22 



a male by the development of testes/or a female by that 

 of ovaria. The ova are impregnated, and develope the 

 monadiform embryo ; this gives origin to the beroiform 

 Pluteus, which produces by e metagenesis/ rather than by 

 metamorphosis, the young Echinus, and the cycle is com- 

 pleted by its growth into the oviparous adult. 



Passing to another great group of Invertebrate animals, 

 the Mollusca, we learn from the observations of M. Milne- 

 Edwards*, Dalyellf and Sars J, that the embryo developed 

 from the ovum of the compound Ascidiae quits the recep- 

 tacle of the fixed parent in the form of a large Cercaria, 

 i.e. with a caudate appendage, as a freely swimming animal- 

 cule ; then finally settles, becomes attached to some foreign 

 body, developes the organization of the Ascidian, and pro- 

 pagates by gemmation the compound group of such Mol- 

 lusca, which in their turn develope ova producing freely- 

 swimming tadpole-like larvae. 



Chamisso was long ago led by his observations during 

 Kotzebue's circumnavigatory Voyage to the conclusion, 

 that the solitary Salpa3 gave birth to embryo Salpae of a 

 different form, and connected together in chains ; from 

 each individual or link of which a solitary embryo is pro- 

 duced, which grows and assumes the characters of that 

 Salpa from which the chain of embryo Salpae again pro- 

 ceeds. 



The Nais and the young of many marine Annelides pro- 

 pagate by gemmation from the terminal segment of the 

 body a succession of other individuals like themselves, and 

 the characteristics of the head of the parent, whether a 

 proboscis or eye-specks, antennae or branchiae, are recog- 

 nisably developed upon the anterior segment of the still 

 attached offspring. Before this is cast off, the gemmiparous 

 segment of the parent repeats its procreative office, and a 



* 'Observations sur les Ascidies Composees,' 1841. 

 t Edinb. New Philosophical Journal, 1839. 

 I Cited by Steenstrup, op. cit. p. 48. 



