CRA YFISH. 



277 



connection with the second maxillipede there is a single arthrobranch ; 

 in connection with each of the five following appendages there are two ; 

 so that there are eleven arthrobranchs altogether. There remain three 

 pleurobranchs, one on the epimeron of the fifth large limb, and two 

 others quite rudimentary on the two preceding segments. The bases 

 of the podobranchs bear long setae. 



In Nephrops, the podobranchs are represented by a small rudiment 

 on the second maxillipede, and by five well-developed gills on the next 

 five appendages ; there are eleven arthrobranchs, the most anterior 

 being small ; and there are four large pleurobranchs. 



Excretory system. A kidney or "green gland" lies 

 behind the base of each antenna, and its opening is marked 

 by a conspicuous knob on the basal joint of that appendage. 

 Each kidney consists of a dorsal sac communicating with 

 the exterior, and of a ventral coiled tube which forms the 

 proper renal organ. The 

 latter is supplied with blood 

 from the antennary and 

 abdominal arteries, and 

 forms as waste products 



urc acid and greenish 

 guanin. 



Each kidney may 

 be regarded as homologous 

 with a nephridium. 



The crayfish has also, near the 

 gills, small branchial glands which 

 excrete carcinuric acid from the 

 blood, and also help in phago- 

 cytosis, that important process in 

 which wandering amoeboid cells 

 resist infection and help to repair 

 injuries (cf. possible function of 

 thymus in Fishes). 



Reproductive organs. 



The male crayfish is distin- 

 guished from the female by 

 his slightly slimmer build, 

 and by the peculiar modi- 

 fication of the first two pairs 



of abdominal appendages. In both sexes the gonads are 

 three-lobed, and communicate with the exterior by paired 

 ducts. 



The testes consist of two anterior lobes lying beneath and 



FIG. 128. Male reproductive organs 

 of crayfish. After Huxley. 



f., Testes; I'd. , vas deferens ; rd'., open- 

 ing of vas deferens on last walking 

 leg. 



