PROTECTIVE MOVEMENTS 



365 



Even among the mammals we find the primates (monkeys, 

 apes, man) using their front limbs in food-getting quite as 

 much as in locomotion, 

 or even more. 



410. Migration. A 

 very interesting prob- 

 lem in connection with 

 the protective move- 

 ments of animals is that 

 of migration. The mi- 

 grations of the common 

 birds are more or less 

 familiar to all of us. 

 Those of us who live in 

 the northern latitudes 

 are likely to look upon 

 bird migration as " go- 

 ing south in the winter 

 to get away from the 

 cold," or as " going 

 south to get food." If 

 we live in the south we 

 may well ask why the 

 birds ever go north ; 

 and we can think of 

 no advantage to their 

 migration except that 

 of finding a breeding 

 place for the young in 

 a region free from the 

 usual enemies or other 

 obstacles (see Fig. 184). 



FIG. 183. The appendages of the lobster 



In the Crustacea all the appendages are built on the 

 same plan, but each segment of the body (repre- 

 sented by Roman numerals) has a distinctive organ, 

 /and //are sensory; ///-F combine sensory func- 

 tions with food-getting; VI-VIII are chiefly food- 

 getters, but are also related to breathing ; IX is the 

 nipper ; X and XI are both grasping and locomotor 

 organs ; XII and XIII are walking legs. The ab- 

 dominal appendages XIV-XVIII are called swim- 

 merets and probably assist in slow swimming. XIV 

 and XVare also related to reproduction in the male, 

 and in the female all the swimmerets carry the hatch- 

 ing eggs and larvae. XIX and XX spread out into a 

 flat tail-paddle, used in swimming backward suddenly 



It is possible that some species migrate originally with relation to 

 food and weather, and that other species migrate primarily in relation 

 to possible enemies. Whatever the advantage to the species, it is 



