3G4 LIFE ri;(_)CEs^;K.s of aalmals 



parts of the sponfi;o's body, while in the Ixidios of the hydrse, 

 sea anemones, and polyps the food cii'culates in the fluid 

 of the bod)- caA'ity. But in the jell^dishes there is a system 

 of tubes (not blood vessels) brandling olT fr<jm the stomach, 

 as we have seen, through which the food is cai-ried directly 

 to the various jiarts of the jjod}'. The first ai)])roacIi to a 

 true circulatory system is made l>y the starfish and sea 

 urchins. The earthworm possesses a lietter defined cir- 

 culatory system but has no true heai't. Jlany members 

 of the Arthro])0(la and Mollusca piesent a well-dehned cir- 

 culation, but it is among the vertef)rates, especially birds 

 and mannnals, that we hnd the higliest t>'pes of circulation. 



Reproduction. — To insure its existence and to prevent 

 its extinction cverj^ species of animal is endowed mth the 

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

 production among animals; namely, n.^exiial and sexual. 

 Only the lo\\'er animals reproduce asexually. The great 

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

 are also two methods of asexual reproduction; namely, 

 fission and hiiddirig. In asexual reproduction by fission 

 the animal simply di\'ides in two parts. Fission takes place 

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

 In asexual re])ro(Kicti(in by budding, a budlike ])rotuberance 

 forms on the side of the liody (jf the jjarent animal. The 

 bud grows and gradually develojis into a mature form which 

 may or may not remain attached to the ])arcHt animal. 

 Reproduction by ])uddiiig takes ]>lace among the sponges, 

 Cadento'ata, some wni-ms, and ascidians. 



The most univei'sal and imjiortaiit metliod of I'cprodue- 

 tion among animals is the sexual mc1li(i(h wliich consists 

 in the vmion of twd cells, the s]kmiii (male) cell and the egg 

 (female) cell. Among the vei-lcbi-ales, in most arthropods 



