64 THE PHYSIOLOGY OF THE COMMON CRAYFISH. 
them; but what has been made out is very interesting, 
and proves that considerable differences exist between 
crayfishes and the higher animals in this respect. 
The physiologist calls those organs, the function of 
which is to prepare and discharge substances of a special 
character, glands; and the matter which they elaborate 
is termed their secretion. On the one side, glands are 
in relation with the blood, whence they derive the 
materials which they convert into the substances 
characteristic of their secretion; on the other side, 
they have access, directly or indirectly, to a free surface, 
on to which they pour their secretion as it is formed. 
Of such glands, the alimentary canal of the crayfish 
is provided with a pair, which are not only of very large 
size, but are further extremely conspicuous, on account 
of their yellow or brown colour. These two glands (figs. 
12 and 13, lr) are situated beneath, and on each side of, the 
stomach and the anterior part of the intestine, and answer 
in position to the glands termed liver and pancreas in the 
higher animals, inasmuch as they pour their secretion into 
the mid-gut. These glands have hitherto always been re 
garded as the liver, and the name may be retained, though 
their secretion appears rather to correspond with the 
pancreatic fluid than with the bile of the higher animals. 
Each liver consists of an immense number of short 
tubes, or ceca, which are closed at one end, but open at 
the other into a general conduit, which is termed their 
duct. The mass of the liver is roughly divided into 
