772 



CRUSTACEA. 



led to their ordinary denomination of maxil- 

 lary limbs or feet (jigs. 408, 409, 410.) 



Fig. 408. 



Fig. 409. 



The forms and dimensions of 

 these organs vary considerably, 

 and are obviously in harmony 

 with their uses ; they are by so 

 much the shorter and flatter as 

 they are more peculiarly appor- 

 tioned to the oral apparatus, a 

 disposition which is nowhere 

 more conspicuously displayed 

 than among the short-tailed De- 

 capods, in which they resemble 

 horny laminee, armed with teeth 

 or serrse of various sizes, and 

 supporting an articulated palp (6, 

 Jig. 408) as well as a flabellifbrm 

 or whip -shaped appendage (c), 

 which penetrates into the interior 

 of the branchial cavity. The last 

 pair of all (jig. 410) presents 



Fig. 410. 



amounts to three pairs, and in the Phyllo- 

 soma to two pairs only. 



To conclude, the Limuli, a group of Crusta- 

 ceans of the most singular conformation, are at 

 the bottom of the scale in this respect; for in 

 them (fg. 41 1) the anterior ambulatory extre- 

 mities themselves surround the mouth, and 

 their basilar articulations perform the office of 

 jaws. 



The organs of which we have just made 

 mention, are, according to the modifications 

 they undergo, adapted in a more or less espe- 

 cial manner to seize, to hold fast, and to 

 comminute the alimentary matters upon which 

 the animal lives. Moreover the thoracic ex- 

 tremities in many species are themselves calcu- 

 lated to accomplish one or all of these offices 

 with various degrees of success, according to 

 their form, their extent, and the mode in which 



Fig. 411. 



Limulus polyphemus, (ventral aspect.) 



a, carapace ; b, frontal portion of the carapace ; c, thorax ; 

 d, chelifera , e,f,g,h,i,.j, legs, the basilar portions of which 

 surround the mouth and act as mandibles ; I, under-lip ; 

 m, branchial or lamelliform appendages ; n, mouth. 



itself under the 

 shape of two thin 

 and much expand- 

 ed laminae which 

 serve as a kind of 

 broad operculum to 

 cover the whole of 

 the oral apparatus. 



Starting from this complication of structure, 

 the greatest in the series, we shall see the ap- 

 paratus degenerating by successive degrees, at 

 the same time that in any given group its com- 

 position presents much less of constancy or 

 regularity. The Sergestes among the Decapods 

 have one pair of maxillary feet fewer than the 

 highest number; the Edriophthalmians have 

 no more than a single pair, whilst in the 

 Thysanopoda and the generality of the Sto- 

 mapoda the number of oral appendages 



they are terminated. The most favourable 

 disposition to these ends is observed in the 

 lobsters, crabs, &c.; in a word in a very great 

 number both of the short and long-tailed De- 

 capods, in which the anterior thoracic extre- 

 mities terminate in pincers of greater or less 

 strength, armed with teeth and sharp hooks 

 which give them increased powers of pre- 

 hension. This form results mainly from the 

 state of extreme development in which the pe- 

 nultimate articulation frequently occurs, and 

 its assumption of the shape of a finger, by the 

 prolongation of one of its inferior angles. 

 Against the finger-like process thus produced, 

 which is of great strength and quite immove- 

 able, the last articulation can be brought to 

 bear with immense force, as it is put into mo- 

 tion by a muscular mass of great size, and in 

 relation with the extraordinary size of the pe- 



