HYDRA AND OTHER FRESH-WATER HYDROZOA 317 



granular opaque entoderm, and a narrowed paler aboral part 

 which is termed the stalk. In other species designated in the 

 key as " not stalked," there is no clearly marked division into 

 such regions. Highly contractile fibers formed by certain cells in 

 both ectoderm and entoderm may bring about either a great 

 elongation of the body and tentacles to thread-like proportions or 

 their contraction to an almost globular form. Certain kinds of 

 ectoderm cells, which are most abundant in the adoral half of the 

 body, especially in the tentacles, give rise to the characteristic ne- 

 matocysts or " nettling cells " of different shapes and sizes. These 

 contain a fluid secretion which passes out through a thread-like 

 extension of the sac wall, that is forced out when the cell is stimu- 

 lated. The combined action of a number of these nematocysts 

 on the small organisms encountering them results in the loss of 

 activity or even death of the organisms and so permits their cap- 

 ture and appropriation as food by the hydra. 



Spermaries and ovaries develop in the ectoderm layer and at a 

 time of year which seems to be fairly constant for a given species 

 but differs in different species. After fertilization the ovum passes 

 through the early stages of development while still in the ovary 

 and becomes enclosed by a chitinous envelop which has a charac- 

 teristic shape and surface for each species. This envelop which 

 often is spiny is referred to in the key as the embryonic, chitinous 

 membrane. In some species the embryos are freed from the parent 

 organism and drop to the bottom, while in others they are fastened 

 by the parent to the substratum to which it adheres. The develop- 

 ment is direct. In one species (Hydra oligactis) the individuals are 

 said to be of separate sexes, or dioecious, but in others hermaphro- 

 ditism prevails. Asexual reproduction by budding is the preva- 

 lent mode of multiplication and very rarely the formation of two 

 individuals by a process of fission has been observed. 



Hydra has long been an object of interest and experiment because 

 of its notable powers of regeneration and form regulation and there 

 is now an extensive literature dealing with these phenomena. 



Hydra individuals ordinarily maintain an independent existence 

 but in various related groups colonies which often include many 

 individuals arise by asexual reproduction. In some such colonies, 



