19 



fact, we meet with every gradation between pediform and opercular external-maxillipeds. On 

 the other liand, there are a few Decapods outside the limits of the Brachyura, in which the 

 external maxillipeds are opercular. 



In no crabs above the level of the Dromides are epipodites or podobranchia? 

 found on any of the last five thoracic appendages. 



N.B. — In the larger chelipeds of crabs the meropodite is frequently spoken 

 of as the " arm," the carpopodite as the " wrist," and the propodite and dactyl- 

 ns together as the " hand," the body of the propodite being the " palm," its 

 process being the " fixed finger," and the dactylus being the " movable finger." 



The appendages of the abdomen, like the abdomen itself, are of a rundimen- 

 tary or, at least, subordinate character. 



In the male they are present on the first two abdominal somites only, and 

 are completely modified to form uniramous organs of copulation. 



In the female they are present on the 2nd, 3rd, 4th and 5th somites as 

 slender setose biramous appendages to which the eggs, when laid, are attached. 

 In the Dromides alone the 1st abdominal somite has a pair of appendages, which 

 are slender short and uniramous. 



In no crab does the 6th abdominal somite carry appendages : in the Uromn- 

 dea alone are these appendages represented— and then in a much modified form 

 — by a pair of little plates intercalated between the 6th and 7th abdominal 

 somites. 



In certain other Decapods, besides crabs, the abdominal appendages are reduced in size and 

 number in one or both sexes. Instances occur among the Loricata, the Pagurids, and the 

 Galatheids ; but here we always tind the appendages of the 6tli somite present. Among the 

 Lithodidse however even the appendages of tlie 6ih somite are absent. 



3. Of the Oir/uns of Uespiration of the Brachyura. 



As in Nephrops, the gills are attached on either side of the body to the epi- 

 niera and appendages of certain of the thoracic somites, and lie in a branchial 

 chamber formed by the overlapping of the carapace on either side. Into this 

 chamber the water used in breathing finds its way, usually but not invariably 

 by a valvular orifice situated between the basal joints of the chelipeds and the 

 neighbouring margin of the carapace, and from it the water, when done with, is 

 expelled by a channel running in either side of the buccal cavern, the current 

 being maintained by the scaphognathite of the 2nd maxilla and the epipodites of 

 the maxiUipeds, especially those of the 1st pair. The entry to the chamber 

 is the afferent branchial orifu-r, the exit from it is the eff'erent branchial canal. 

 In the Oxystome crabs the efferent branchial canals unite and traverse the endos- 

 torae in the middle line, and in many Oxystomes the afferent branchial canals 

 lie on either side of the endostome. 



