20 



The number of gill-plumes in either brancliial chamber varies from 20 to 6, 

 but the typical number is 9, arranged as follows : — 



The Branchial Formula of Scylla serrata. 



Total 2+ Sep. 3 2 2= 9 + 3ep. 



In the typical crabs the gills are phyllobranchise ; that is to say, the gill- 

 elements are broad thin leaves arranged in two series along a central stem, like 

 the barbs of a feather-vane. There are however, among the Bromides, to be 

 found trichobranchise and gills of an intermediate character. 



In certain land-crabs although functional gills are present, aerial respiration 

 is in part carried on by the remarkably thick and vascular lining-membrane of 

 the branchial chambers. 



4. Of the Vinceral Systems. 



Except for the better definition of the buccal cavern and for the valviform 

 modification of the external maxillipeds, the Digestice System, of the Brachyura 

 is not essentially different from that of Nephrops. 



Crabs feed in a very human fashion. Holding the food in one of the large chela% they tear 

 ofi pieces with the other and carry the morsels to the mouth. 



The Circulatory and Renal Excretory Systems do not differ essentially from 

 those of Nephrops. Nor does the Central Nervous System, except that, in cor- 

 respondence with the shortening-up of the body, the ganglia of the ventral chain 

 are all united into a central mass or ring from which the nerve-cords radiate. 



In the Bromides, however, this concentration of the ventral ganglia is not 

 quite complete. 



5. Of Beiyrudnction, among Brachyura. 



The reproductive organs resemble those of Nephrops. In the male their 

 ducts usually perforate the coxopodite of the last pair of thoracic legs, but 



