332 



PHYLUM ARTHROPODA 



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 hmb, and two 

 others quite rudimentary on the two preceding segments. The bases 

 of the podobranchs bear long setas. 



In Nepkrops, 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 open- 

 ing is marked by a con- 

 spicuous 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 sup- 

 plied with blood from the 

 antennary and abdominal 

 arteries, and forms as 

 waste products uric acid 

 Fig. 176.— Male reproductive organs ^nd greenish guanin. Each 

 of crayfish.-After Huxley. j^-^^^^y ^^^ ^^ regarded 



^, Testes; v.d., v^s deferens; i;.^'., open- Vinmnlnoriim with a 



ing of vas deferens on last walking leg. aS nomOlOgOUS WUn a 



nephridium. 



The crayfish has also, near the gills, small branchial glands which 

 excrete carcinuric acid from the blood, and also help in phagocytosis, 

 that important process in which wandering amceboid cells resist 

 infection and help to repair injuries (cf. possible function of thymus 

 in Fishes). In not a few invertebrates there are scattered groups of 

 excretory cells or nephrocytes, and it seems that the endothelial cells 

 of the lymphatic vessels and renal capillaries in tadpoles have a similar 

 function. 



Reproductive organs. ^ — The male crayfish is distin- 

 guished from the female by his slightly slimmer build, 

 and by the peculiar modification of the first two pairs 

 of abdominal appendages. In both sexes the gonads 



