342 ANIMAL BIOLOGY. [Part II. 



of development, and the product to which it gives rise. In the 

 first place we may note the single ovum as the product of the 

 ovary ; (2) its amoeboid movements ; (3) the modification of part 

 of the embryo to form a chitinous case for the rest; (4) the 

 curious histolysis after a certain stage of histogenesis has been 

 reached; (5) the formation of the primitive body-layers, and 

 the contained digestive cavity, not by a process of invagination, 

 but by delamination of the walls of a hollow sac-like embryo ; 

 (6) the disruptive formation of the mouth ; (7) the absence in 

 the perfected organism of any body-cavity. This distinguishes 

 the hydra and its allies (ccelenterata) from all other metazoa 

 (ccelomata), which have at some period a body-cavity. 



It will be seen, also, that the development of hydra is con- 

 tinuous, There is no alternation of generations. In this the 

 hydra is peculiar among the group of hydroid polyps to which 

 it belongs. It differs also from allied zoophytes in being solitary, 

 and not a member of a colony, group, or stock (hydrosome). Such 

 a stock consists, in general, of a stem and perhaps ramifying 

 branches, from which the polyps of the colony (hydranths) are 

 offshoots. The stem and branches are tubular, and the somatic 

 cavity within them is in free communication with the digestive 

 cavity of the hydranths. The generative organs (gonophores) 

 arise in special regions, and arise as buds either from the polyp 

 or from the tubular stem or branches. In either case it is at 

 first a sac-like diverticulum of the somatic or gastric cavity. 

 Within this diverticulum the generative products accumulate. 

 The genital cells may, however, arise in parts other than those 

 in which they are finally lodged, or even in some cases in the 

 body-wall of the polyp before the gonophores have made their 

 appearance. In such cases the generative cells subsequently 

 migrate into the gonophores. The development of the oospores 

 in some cases precedes that of the spermatospores ; and the ova 

 and spermatozoa are, as a rule, finally lodged in different gono- 

 phores. 



So far we have regarded the gonophore as a simple bud or 

 diverticulum attached to the polyp, or to the axial branches of 

 the stock. But in the higher forms this develops into a medusa, 



