206 CllUSTACEA. 



To the Chelce succeed four pairs of slender legs, 

 scarcely at all serviceable for locomotion, but the 

 two anterior being terminated by feeble forceps, 

 are auxiliary instruments of prehension. 



The articulated appendages situated beneath the 

 tail are so rudimentary, that they are no longer re- 

 cognizable as instruments of progression, and it is at 

 once evident, when we examine the manner in which 

 these creatures use their tails in swimming, that any 

 large limbs in this position would materially impede 

 the movements of animals presenting such a con- 

 struction. The false feet, as these organs are called, 

 are merely available as a means of fixing the eggs 

 wdiich the female lobster attaches to them. 



The tail is the great agent of locomotion, and for 

 this purpose it is terminated by a fin formed of broad 

 plates, so arranged, that w^hile they will close to- 

 gether during the extension of the tail, and thus 

 present the least possible surface to the water, they 

 are brought out to their full expansion by its down 

 stroke ; and such is the impulse thus given, that a 

 lobster will dart backwards to a distance of twenty 

 or thirty feet by one sweep of this remarkable loco- 

 motive instrument. 



The Crustacea are divided into several important 

 sections, each of w'hich will require separate notice. 



The first great division of this extensive class has 

 received tlie name of 



Decapoda* (ten legs), from the circumstance that 

 their locomotive limbs are always ten in number. 

 The Decapods are divided into three groups, known 

 by the somewhat unpronounceable names of Macroura, 

 Brachyura, and Anomoura; when translated into 

 English, however, these formidable looking words 

 simply mean Big-tails, Little-tails, and ftueer-tails. 



The division Macrourat {big-tails), includes the 

 Lohstei'S, Crag-fishes, SJtrimps, and Prawns. 



* ZiKa, deca, ten ; irous, Todhs, pous, podos, a foot.. 

 t ixaKpos, macros, lomj ; ovpa, oura, a tail. 



