30 CCELENTERATES. 



Sexual Reproduction consists in the production of male 

 and female reproductive elements, sperms and ova, and in 

 their union to form oosperms, and in the development of 

 the oosperms into new individuals. At first certain cells 

 are separated off from the other cells of the body, and in 

 certain parts of the body, specialized for the purpose, are 

 developed into sperms and ova. Then the sperms of the 

 hydra escape out into the water, and swim actively about 

 until some of them come into contact with the ova of 

 another hydra. One sperm enters and coalesces with 

 each ovum, changing it into an oosperm. Fertilization 

 is the name for this part of the process. The resultant 

 oosperm hardly differs from the ovum in appearance, yet 

 vitally differs from it, in that the oosperm has the poten- 

 tiality to become, under proper conditions, a new hydra. 



When both kinds of sexual elements are produced in a 

 single individual, the individual is said to be hermaphro- 

 dite. In the hydra, spermaries are developed first; and 

 not until after they are ripened, and their sperms dis- 

 charged, do the ovaries mature their ova. The ova can- 

 not, therefore, be fertilized by sperms from the same 

 individual, but Avill be fertilized by sperms of a later 

 growth from some other individual. This is called cross 

 fertilization : it seems to be essential to the healthy con- 

 tinuance of most animals. 



The Life Process. I. Nutrition. In the hydra we 

 have our first example of an animal with a distinct gastric 

 cavity, in which food may be received and retained until 

 digested. We find, also, better means of securing food than 

 in any protozoans. After the food is taken by the tenta- 

 cles and the extensible mouth, it is pressed down into the 

 digestive cavity, and there the endoderm cells do most, if 

 not all, of the work of reducing it to soluble form. These 

 lining cells are large, and capable of considerable amoeboid 



