ON OUR PRESENT KNOWLEDGE OF THE CRUSTACEA. 89 



labium or movable Kp. It is very conspicuous in young animals, and fre- 

 (jucntly iu adult forms, particularly among the Amphipoda. It is represented 

 by two small osseous disks in Palimtrus, and a single small triangular plate 

 in Cancer, Corresponding with this labium posteriorly is another that 

 protects the opening between the mandibles in this direction. This is also 

 supported frequently by osseous plates ; bat this organ is not constantly 

 developed beyond a limited extent, except in a fe^v instances. In Pallnurus 

 it consists of a central osseous plate, having a suture through the median 

 line ; from this base it projects in two long membranous sacs, supported on 

 the outer or posterior surface by one or two osseous plates (PI. III. fig. 22). 

 It is this organ, it appears to me, that represents and is homologous with the 

 lip-plate or metastoma in Eurypterus, Pteri/gotus, &c., that has been so fully 

 described by Huxley, Woodward, and Salter. 



The fifth or next succeeding pair of appendages is that which Prof. Milne- 

 Edwards has called the deutognathe. It is what has been known in popular 

 carcinology as the first pair of foot-jaws, and first maxilla or siagnopoda in 

 the ' History of the British Sessile-eyed Crustacea,' the latter name being sug- 

 gested by Prof. Westwood " as the Greek equivalent for the Latin name of 

 the five pairs of appendages succeeding the mandibles, which were collectively 

 teTxaed pattes-mdcJwires by Cuvicr, Savigny," &c. 



The deutognathe in all known form^ of Crustacea exists in the adidt stage 

 in an embryonic condition ; it is small in size, feeble in power, and consists, in 

 different genera and families, of a varying number of thin squamiform i:)lates. 

 Each joint of the typical limb, as far as present in the adult condition (PI. III. 

 fig. 23), offers no very exceptional distinction from the same iu the embry- 

 onic stage (fig. 24). 



The ti-itognatJw, or sixth pair of appendages, supports the idea of the adult 

 form bearing a close resemblance to that of the zoea or embryonic condition 

 stiU more decidedly (PI. III. figs. 25 & 26), 



The seventh pair of appendages, the teiarfognatJie of Mihie-Edwards's 

 nomenclature, is the first pair of muclioives aiuviliaires of Savigny, or the 

 anterior mdclwires or foot-jaws of most authors. 



These, in the adult Brachyura, are still more embryonic in appearance. 

 In Maia and Cancer they are very reduced in size and apparent importance 

 (PI. III. fig. 27) ; but in some less highly developed types, such as the Am- 

 phipoda and Isopoda, where they are generally recognized under the name of 

 maxUlipeds (PI. III. fig. 28), they assume a more important feature, and 

 bear a not very distant resemblance to the typical form from which they 

 are supposed to depart. In NihuUa they closely resemble the posteriorly 

 succeediug pairs of limbs ; but in this genus the whole of these gradually 

 degenerate to the embryonic condition as they recede from this point. 



In the larval or zoea stage of Crustacea they are wanting in the higher 

 forms. 



These three pairs of limbs appear to me to offer an interesting and valuable 

 example of the manner in which any great changes in the variation of the 

 structure of an animal takes place. The crowding together so to speak of 

 the three posterior somites of the ccphalon, so as to bring, as much as possible, 

 the several pairs of appendages within the limits of the oral region, so crushes 

 them in their position, that their usefulness as separate organs must be much 

 impeded. It would therefore appear that the crowding of appendages 

 together interferes with and arrests the progress of their development, while 

 they are best suited to exist under the altered conditions where they are 

 the least inconvenient. That they are of little or no importance iu the 



