27 



The flagella, when present, of the exopodites of the maxilhpeds are ahnost 

 always bent inwards nearly at a right angle, and the epipodites of the maxillipeds 

 are ahnost always very large, 



Tn a great majority of crabs the ischium and merus of the external maxilli- 

 peds are broad plates, which completely close the buccal cavern like doors and 

 conceal the other mouth-parts. 



The thoracic legs are six-jointed, their basipodite and ischium being fused 

 to form a single piece. None of these legs have exopodites, and it is only in one 

 small primitive group that any of them carry epipodites or podobranchifp. The 

 1st i^aii', which ax-e often enlai'ged, are chelate; the 2nd and 3rd pairs are mono- 

 dactylous ; the 4th and 5tli pairs may either be monodactylous, or sometimes one 

 or both of them may be subchelate and reduced in size. 



Tn the male, modified uniramous appendages are present on the 1st two 

 abdominal somites. In the female biramous setose appendages are present on 

 the 2nd-5th abdominal somites. In the Divmidcx alone a pair of small uni- 

 ramous appendages are also present on the 1st abdominal somite, and in the 

 Bromiidea alone a pair of plates intercalated between the 6th and 7th abdominal 

 somites are the modified horaologues of apj^endages of the Gtli somite. 



The ducts of the male reproductive organs open either on the coxopodites 

 of the last pair of thoracic legs, or, less commonly, on the corresponding seg- 

 ment of the sternum itself. The oviducts almost always open on the sternal 

 segment corresponding with the antepenultimate pair of thoracic legs, rarely 

 upon the coxjb of these legs themselves. 



The gills are almost always phyllobranchias : in the Dromiidea alone they 

 may sometimes be trichobranchia?, or of an intermediate character. 



Five main sections of the Brachyura are generally recognized, as is shown in 

 the following table : — 



I. The efferent brancliial channels run along the middle line of the endo- 

 stome, so that the buccal cavern or, at any rate, that part of it which 

 corresponds with the endopodites of the external maxillipeds, is 

 elongate-triangular, its apex traversing or invading the extremely 

 narrow epistome. The 1st abdominal somite of the female is without 

 appendages. No appendages of any sort to the 6th abdominal somite OxvaiOMA. 

 II. The efferent branchial channels run along either side of the endostome, 

 so that the buccal cavern is more or less quadrangular, never acutely 

 triangular : — 



1. The 1st or true basal joint of the antennal peduncle is a large, 

 exposed, and generally a freely-movable joint. Either 

 Unem anovuiriae are present on the carapace, or the append- 

 ages of the 6th abdominal somite are present as a pair of 

 plates intercalated between the 6th and 7th somites. The 

 1st abdominal somite of the female carries a pair of slender 

 uniramous appendages. The oviducts open on the coxo- 



