46 Animal Life and Intelligence. 



aurelia. Thus the fertilized ovum of aurelia develops, not 

 into one, but into a number of medusas,* passing through 

 the Hydra tuba condition as an intermediate stage. 



Many of the hydroid zoophytes, forming colonies of 

 hydra-like organisms, give rise in the warm months to 

 medusoid jelly-fish, capable of producing ova and sperms. 

 Fertilization takes place; and the fertilized ova develop 

 into little hydras, which produce, by budding, new colonies. 

 In these new colonies, again, the parts which are to become 

 ■ovaries or testes float off, and ripen their products in free- 

 swimming, medusoid organisms. Such a rhythm between 

 development from ova and development by budding is 

 spoken of as an alternation of generations. 



The fresh-water sponge (Spongilla) exhibits an analogous 

 rhythm. The ova are fertilized by sperms from a different 

 short-lived individual. They develop into sponges which 

 have no power of producing ova or sperms. But on the 

 approach of winter in Europe, and of the dry season in 

 India, a number of cells collect and group themselves into a 

 so-called gemmule. Round this is formed a sort of crust 

 beset with spicules, which, in some cases, have the form of 

 iwo toothed discs united by an axial shaft. "When these 

 gemmules have thus been formed, the sponge dies ; but the 

 gemmule s live on in a quiescent state during the winter or 

 the dry season, and with the advent of spring develop into 

 sponges, male or female. These have the power of pro- 

 ducing sperms or ova, but no power of producing gemmules. 

 The power of producing ova, and that of producing gem- 

 mules, thus alternates in rhythmic fashion. 



But one more example of these modified forms of 

 reproduction can here be cited (from the author's text-book 

 on "Animal Biology"). The liver-fluke is a parasitic 

 organism, found in the liver of sheep. Here it reaches 

 sexual maturity, each individual producing many thousands 

 of eggs, which pass with the bile into the alimentary canal 

 of the host, and are distributed over the fields with the 



* On the other hand, three ova of the crustacean Apiis are said to coalesce 

 to form the single ovum from which one embryo develops. 



