Perhaps it would be better to say tbat, whatever other elements maj- enter into the formation 

 of that part of the carapace which lies in front of the cervical groove, the pleura and terga of the 

 third or antenuular somite can, in many Macrarous and Braohyurous Crustacea, be recownizod as 

 forming the preponderating element. 



Each antenna consists of a peduncle, a scale (exopodite), and a lash or 

 fiagellum (endopodite). The peduncle is constituted of a rather zig-zag series of 

 five joints, the first two of which correspond with a protopodite : on the ventral 

 surface of the first joint is a prominent orifice, the renal tubercle, at which the 

 green gland, or excretory organ, opens. The antenna! scale, which is a broad 

 thin plate, is articulated to the end of the 2nd joint of the peduncle. The 

 fiagellum, which is slender and of great length, is made up of a succession of 

 little rings, and articulates with the end of the peduncle : it is an organ of touch. 



The fourth segment is the mandibular somite and its appendages are the 

 viandihles : its modifications take the same turn as do those of the antennal somite. 

 Its sternum is recognizable as forming the posterior part of the epistome ; its 

 pleura and tergum probably constitute the preponderating elements of that part 

 of the carapace which lies behind the cervical groove. 



It is difficult to avoid the inference that in the carapace of most Macrura and Bracltyura 

 the part whicli lies behind the cervical groove belongs to the mandibular somite. In most 

 Anoviura, however, and in some Macrura, the mandibular soniite seems to enter into the 

 formation of the sidewalls of the carapace only. Furthermore, the sutures of the carapace of 

 Thalasdna forcibly suggest that other terga and pleura, besides those of the antennal and mandi- 

 bular somites, may take a considerable share iu the formation of both the roof and the sidewalls 

 of the carapace. 



Each mandible consists of (1) a strongly calcified protopodite placed obliquely, 

 and (2) a cm"ved three-jointed palp (eiidopodite). The body of the mandible, 

 or protopodite, is shaped something like a tooth : its distal end is broadened and 

 is grooved fore-and-aft, the ventral edge of the groove forming a salient and 

 trenchant incisor process, while the concealed dorsal edge of the groove forms a 

 broadened molar process. 



The fifth and sixth segments are only visible after the anterior appendages 

 of the thorax ai'e removed, and even then all that is seen of them is a little plate 

 lying in the middle line, behind the mouth, and representing their conjoint sterna, 

 to which they are reduced. It is also possible that their pleura are represented 

 — though their individuality is, of course, completely lost — by a part of the edge 

 of the carapace in their own immediate neighbourhood. Both these somites are, 

 however, represented by appendages. 



The appendages of the fifth somite are the \st maxillx! They are thin 

 foliaceous chewing-organs, and lie in contact with the mandibles. Each consists 

 of (1) a leaf-like coxopodite and basipoditc, both of which have their long axis 

 nearly transverse and their free (distal) end fringed with set«, and (2) a two- 

 jointed endopodite or palp. The exopodite is not present. 



