Chap. XII.] THE CRAYFISH. 245 



short section of the canal has naked epithelial walls. This is the 

 mid-gut (76, C.,M. G.). Near its commencement there is on each 

 side the wide opening of the duct (h. d.) of the digestive gland, 

 and dorsally a short caecum (cce.). 



Behind the mid-gut there are six squarish elevations with a 

 chitinous investment. From them longitudinal ridges pass back- 

 wards, with a slight spiral twist, to the anus. The ridges are 

 beset with small papillae. All this part of the intestine is known 

 as the hind-gut, and is lined throughout with a chitinous mem- 

 brane continuous with the cuticle of the exterior. 



The digestive gland is made up of an immense number of 

 caeca, It occupies a large space on either side of the mid-gut, 

 each lateral mass being slightly three-lobed. Each lobe ha*s a 

 main duct, the three uniting to form the common duct of that 

 side. The organ, as a whole, is to be regarded as a much-divided 

 diverticulum of the mid-gut. 



Its product is a yellow fluid, which has a slightly acid reaction, 

 converts proteids into peptones, acts on starch, converting it into 

 sugar, and breaks up olive oil into an emulsion. The producing 

 cells are of two kinds; (1) hepatic cells containing oil-globules, 

 and giving rise to animal colouring matter; and (2) ferment 

 cells giving rise to the digestive secretion. It has been proposed 

 to call this digestive gland the hepato- pancreas. The student 

 must not regard it as homologous with these glands in the verte- 

 brata, but merely as partly performing the same functions. 



On each side of the stomach (at g., Fig. 76, C.), there is formed 

 during the summer months a calcareous body called the gastro- 

 lith. During ecdysis these are shed into the interior of the 

 stomach, and there broken up and dissolved. If they are not 

 present, ecdysis would seem to be unsatisfactorily performed. It 

 is said that during exuviation a layer of processes, or " casting 

 hairs," is formed in such a way as to aid in loosening the old 

 shell from the new. 



The Heart and Circulation. In a recently-killed crayfish the 

 systole and diastole (contraction and relaxation) of the heart 

 may be seen. This organ has the form shown in Fig. 75, k., 



