3 oo 



AN INTRODUCTION TO ZOOLOGY 



antenna, and the larval Sacculina sheds its appendages, shell, 

 muscles, and excretory organs. What remains (E) pass* 

 through the antenna into the body of the crab (F), and finally 

 reaches the body cavity. A system of roots now grows out in 

 directions. When the crab molts, the saclike body of the para- 

 site penetrates to the outside, resembling a tumor consistii 

 almost entirely of reproductive organs (237). 



(d) Social Life 



That there is a continual competition among individuals am 

 species has been pointed out in the course of our discussion of 

 geographical distribution (p. 275), and the struggle for existem 

 (p. 283). However, there is another law of nature, namely 

 sociability, not so far-reaching, perhaps, but of undoubted im- 

 portance to the welfare of many and diverse species of animals 

 Certain of the social species have already been mentioned (\ 

 264), and the activities of the honeybee, an insect with a vei 

 complex social life, have been described in some detail (p. 233). 



The beavers, Rocky Mountain sheep, and prairie dogs (mam- 

 mals), the swallows, herons, and sea gulls (birds), and the ants, 

 wasps, and termites (insects) are famous for their community 

 life. 



The advantages of living together are many, and the lives 

 most of the gregarious animals depend upon their instinct t( 

 form bands and colonies. Darwin recognized the value of sock 

 life, although his theories were based largely upon the effects oi 

 the struggle for existence. He says that " the individuals whicl 

 took the greatest pleasure in society would best escape various 

 dangers; while those that cared least for their comrades, an< 

 lived solitary, would perish in greater numbers." Deer, fc 

 example, when banded together may escape from their enemh 

 whereas single individuals would be destroyed. The Rocki 

 Mountain sheep post sentinels, wh'ch warn the main herd of 

 approach of danger, and enable all to seek places of safety. 



