PHYSIOLOGY OF THE INVERTEBRATA. 409 



The Ccelenterata. 



The modes of reproduction in Hydra are by gemmation, 

 fission, and sexual reproduction. 



Gemmation is the most common mode of multiplication. 

 The buds may remain attached, or may become separated from 

 the parent ; and consequently lead an independent life. The 

 bud of Sydra " consists always of a simple fold of the wall of 

 the stomach and the skin, so that the stemach of the young 

 individual is in direct communication with that of the parent, 

 and the chyme (nutritive fluid) can pass freely from one to 

 the other." When the foot of this new being has acquired a 

 proper development, it is completely detached at its inferior 

 extremity. 



In regard to the second mode of reproduction — by natural 

 fission — it may be stated that it is comparatively rare. Fission 

 takes place longitudinally or transversely, and each part 

 repairs itself, and ultimately develops into a new Hydra 

 identical with the parent. In some forms of the Cwlenterata 

 the fission may or may not be complete. " When it is com- 

 plete the cells of the corallum are definitely limited, as in 

 Astrcea, Favia, and CaryopJvyllia, but when incomplete, the 

 cells are branched, lobulated, and of irregular contour, as in 

 Agaricia, Mceandrina, Monticularia, &c." 



Sexual reproduction takes place in Hydra ; but the animal 

 is hermaphrodite. In the summer, testes are developed at the 

 base of the tentacula ; and one or more ovaries at the base of 

 the column near the disc. The testis is simply a mass of 

 inner ectodermal cells, by the division of whose nuclei, sperma- 

 tozoa are formed. A spermatozoon consists of a small oval 

 head and a long filament. This filament by its rapid move- 

 ments enables the spermatozoon (when liberated) to swim 

 about in the water ; and in this medium it retains its fertilis- 

 ing properties for many days.* 



* The retention of the fertilising properties of spermatozoa after expul- 

 sion from the body, varies in different animals. In trout the property is 



