Some oj the Lower Metazoans 389 



The appearance of ovaries and testes seems to be in response to 

 changes in the temperature of the water. In some species a lowering 

 of the temperature in winter is accompanied by sexual reproduction ; 

 in others it is a response to rising temperature in the spring. In either 

 case, the testes and ovaries are similar in appearance and development. 

 The testis is a small pointed protuberance with a nipplelike opening 

 for the exit of the sperm. In dioecious species, the testes or ovaries 

 are found scattered over the central portion of the animal; in monoe- 

 cious or hermaphroditic species, the testes are usually near the mouth 

 end while the ovaries are closer to the budding zone. 



The ovary consists of modified interstitial cells, one of which en- 

 larges to form the ovum and others which provide nourishment. This 

 single, large ovum which is produced breaks through the epidermis and 

 comes to lie on the surface in a cuplike structure. Fertilization occurs 

 within a short time and is followed by holoblastic cleavage until a gas- 

 trula is formed. This gastrula is a solid sphere with an outer layer 

 of ectoderm and inner mass of entodermal cells. The ectodermal cells at 

 this time secrete two heavy, protective envelopes : an inner gelatinous 

 one and an outer thick shell. This well-protected embryo, known as a 

 cyst, drops from the parent and may attach to any suitable object. After 

 a dormant period of some weeks during which the cyst can endure 

 adverse environmental conditions, the cyst cover softens and a young 

 hydra with short tentacles emerges. 



The Responses of Hydra to Its Environment. — Some of the 

 ways in which hydra responds to changes in its environment have 

 already been indicated. One reaction is the change from asexual to 

 sexual reproduction which occurs when the temperature of the water 

 either rises or falls. The response to this varies with the individual 

 species. Food in the vicinity of the tentacles evokes an immediate 

 response as is evidenced by the discharge of the nematocysts and the 

 grasping of the prey by the tentacles. A sudden touch on a tentacle 

 will result in a withdrawal, while the jarring of a container will bring 

 about a contraction of the entire animal. Light affects the different 

 species in varying manners. Chlorohydra, the green hydra, usually 

 prefers stronger illumination than do the others. Cooler waters are 

 preferred by all of them, with temperatures under 70° F. being most 

 suitable. 



Occasionally hydras undergo degenerative changes which are but 

 little understood. These can occur due to stagnation of water, over- 



