ON OUR PRESENT KNOWXEDGE OF THE CRUSTACEA. T7 . 



Tn the Anomura, of which wo may take Litliodes (PI, II. fig. 3) as an 

 example, the coxaj of the legs are so closely compressed together laterally 

 that, without coalescing or being fused together, thoy are apparently united, 

 while the inferior part of each coxa is completely fused with its neighbour 

 for about half its extent. 



This is carried still farther in the true Brachyura (PI. II. fig. 4), where 

 the first joints of the legs are all consolidated into a tolerably perfect 

 mass of calcareous structure, and resemble the nature and character of a 

 sternum. 



The ventral plastron, therefore, is formed of the first joint of the leg, and 

 the inferior arc of these seven somites is wanting iu the true Brachyura in 

 the adult stage, the inferior surface of the legs fulfilling the duty of the sternal 

 plate. As I have already observed, this state can be traced gradually from the 

 Macrura to the Brachyura ; and it may also be observed gradually to assume 

 this condition by following the development of the young, in which the coxal 

 joints may be distinguished separate and individually present, and gradually 

 coalescing as the animal increases in dimensions with age. I am aware that 

 this assertion is not in accordance with the teachings of previous carcinolo- 

 gical anatomists ; but it is one that can be proved to demonstration. 



Milne-Edwards, " Observations sur le Squelette tegumentaire des Crus- 

 tacea de'capodes," Ann. des Sc. Nat. p. 269, 1854, says, " These rings exhibit 

 all the tergal pieces, and are closed above by a carapace, except among a small 

 number of Anomura, as the Cenobitts, where the seventh ring is complete. We 

 can distinguish always a ventral arc, constituted normally by two sternal and 

 two episterual pieces, and a dorsal arc, represented upon the sides of the epi- 

 meral pieces of the sclerodermic prolongations extending between the ven- 

 tral and dorsal arcs of each ring, so as to enclose between them each side of 

 the body, and to circumscribe before and behind the articular cavities destined 

 for the insertion of the corresponding members. When the rings are free, each 

 of these arcs' extremities I shall call artJirodkds, for the sake of bein" dis- 

 tinct ; but when the zones are soldered together it is diff'erent. The anterior 

 arthrodial of each thoracic ring is united to the posterior arthrodial of the 

 preceding zone, and is more or less completely united with it, so that the 

 interarticular space situated between two such legs, instead of presenting 

 two sclerodermic rings, lodges only a single artlirodial prolongation, which 

 becomes common to the two approximating frames, so that it appears to 

 depend more especially upon the last of the two rings so united. To simplify 

 the description, I shall consider these complex arthrodials as if they were 

 formed only by their most important parts, and shall neglect consequently 

 their anterior plate ; but it should be observed that we can nearly always 

 recognize its existence. There is also an interannular symphysis which 

 results from the formation of an interior fold of the sclerodermic lamella, a 

 fold the two plates intimately sustain between them. These processes must 

 be looked upon as if they were produced by the simple lamella of the posterior 

 border of one of the segments so imited bj^ symphysis. 



" It is always in the anterior portion of the thorax of the decapods that 

 consolidation of the integumentary skeleton is carried to the furthest limit by 

 the soldering or fusion of the anatomical elements." 



Now what I contend is, that the structure of the somite has, as a part of 

 the dermal skeleton, ventraUy disappeared in the Brachj-ura, and its place 

 has been taken by the dermal tissues of the first joint of the several legs of 

 the pereion, and the apodema is formed in the various families of Crustacea 

 out of parts that are homologically distinct. 



