CCELENTERATA. 23 



be hermaphrodite. They are only found, as a rule, during the 

 autumn months. 



(1) The Male Organs consist of a varying number (1 to 20) of 

 spermaries (testes) (t, ), usually placed near the distal end of the 

 body. Each is a conical or rounded elevation, the wall of which 

 is formed by large ectoderm cells, while within is an aggregate 

 of interstitial cells. The spermaries are, in fact, projections 

 caused by the increase of these cells at particular points of the 

 bodj*. These contained germinal cells develop into sperms (sperm- 

 atozoa), minute tadpole-like bodies, with oval heads and long 

 vibratile tails. The head of the sperm is mainly formed from 

 the nucleus of the germinal cell, covered by a thin film of its 

 protoplasm, the rest of which is drawn out into the tail. 



(2) The Female Organs are known as ovaries (ov), and are 

 typically developed at the proximal end in the same manner as 

 the spermaries. Hydra viridis usually has but one ovary; H.fusca 

 may possess from one to eight. Each is a rounded projection, 

 much larger than a spermary, and, when mature, contains a 

 single large egg-cell or ovum. The young ovary contains at first 

 a large number of germinal cells. One of these, occupying a 

 central position, grows more vigorously than the others, becomes 

 amoeboid, and uses them as food, amoeba-fashion. This relatively 

 large cell is the ovum. It assumes a spherical form on attaining 

 its full size, and though resembling ordinary cells in structure, 

 its parts receive special names. The protoplasm is termed 

 vitellus, and contains numerous highly refractive bodies, known 

 as yolk-spherules. The large spherical nucleus, or germinal vesicle 

 contains a nucleolus, the germinal spot. 



PHYSIOLOGY. 



As might naturally be expected, the ectoderm is more espe- 

 cially concerned with the function of irritability and spontaneity, 

 while the encloderm carries on digestion. 



1. Nutrition. The food consists of small animals, often of 

 relatively high organization, such, for example, as the active little 

 Crustacea known as water fleas. The tentacles are the agents by 

 which these are conveyed to the mouth, arid their nematocysts 

 play an important part in this connection. The following -de- 

 scription applies to the large forms. In the quiescent state, the 



