It is among those Crustaceans the thoracic rings of whose tegumentary skeleton blend or become consolidated in this manner, and ac quire dimensions so considerable, that the struc ture of this portion of the frame-work also exhi bits the utmost extent of complication, in con sequence of the existence of large apodemata in their interior. These septa are of two kinds ; the one, styled sternal apodemata, arise from the lines of consolidation of the thoracic sternal pieces; the other, named epimeral apodemata, arise in a similar manner from the epimeral pieces of the same rings. They are met with among the Macroura and Anomoura, as well as among the Brachyura ; but it is among these last that they acquire their highest development; their direction, vertical to the internal planes of the rings, and the unions of those that rise from the inferior aspect or floor with those that des cend from the arched superior surface, give rise to the most singular combinations and forms, too multifarious to admit of description in an article of the extent of that in which we are engaged, but the final effect of which is the establishment of cells, divided from one an other by vertical septa, and corresponding to each ring, and further intersected in the direc tion of their height, in a certain number of species, and divided into two stages by means of horizontal reduplications..
It is within these different cells that the muscles and principal vessels of the thorax are lodged in the Brachyura; holes left at the con junctions of these lamince admit of the com munication of the cells two and two, either through the vertical septa or through the hori zontal floors which divide the superposed cells, and it is by means of these holes of conjunc tion that the anastornoses of the vessels of one ring take place with those of the neighbouring ring, as we shall see presently.
In the Macroura, again, this structure does not occur, in consequence of which other means of communication between the vessels of the different segments require to be established, the nature of which we shall also have to inves tig,ate before low,. Generally speaking, the disposition of these cells and of the septa which form them varies considerably in the Bracliyura and the Macroura. Certain pro longations from the superior and internal angle of the sternal apodemata, by their union in the median line, after bending from before back wards, even form a longitudinal canal, which extends through almost the whole length of the thorax. This is the sternal canal, destined to lodge the ganglionic nervous cord, and to serve as the chief venous reservoir.
It has long been admitted as an axiom in animal physics, that when any particular part of the body acquires a very high degree of de velopment, certain other parts stop short of their ordinary state of evolution, as if the former had obtained their unusual increment at the cost ment, or that the forms they assume, in har mony with the uses they serve, render them liable to be mistaken. It is very dif ferent in the embryo; here, in fact, as among the simplest forms of the series, the whole of the extremities are at first similar; and it is only in consequence of ulterior develop ments that each pair finally assumes diver sities of form and character in relation with the various functions to which they are espe cially destined.
In its most perfect state of development, the extremity in the Crustacean consists of three principal parts : the stem (a), which is the most of the latter. This rule, which has been dis cussed by M. Geoffroy St. IIilaire under the title of /a loi de bdancement organique, or law of org,anie equivalents, is found to apply in the present instance ; for the Crustacea in which the cephalic portion of the tegumentary skele ton is developed in the greatest degree, (viz. the Brachyura) present the abdominal portion of the body of very small dimensions; whilst, on the other hand, in the Macroura, or those species in which the abdominal' portion of the body arrives at its maximum of develop ment, and performs a very important office in the business of locomotion, the cephalic por tion is relatively greatly inferior in size.
.With regard to its disposition the abdomen is simple enough ; the rings of which it con sists are in general moveable upon one another, and even when they are consolidated, present no apodemata projecting from their interior. It is also deserving of remark that the elementary pieces of the different rings are not very distinct, and sometimes even appear to be partially wanting.
Let us now go on to examine the portion of the tegumentary skeleton belonging to the extremities or that portion of the external skeleton of the Crustacea which may be re garded as an appendage to the more essen tial covering of the head, thorax, and ab domen.
The Crustacea present this invariable cha racter, that the whole of the appendages belong exclusively to the inferior arc of their tegu mentary rings, a point in which they resemble the Arachnidans, and differ like these from Insects, in which one or two of the thoracic rings generally present a pair of extremities supported by the superior arcs, as in the An nelidans, in which the dorsal segment of each of the rings almost always carries a pair of extremities fashioned in the same manner as those belonging to the ventral arcs.* We have already said that a pair of appendages ought to be found attached to each ring ; but it very frequently happens that many of the pairs are completely checked in their develop essential and most constant part, formed of a variable number of articulations ; the palp (b), an appendage which is detached from one of the three first articulations of the stem, but almost always from the first ; and the whip (fouet ) (e), which is sent off above and to the outer side of tbe palp. It but rarely happens, however, that these three organs exist simultaneously ; occa sionally not more than one of them can be demonstrated ; and sometimes the whole three are altogether wanting.