128 COMPARATIVE ANATOMY 



such passing forward from the cardiac ossicle to be attached 

 to the carapace near the base of the rostrum, another pair 

 passing back from the pyloric ossicle to be attached to the 

 roof of the carapace behind the proventriculus. By the 

 alternate contraction and relaxation of these muscles, the 

 gastric mill is set in motion, and the food contained in the 

 proventriculus is reduced to the necessary state of fineness. 

 At times two peculiar calcareous nodules are found in the 

 lateral walls of the cardiac division of the proventriculus. 

 They are known as gastroliths, or, popularly, as "crab's-eyes " : 

 nothing is really known about their function, though they are 

 supposed to form a reserve of calcareous matter to supply 

 material for the new armour formed after ecdysis. In pre- 

 scientific days they were credited with mysterious curative 

 properties, and were used as sovereign remedies in all sorts 

 of diseases. ^ 



The passage from the cardiac into the pyloric division 

 of the proventriculus is guarded by a large valvular process 

 rising from the floor of the passage between them, and there 

 is a further arrangement of setose ridges projecting into the 

 cavity of the pyloric division, which effectually prevents any 

 but the most finely comminuted food from passing into the 

 mid-gut. The mid-gut, though extremely short, represents 

 the stomach and the greater part of the intestine of Apus. It 

 consists of a short segment of the gut, the internal surface of 

 which is devoid of a cuticle, and is produced dorsally into a 

 pouched process, the caecum. Right and left it receives the 

 wide ducts of the digestive glands. The mid-gut is succeeded 

 by the intestine or hind-gut, a straight tube running back to 

 the anus. It is lined by a chitinous cuticle raised into six 

 prominent longitudinal ridges, covered by small papillae, and 

 this character, as well as its development, shows it to be an 

 invagination of the external ectoderm in other words, a 

 proctodaeum. The digestive glands, more commonly called 

 the " liver," are large lobed, yellow masses lying on either side 

 of the gut in the cephalothorax. Each moiety of the gland is 

 divisible into three lobes, an anterior, a posterior, and a dorsal, 

 and each lobe is composed of a number of short tubules 

 or caeca, which are really digitiform outgrowths of the main 

 duct forming the axis of the lobe. The ducts of the three 

 lobes unite to form the wide hepatic duct opening into the 



