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 
ameeboid cells resist infection 
and help to repair injuries (cf. 
possible function of thymus in 
Fishes). In 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 tadpoles 
have a similar function. 
Reproductive organs.— 
The male crayfish is distin- 
guished from the female by 
his slightly slimmer build, 
of crayfish.—After Huxley. and by the peculiar modi- 
4, Testes; vd., vas deferens; va’., open- fication of the first two pairs 
ing of vas deferens on last walking leg. 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 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, 
Fic. 145.—Male reproductive organs 
