CRUSTACEA. 



757 



tegumentary skeleton presents the most singular 

 forms : thus among the Limmadia ancf the 

 Cypris, the pieces which are analogous to the 

 epimeral or lateral pieces of this cephalic 

 buckler, acquire a great extension, whilst the 

 tergal portion of the arc to which they belong 

 continues rudimentary or proves entirely abor- 

 tive, so that they constitute two large valves 

 covering the whole body of the animal, and 

 bearing considerable resemblance to the shells 

 of certain acephalous Mollusks. The dorsal 

 laminae which in the Pandarus form appendices 

 on the back similar to Elytra, and those which 

 in the Anthostomata form a kind of sheath 

 around the posterior part of the body, are also 

 formed by the anomalous development of cer- 

 tain parts of both the dorsal and ventral arcs of 

 the two posterior thoracic rings. 



The inferior arcs of the thoracic rings of the 

 tegumentary skeleton of the Decapoda, by 

 their intimate union, form a kind of ventral 

 shield, named sternal plastrum, upon which 

 lines of conjunction indicate the respective 

 limits of the greater number of the segments, as 

 well as of the sternal and episternal pieces of 

 which these are composed. In the Decapoda 

 Macroura and Anomoura, this plastrum is in 

 general very narrow, but in the Brachyura it is 

 expanded to such a degree as frequently to con- 

 stitute a great and nearly circular disc. In the 

 whole of these Crustaceans, the lateral pieces of 

 the thoracic rings are conjoined, like those of 

 the inferior arc of the same segments, and form 

 on either side of the middle portion of the 

 body a septum which is covered by the cara- 

 pace, and which is known among anatomists 

 under the name of the vault of thejiancs. In 

 the Macroura this septum is nearly vertical, 

 but in the Brachyura it is oblique, or even 

 almost horizontal. 



Fig. 383. 



Lateral portion of the thorax of a Decapod. 



a, the epimeral pieces united to form the vault of 

 the flancs ; b, the sternum ; c, the apodemata 

 rising from the sternum and separating the in- 

 sertions of the legs. 



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 apodcmata, 



Vertical section of a portion of the tliorax of one of 

 t/ie Brachyura. 



a, sternum, with a sternal apodema rising from it; 

 b, epimera from the inner surface of which an 

 epimeral apodema descends to join the sternal 

 apodema,' and thus form a septum between the 

 thoracic cells. 



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 Brachyuva ; 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 laminae 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 anastomoses 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- 

 tigate before long. Generally speaking, the 

 disposition of these cells and of the septa 

 which form them varies considerably in the 

 Brachyura 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 



