AETICULATED APPENDAGES TO SEGMENTS. 389 



To notice all the varieties which occur in the extensive class before us, 

 would be to weary the reader with tedious and unnecessary details ; 

 we shall therefore select the Decapod* division of these animals, as 

 abundantly sufficient for the illustration of this part of our subject. 

 This division, which includes the most highly organized forms, has been 

 divided by writers into three extensive families: the Macroura, or 

 Swimming Decapods ; the Anomoura, which inhabit the empty shells 

 of Mollusca ; and the Brachyura, or short-tailed species, of which the 

 Crab is a familiar specimen. If we take the common Lobster as an 

 example of the first of these groups, we shall find that there are five 

 pairs of articulated limbs placed upon each side of the mouth, which are 

 evidently adapted to assist in seizing and conveying into the stomach 

 substances used as food. These singular organs, although entitled to 

 be considered as jaws so far as their use would indicate the name be- 

 longing to them, are no less obviously merely modifications of articu- 

 lated feet ; and the term foot-jaws has now, by common consent, become 

 the appellation by which they are distinguished. 



(995.) 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 

 (chelce) ; 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 Lobster holds fast by some submarine 

 fixed object, and thus prevents itself from being tossed about in an 

 agitated sea ; the other is apparently a cutting instrument for tearing 

 or dividing prey. 



(996.) To the chelce succeed four pairs of slender legs, scarcely at 

 all serviceable for the purposes of locomotion ; but the two anterior 

 being terminated by feeble forceps, they become auxiliary instruments 

 of prehension. 



(997.) The articulated appendages belonging to all the abdominal 

 segments are so rudimentary that they are no longer recognizable 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 development of large organs in this position would materially impede 

 the progress of animals presenting 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 

 beneath her abdomen. 



* So called from the circumstance of their having five pairs of limbs BO largely 

 developed as to become ambulatory or prehensile organs. 



