DECAPODA. 519 



Sergestes 12, but the deep-sea form Benthesicymus has 24 

 gills.* 



There are two main forms of branchiae in the Decapods, 

 phyllobranchiae, consisting of simple leaflike lobes set on either 

 side of a main axis, and trichobranchiae in which linear offsets 

 are arranged around a central stem. A third type, the dendro- 

 branchiate, may be regarded as a modification of the phyllo- 

 branchiate by the sub-division of the lateral lobes of the gill. 

 It is met with, under somewhat different forms, in the Penaeidea. 

 The Caridea and Brachyura have phyllobranchiae, the majority 

 of the Macrura trichobranchiae, but in the Paguridea and 

 Thalassinidea both kinds are found. 



The branchiae are usually completely enclosed between the 

 lat?ral fold of the carapace (branchiostegite) and the body wall, 

 though their bases 

 are often exposed in 

 some Caridea. The 

 water about them is 

 renewed by the 

 movements of the 

 scoop-shaped exopo- 

 dite of the second 

 maxilla, known as 



the SGCL'pllOfrntttflite FIG. 318. -Cephalothoraxof Aslacus fluviatilis, after removal 



of the branchiostegite. K podobranchiae ; Mp scapho- 



wllicll lies at the an- guatlute of second maxilla; Mxf" third maxilliped ; 



stalked eye ; R rostrum. (From C'laus, after Huxley.) 



terior end of the 



branchial chamber and drives out the water which in the Macrura 

 enters behind and below the branchiostegite (Fig. 318). In the 

 Crabs the water usually enters the branchial chamber by an 

 aperture situated behind the pterygostomial region, and in front 

 of the bases of the chelipeds. The epipodite of the third maxilli- 

 ped is dilated at its base into an oval plate, which regulates 

 the flow of water into the chamber through this aperture. The 

 sides of the carapace and the bases of the limbs in the neigh- 

 bourhood of the inhalent aperture are, in eand-haunting crabs, 

 plentifully clothed with hairs, which form, when the legs are in the 



* It is customary to sum up the number of gills on one side of the body in a 

 " branchial formula" thus: Pandalus 12 + 7 ep ; Penaeus 17 ( +r) + 5 ep. 

 ep stands for epipodite or flagellum in the sense defined on p. 518, and r 

 for a vestigial branchia. 



