CRUSTACEA. 323 



arc no less obviously merely modifications of articulated feet ; and 

 the term foot-jaws has now, by common consent, become the appel- 

 lation by which they are distinguished. 



The pair of legs which succeeds to the remarkable members last 

 referred to, is appropriated to widely different offices. The organs 

 in question are developed to a size far surpassing that attained by 

 any of the other limbs and are endowed with proportionate strength. 

 Each of these robust extremities is terminated by a pair of strong 

 pincers (chelee) ; but the two are found to differ in their structure, 

 and are appropriated to distinct uses. That of one side of the 

 body has the opposed edges of its terminal forceps provided with 

 large blunt tubercles, while the opposite claw is armed with small 

 sharp teeth. One, in fact, is used as an anchor, by which the lob- 

 ster holds fast by some submarine fixed object, and thus prevents 

 itself from being tossed about in an agitated sea ; the other is ap- 

 parently a cutting instrument for tearing or dividing prey. 



To the chela succeed four pairs of slender legs, scarcely at 

 all serviceable for the purposes of locomotion ; but, the two ante- 

 rior being terminated by feeble forceps, they become auxiliary 

 instruments of prehension. 



The articulated appendages belonging to all the abdominal 

 segments are so rudimentary that they are no longer recognisable 

 as assistants in progression ; and it is at once evident, when 

 we examine the manner in which the Macroura use their tails 

 in swimming, that the developement of large organs in this 

 position would materially impede the progress of animals pre- 

 senting such a construction : the false feet, as these organs are 

 called, are therefore merely available as a means of fixing the 

 ova which the female lobster carries about with her attached be- 

 neath her abdomen. 



The tail is the great agent of locomotion in all the Macroura 

 or large-tailed Decapods, and for this purpose it is terminated 

 by a fin formed of broad calcareous lamellae, so arranged, that 

 while they will close together 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 the down-stroke of the 

 abdomen ; and such is the impulse thus given, that, as we are 

 credibly informed, a lobster will dart itself backwards to a dis- 

 tance of eighteen or twenty feet by one sweep of this remarkable 

 locomotive instrument. 



If we now pass on to the consideration of the Anomourous De- 



