142 CRUSTACEANS. 



student can so arrange his course of observation, that the Crustaceans 

 are in a manner brought under such control, that its occurrence may be 

 anticipated Avithin given intervals. 



The Crustaceans are provided with a number of limbs, whereon their 

 subdivision into genera is principally founded. Two of these, in some 

 genera, terminate in strong claws, each with a forceps, employed as or- 

 gans of otfence or defence, and as auxiliaries in feeding. 



The Decapodes, or those with ten limbs, are, of all Crustaceans, the 

 most famiUar to us, and on which most of our information regarding the 

 whole race is founded. Some are of personal use to mankind. The 

 characteristics of the majority of the others are not equally well kno^vn ; 

 nor have they received as much notice. 



In general, the motions of the Crustaceans is walking or running : 

 few of them are framed for swimming, to which the structure of the 

 l)ody, the subordinate parts, and its gravity, are adverse. Nevertheless, 

 some can spring with great force through the water, and a few can sus- 

 tain themselves permanently amidst it. They harbour chiefly among 

 rocks and stones, or among sand, more rarely dwelhng among vegetable 

 matter. But the Asellus and Limnoria seem to have no other habita- 

 tion. 



The senses of many Crustaceans are very obtuse, especially sight 

 and smelling. They are quite sensible of light, of shade, or obscuration. 

 They seem to distinguish the approach and the presence of persons to 

 whom they are accustomed, especially if receiving food at their hands. 

 But whether they have any distinct and correct vision of particular 

 forms, is questionable. For a long time I was inchned to rest this doubt 

 on believing that some integument or covering of the eyes separated on 

 exuviation, which I am now satisfied was erroneous. 



Their sense of smelling is certainly very obtuse. It is only when 

 in tlieir vicinity that they are aware of the presence of food. Even then 

 the search after it is wavering and indirect. The eyes are neither turned 

 towards its place, nor does the animal advance for its seizure. It ap- 

 pears to be aroused by nothing short of actual contact of foreign sub- 

 stances with the forceps, where feeling seems principally to reside. 



