 

 multitude of individual Ascidians are in this 

 form collected together in a mass of various 

 shape, in which the circulation of fluids is for 

 a time common among the different indivi- 

 duals. The individual animals produced from 

 the stock by gemmation attain to sexual com- 

 pleteness, and propagate by means of ova, in 

 the same manner as the solitary or distinct 

 Ascidiae do. 



The young of these animals undergo a re- 

 markable metamorphosis : they are first ex- 

 cluded from the egg in the form of a moving 

 tailed body, somewhat like a minute tadpole^; 

 and this caudal organ of motion is lost pre- 

 vious to their becoming fixed, and the deve- 

 lopment of the more complex organisation.f 



Although the changes to which both the 

 Bryozoa and Ascidian Tunicata are subject in 



* Van Beneden, in Mem. de Bruxelles, torn, xviii. 

 See the Article Polypifera, for an account of these 

 researches. 



t See Mr. Rupert Jones's excellent Article TUNI- 

 CATA for an account of these phenomena, and the 

 special Memoirs of Milne-Edwards, sur les Ascidies 

 Composees, &c., Paris, 1832; Lchvig and Kolliker, 

 in Ann. des Sc. Nat, April, 1846; Van Beneden, 

 sur les Ascidies Simples, Brussels, 1847. 



their early state present some very striking 

 phenomena of metamorphosis, yet there is 

 nothing in either which fully deserves the name 

 of alternate generation, for all the individuals 

 of which these compound animal structures 

 consist are alike sexually perfect, and there 

 does not appear to subsist any necessary con- 

 nection between the nonsexual process of 

 multiplication, and the subsequent exercise of 

 the sexual function. There are, in fact, scarcely 

 any intermediate stages of non-sexual exist- 

 ence such as are described in the true in- 

 stances of alternate generation. It is deserving 

 of notice, however, that Lowig and Kolliker 

 are of opinion that in some of the Botryllidae 

 numerous embryoes are at once developed 

 from a single ovum by its division, these indi- 

 viduals subsequently multiplying by gemma- 

 tion into the perfect sexual animals. 



Fig. 20, 



Bowerbankia densa. (After Jarre.) 



a, one of the animals fully expanded. 



b, a similar animal completely retracted. 



c, an immature animal. 



d, one of the gemma? in its earliest state. 



SalpidfB. The most marked example of 

 alternating generation among the Tunicata 

 is that which, since its first discovery by 

 Chamisso, in 18 19, has been known to occur 

 in the Salpidae. This process has been so well 

 and fully described in the article TUNICATA, 

 that it is not necessary to give more than a 

 short outline of it in this place. These animals 

 are known in two states, viz. solitary and ag- 



c 4, 



