ARTHROPODA. 95 



cuticle similar to that covering the outside of the body. Outside 

 the epithelium, connective tissue and muscular layers are present. 



Each digestive gland is made up of a very large number of 

 short caeca, lined by glandular epithelium. In each lobe these 

 open into a central duct, and the three central ducts unite to form 

 the main duct. 



The crayfish feeds upon various substances, vegetable and 

 animal, which are seized by the chelate appendages and torn 

 into small fragments. These are passed on to the third foot- 

 jaws, by which, and the other mouth-appendages, they are still 

 further reduced. All the jaws work from side to side, and not 

 up and down, as in backboned animals. Mastication is completed 

 by the gastric mill. The pyloric strainer prevents any but small 

 particles from passing on to the mid-gut, and parts incapable of 

 sufficient reduction are ejected from the mouth. During the 

 action of the gastric mill, probably, and certainly after reaching 

 the mid-gut, the food is acted upon by the secretion of the 

 digestive glands. This is an alkaline fluid, containing ferments 

 which convert starch into sugar, and proteids into peptones, also 

 emulsifying fats (i.e., converting them to a state of fine division). 



The ridged and papillated lining of the intestine affords a large 

 absorptive surface. Owing to the action of the muscle in the 

 wall of the alimentary canal, the food is gradually passed back- 

 wards, and the undigested parts are ejected from the anus. 



The digested parts of the food, which are either dissolved or 

 emulsified, diffuse out of the digestive organs into the blood- 

 system, by which they are distributed over the body. 



5. The Circulatory Organs consist of a blood-system only, 

 which can be divided into heart, arteries, capillaries, and more 

 or less definite channels and spaces (sinuses) in various parts of 

 the body. All these are in continuity, and they contain a nearly 

 colourless blood. In those invertebrates which, like Crustacea, 

 insects, &c., possess jointed lateral appendages and are known as 

 Arthropods, the coelomic system seems to have been almost 

 aborted as a result of exuberant development of the blood- 

 system. 



The blood is of a faint bluish tint, owing to the presence of 

 hcemocyanin, a complex compound of the nature of a proteid and 

 playing the same physiological part that haemoglobin does in red 

 blood. It contains, however, copper instead of iron. The blood 



