CRUSTACEA. 



771 



Fig. 404. 



Fig. 405. 



stems of very various lengths, and which every 

 consideration leads us to view as the limbs or 

 appendages of the first cephalic ring. It some- 

 times even happens (Jig. 404) that in these 

 animals, between the outer edge of the cara- 

 pace and the base of the antennae, there occurs 

 a furrow or cavity within which the eyes may 

 be withdrawn or laid flat, so as to be out of 

 the way of injury ; this groove or cavity is 

 generally spoken of under the name of* the 

 orbit. 



3. Apparatus of Nutrition. 



In the study of this apparatus we shall have 

 to consider successively the organs of digestion, 

 of circulation, of respiration, and of secretion. 



A. Apparatus of digestion. The organs 

 concerned in the digestion of the food among 

 the Crustacea may be divided into three 

 orders, according to the functions they fulfil, 

 to wit, 1st, the apparatus for the prehension 

 and mastication of the food; 2nd, the alimen- 

 tary canal ; 3rd, the various secreting organs 

 associated with the intestine. 



The Crustacea are divided into two grand 

 sections in conformity with their habits and 

 the nature of their food : the masticators, 

 which generally live apart from their prey, 

 pursue it, and seize it in proportion as they 

 are admonished by their wants or appetite to 

 do so; and the suckers, considerably fewer in 

 number, and which in their state of perfect 

 growth live almost invariably attached to their 

 prey without executing any other motions than 

 such as are performed by the latter. 



The masticating Crustacea being the highest 

 in point of organization, we shall commence 

 our description with them,* and we shall even 

 select for our particular consideration the spe- 

 cies among these which have the class of 

 organs about to be investigated of the most 

 complex structure, namely, the Decapoda 

 brachyura. In these animals the mouth is 

 constantly situated on the inferior surface of 

 the cephalic portion of the body. Two lips 

 close it anteriorly and posteriorly ; the upper 

 lip or labrum (a, fig- 405) is a median piece in 

 the form of a simple fold, and the lower lip 

 or languette (c) is for the most part bifid. Be- 

 tween these two pieces and on their sides are 

 the mandibles, (jig. 406,) appendices of the 

 fourth cephalic ring, modified so as to serve for 

 mastication. As in the whole tribe of articu- 



* On this subject consult Savigny Memoires sur 

 les Animaux sans Vertebres, Ire fascicule ; La- 

 treille, Hist. Nat. des Crustaces et Insectes, &c. ; 

 Cuvier, Regue Animal ; Desniarest, Considerations 

 sur les Crustaces ; Milne Edwards, Hist. Nat. des 

 Ciustaces, t. i. p. 61. 



Masticatory Organs of tJie Phyllosomu. 



a, upper lip j b, mandibles j c, lower lip j 



(I, maxillae. 



lated animals, these organs act laterally, and not 

 upwards and downwards in the line of the axis 

 of the body as in the vertebrate series univer- 

 sally. They do not vary 

 Fig. 406. much in point of form 



among the Decapoda; 

 in almost every one of 

 these they are seen pos- 

 sessed of a principal 

 part terminated by a 

 cutting edge, or a sur- 

 face adapted for tritu- 

 ration; and an appendage which appears to 

 fasten the food and keep it steady during 

 the process of mastication. The mandible 

 itself, which is of extreme hardness, appears 

 to be neither more nor less than the basilar 

 piece of the member or appendage, of great 

 strength and toothed. The articulated palp 

 which it supports, in this mode of viewing 

 the structure, would turn out to be a mere 

 continuation of the stem (tige), and not a 

 proper palp, as its name seems to imply, 

 but which it has only acquired from its resem- 

 blance to the appendage to which the term of 

 right belongs. 



Such is the structure of the mouth among 

 a certain number of the inferior Crustacea; but 

 among those to which we now turn our atten- 

 tion, we remark an addition of as many as five 

 pairs of modified appendages situated behind 

 the under lip, and all subservient to the pre- 

 hension and the mastication of the food. The 

 two first (figs. 406 and 407) are the most con- 

 stant; and even when we get low in the series, 

 and they have lost their special functions, they 

 can still be traced, although of course only in 

 a rudimentary state. When well developed 

 they are without palps and are designated 

 by the name of jaws. The three other pairs, 

 again, soon cease to 

 appear as part of the 

 implements of digestion, 

 in order to show them- 

 selves among the instru- 

 ments of locomotion ; 

 sometimes, however, 

 they seem to serve for 

 both kinds of function, 

 a circumstance which has 



Fig. 407. 





