64) THE PHYSIOLOGY OF THE COMMON CRAYFISH. 



them ; but what has been made out is very iiiterestmg, 

 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 indirectl}^, 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, Ir) 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 alwa3"s been re- 

 garded as the livei', 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 C(sca, which are closed at one end, but open at 

 the other into a general conduit, which is teimed their 

 duct. The mass of the liver is roughly divided into 



