64 CRUSTACEANS AND RELATED FORMS 
it has gained in its course through the body. The method 
of purification of the blood in these glands is much the 
same as in the kidneys of the higher animals. 
48. Circulatory System. — The crayfish has a well-devel¬ 
oped heart from which extend several arteries that carry 
blood to the various 
parts of the body. The 
blood returns to the 
heart through veins and 
through several irregular 
ducts called sinuses (si'- 
nus-es). As the blood 
flows through the body 
it loses oxygen and re¬ 
ceives carbon dioxide. 
Fresh oxygen is absorbed 
by means of the gills, 
which, at the same time, pass off carbon dioxide from the 
blood into the water. 
49. The Nervous System. — In the crayfish this is made 
up of a brain, ventral nerve chain, and many nerves. The 
eyes are borne on a pair of short movable 
stalks. The special senses are well devel¬ 
oped, and the sense of taste is keener than 
that of most lower animals. 
50. Life History. — The sexes are dis¬ 
tinct. The males may be distinguished 
from the females by the larger tubular 
appendages on the first and second seg¬ 
ments of the abdomen. The eggs of the 
female are carried for some time by the 
appendages of the abdomen, where they 
pass through their early stages of de- 
velopment. The young crayfish is un- common under 
like the adult in form, and approaches boards and sticks. 
Figure 52. — Soft-shell Crab. 
This is the common form sold in the 
markets. 
