364 LIFE PROCESSES OF ANIMALS 



parts of the sponge's body, while in the bodies of the hydrse, 

 sea anemones, and polyps the food circulates in the fluid 

 of the body cavity. But in the jeUyfishes there is a system 

 of tubes (not blood vessels) branching off from the stomach, 

 as we have seen, through which the food is carried directly 

 to the various parts of the body. The first approach to a 

 true circulatory system is made by the starfish and sea 

 urchins. The earthworm possesses a better defined cir- 

 culatory system but has no true heart. Many m.embers 

 of the Arthropoda and MoUusca present a well-defined cir- 

 culation, but it is among the vertebrates, 'especially birds 

 and mammals, that we find the highest types of circulation. 



Reproduction. — To insure its existence and to prevent 

 its extinction every species of animal is endowed with the 

 power to reproduce itself. There are two methods of re- 

 production among animals; namely, asexual and sexual. 

 Only the lower animals reproduce asexually. The great 

 majority of animals, if not all, reproduce sexually. There 

 are also two methods of asexual reproduction; namely, 

 fission and budding. In asexual reproduction by fission 

 the animal simply divides in two parts. Fission takes place 

 in the Protozoa, in some ccelenterates, and in some worms. 

 In asexual reproduction by budding, a budlike protuberance 

 forms on the side of the body of the parent animal. The 

 bud grows and gradually develops into a mature form which 

 may or may not remain attached to the parent animal. 

 Reproduction by budding takes place among the sponges, 

 Ccelenterata, some worms, and ascidians. 



The most universal and important method of reproduc- 

 tion among animals is the sexual method, which consists 

 in the union of two cells, the sperm (male) cell and the egg 

 (female) cell. Among the vertebrates, in most arthropods 



