THE DECAPODA 277 



Astacidae, however, the axis of the gill coalesces with the mastigo- 

 branchia, which has the form of a folded membranous lamina from 

 which the branchial filaments spring directly. In the Parastacidae 

 this lamina is greatly reduced or disappears. 



In the Caridea, the mastigobranchiae, when present, have usually 

 the form of short curved rods, directed backwards, each ending in 

 a hooked process which grasps a tuft of long slender setae on the 

 coxopodite of the next succeeding appendage. This tuft of setae, 

 which is also present in some Reptantia, springs from a small 

 papilla which Coutiere has compared with the setiferous epipodial 

 process found in Gnathophausia (Mysidacea) (Fig. 106, ep, p. 1 76), and 

 which he regards as a distinct element of the branchial system 

 (setobrandiia of Borradaile). In a species of Ekonaxius (Thalas- 

 sinidea) Coutiere has found that the coxopodite of the first leg 

 bears two podobranchiae, one attached as usual to the base of the 

 mastigobranchia, the other close to, if not actually inserted on, the 

 setobranchia. In no other Decapod is more than one podobranchia 

 found on any limb. 



As regards their structure, each branchia consists of a stem or 

 axis which is attached at or near one end and bears numerous 

 lateral branches. According to the form and arrangement of these 

 latter, three main types of gills have been distinguished, which, 

 however, are connected by intermediate forms. In the tricho- 

 branchiate type (Fig. 165, B) the branches are filamentous, and are 

 arranged in several series around the axis. In the phyllobranchiate 

 type (Fig. 165, C) the branches are flattened laminae, and as a 

 rule only two opposite series are present. The dendrobranchiate 

 type (Fig. 165, A) is characterised by the fact that the biserial 

 primary branches are themselves ramified, sometimes in a very 

 complex fashion. The dendrobranchiate type is peculiar to the 

 Penaeidea, but each of the other two types recurs in widely 

 separated groups. Thus the Caridea have phyllobranchiae, as 

 have also all the Brachyura, with the exception of some of the 

 primitive Dromiacea, which have trichobranchiae. The Stenopidea, 

 Palinura, and Astacura have trichobranchiae. Among the Anomura, 

 phyllobranchiae are the rule, but Aeglea among the Galatheidea, 

 and the Pylochelidae, with several genera of Paguridae among the 

 Paguridea, have trichobranchiae, and the gills of some Thalassinidea 

 are intermediate in character. 



In the number and arrangement of the gills very great differ- 

 ences exist, which afford valuable systematic characters. At the 

 same time, the important divergences sometimes presented by 

 closely allied forms render it necessary to use caution in estimating 

 the value of these characters (compare, e.g., Caridina and Limnocari- 

 dina, or Pandalus and Pandalina). The last thoracic somite is 

 invariably destitute of mastigobranchia, podobranchia, or arthro- 



