OVUM. 



17 



of these animals in the zoological system than 

 that which has hitherto been followed. Most 

 Naturalists are now disposed to separate from 

 the true Polypina the Bryozoa, or so-called 

 C'iliobrachiate Polypes, which, though pre- 

 senting a considerable resemblance to the 

 Polypes in their external anthoid appearance, 

 yet approach much more nearly to the Tu- 

 iiicated Acephalous Mollusca by their internal 

 organisation; and remarkable affinities have 

 been pointed out between some of the Poly- 

 pina and Acalephce, which show that these 

 classes, though very dissimilar in their external 

 forms and mode of life, are in reality very 

 closely allied in structure. 



The greater number of the Polypina are ag- 

 gregate^or compound animals, that is, consist 

 naturally of groups of individuals united or 

 associated together on a common stem and 

 branches, or on a more solid stock. But the 

 common fresh water Polype, or Hydra, and 

 the various Actinias of the sea coast, are, to a 

 certain extent, exceptions to this general rule, 

 and, as we shall see, differ also in regard to 

 their mode of reproduction from most of the 

 other families of this division of animals. The 

 Actinia is usually a single animal : no doubt it 

 is multiplied occasionally by buds, but these are 

 thrown off and become developed usually in 

 an isolated position. The Hydra sometimes 

 occurs as a single animal, but more frequently 

 during summer, and when well nourished, as a 

 compound one ; the multiple individuals being 

 developed by gemmation from the first or 

 principal stock, and also themselves forming 

 younger progenies by budding; but the indi- 

 viduals so formed on the Hydra generally 



/ Fig. 10. 



the hermaphrodite parent ; but in Hydra it 

 would appear that it is principally in the au- 

 tumn, on the approach of cold weather, that 

 the sexual mode of propagation is substituted 



Fig. 11. 



Hydra viridis in different stages of extension and 

 contraction, reproducing gemmiparously, attached 

 to the roots of duck-weed. (From Eoesel.} 



separate from the parent stock when they 

 have attained to maturity, migrate, and esta- 

 blish themselves as independent animals, to 

 form new buds. 



Both of these animals are capable of pro- 

 pagation by ova formed in the sexual way : in 

 Actinia this seems to be the more common 

 mode of its multiplication, the ova being 

 fecundated and developed within the body of 



Supp. 



Hydra viridis. 



A, Hydra of autumn, bearing an ovum, o,and two 

 spermatic capsules, s, s. B, spermatic capsule burst 

 artificially, showing spermatozoa. 



c (from Laurent), ova with young Hydra in 

 various stages of development hanging out of them. 



D, D' (from Laurent), portions of the body of 

 summer Hydra, with a bud sprouting. D, the ear- 

 liest ; D', more advanced, showing the texture to 

 be the same as the rest of the body. 



c 



