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 investigfated of the most complex structure, namely, the Decapoda brachyum. 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 /ower /ip or lunguette (c) is for the niost part bifid. Be tween these two pieces and on their sides are the mandibles, (fig. 406,) appendices of the fourth cephalic ring, modified so as to serve for mastication. As in the whole tribe of articu 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 sally. They do not vary 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 tion, we remark an addition of as many as five pairs of modified appendages situated behind the under lip, and all subservient to the hension and the mastication of the food. The
two first (figs. 406 and 407) are the most 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, aaain soon cease to 0 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 led to their ordinary denomination of maxil lary limbs or _feet (figs. 408, 409, 410.) 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 tbey 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 laminze, armed with teeth or serne of various sizes, and supporting an articulated palp (b, fig. 408) as well as a flabelliform or whip -shaped appendage (c), which penetrates into the interior of the branchial cavity. The last pair of all (fig. 410) presents itself under the shape of two thin and much expand ed laminw 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 amounts to three pairs, and in the Phyllo soma to two pairs only.