THE ARTHROPODS 125 



bottom. It is yery cautious and cunning, capturing its 

 prey by stealth, and with weapons which it knows how to 

 conceal. Lying hidden in a bunch of seaweed, in a crevice 

 among the rocks, or in its burrow in the mud, it waits until 

 its victim is within reach of its claws, before striking the 

 fatal blow. The senses of sight and hearing are probably 

 far from acute, but it possesses a keen sense of touch and 

 of smell, and probably also a sense of taste." 



Although enclosed in a horny and often very thick and 

 strong armor, the sense of touch is very keen in the 

 Crustacea and in arthropods generally. On many of the 

 more exposed portions delicate hairs or pits connected 

 with the nervous system occur in great abundance. Some 

 of these, usually on the antennae, undoubtedly serve in 

 detecting odors, but the remainder are considered to be 

 tactile. In the higher Crustacea, such as the crayfish, 

 lobsters, and crabs, ears are usually found, consisting of 

 sacs lined with similar delicate hairs, and containing sev- 

 eral minute grains of sand, which in many cases make their 

 way through the small external opening. Vibrations com- 

 ing through the water gently shake the grains of sand, 

 causing them to strike against the hairs which communi- 

 cate with the nervous system a very simple ear, yet suffi- 

 cient for the needs of the animals. 



The eyes of the Crustacea and arthropods in general are 

 either simple or compound. The simple and frequently 

 single eyes usually consist of a relatively few cells embedded 

 in a quantity of pigment and connected with the nervous 

 system. It is doubtful whether they perceive objects as 

 anything more than highly blurred images, and perhaps 

 they merely recognize the difference between light and 

 darkness. The compound eyes, on the other hand, are 

 remarkably complex structures, often borne on the tops of 

 movable stalks, as in the common crabs and crayfishes. 

 Each consists of an external transparent cornea, divided 

 into numerous minute hexagonal areas corresponding to as 



