27 8 



THE CRUSTACEA 



branchiae, though it may carry a pleurobranchia and a setobranchia. 

 As a rule no gills are present on the first thoracic somite, but 

 in some Penaeidea, Stenopidea, Astacura, and Thalassinidea, a 

 minute arthrobranchia (1 pleurobranchia) is present, while in some 

 Parastacidae the epipodite bears some branchial filaments and is, in 

 fact, a rudimentary podobranchia. 



On the remaining somites the podobranchiae are the most 

 frequently suppressed. It is characteristic of the Scyllaridea and' 

 Astacura that they possess a full series of, podobranchiae, and less 

 complete series are found in the more primitive Penaeidea (Cerata- 

 spinae and Aristeinae), in the Eryonidea and some Thalassinidea, 

 and in the primitive Homolodromiidae among the Dromiacea. In 



b.c. 



C. 



FIG. 105. 



Branchiae of Decapoda. The lower figures show the complete branchiae, the upper tigures 

 transverse sections of the same. A, dendrobranchiate type (Peitaeus canaliculotus). IB, tricho- 

 branchiate type (Astcums fluviatilis). C, phyllobranchiate type (Palaemon tar), b.c, blood- 

 channels in axis of branchia. 



all other Decapods they are absent from the legs and, except in 

 Brachyura, from the third, though not uncommonly present on the 

 second, maxillipeds. Apart from the podobranchiae, the mastigo- 

 branchiae and setobranchiae may persist in a more or less complete 

 series, especially in the Caridea. The pleurobranchiae are stated 

 to extend forwards to the somite of the third maxilliped in some 

 Caridea, and to that of the second in the Penaeidae ; but it must 

 be noted that the distinction between pleuro- and arthrobranchiae 

 in the crowded anterior part of the branchial chamber is often 

 obscure. They form the chief part of the gill-system in the Caridea, 

 where five are usually present, and, on the other hand, they are 

 quite wanting in most of the Thalassinidea. In the Brachyura a 

 formula of nine branchiae on each side is found in all the main sub- 

 divisions ; but while it is practically universal in the Oxyrhyncha 



