292 



PHYLUM ARTHROPODA. 



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 uric 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 phagocytosis, that important 

 process in which wandering 

 amoeboid cells resist infection 

 and help to repair injuries (cf. 

 possible function of thymus in 

 Fishes). Jhi not a few inverte- 

 brates there are scattered groups 

 of excretory cells or nephrocytes, 

 and it seems that the endothelial 

 cells of the lymphatic vessels 

 and renal capillaries in tadpole. c 

 have a similar function. 



Reproductive organs. 



The male crayfish is distin- 

 guished from the female by 

 his slightly slimmer build, 

 f d . b y the peculiar modi- 

 , , , , fication of the first two pairs 



; va., vas deferens ; va., open- r i j i j 



ng o vas deferens on last walking leg. OI 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 in front of the heart, and of a median lobe extending 

 backwards. Each lobe consists of many tubules, within 

 which the -spermatozoa develop. From the junction of 

 each of the anterior lobes with the median lobe, a genital 

 duct or vas deferens is given off. This has a long coiled 

 course, is in part glandular, and ends in a short muscular 

 portion opening on the last thoracic limb. The spermatozoa 

 are at first disc-like cells ; they give off on all sides long 

 pointed processes like those of a Heliozoon, and remain 

 very sluggish. The seminal fluid is milky in appearance, 



