PHYLUM ARTHROPODA 507 



the cardiac division of the stomach the chitinous lining is 

 kened and calcified in certain parts, so as to form a complex 

 culated framework, the gastric mill, on which are borne a 

 median and two lateral teeth, strongly calcified and projecting 

 into the cavity of the stomach. Two pairs of strong muscles 

 arise from the carapace, and are inserted into the stomach : when 

 they contract they move the mill in such a way that the three 

 teeth meet in the middle and complete the comminution of the 

 food begun by the jaws. The separation of the teeth is effected 

 partly by the elasticity of the mill, partly by delicate muscles in 

 the walls of the stomach. The pyloric division of the stomach 

 forms a strainer: its walls are thickened and produced into 

 numerous setae, which extend quite across the narrow lumen and 

 prevent the passage of any but finely divided particles, into the 

 intestine. Thus the stomach .has no digestive function, but is 

 merely a masticating and straining apparatus. On each sid<T~oT 

 the cardiac division is found at certain seasons of the year a 

 plano-convex mass of calcareous matter, the gastrolith. 



The digestion of the food and to some extent the absorption of 

 the digested products are performed by a pair of large glands (lr.), 

 lying one on each side of the stomach and anterior end of the 

 intestine. They are formed of finger-like sacs or cceca, which 

 discharge into wide ducts opening into the small intestine, and 

 are lined with glandular epithelium derived from the endoderm 

 of the embryo. The glands are often called livers, but as the 

 yellow fluid they secrete digests proteids as well as fat, the name 

 hepato-pancreas is often applied to them, or they may be called 

 simply digestive glands. The Crayfish is carnivorous, its food con- 

 sisting largely of decaying animal matter. Microscopic glands 

 occur in the wall of the gullet. 



The digestive organs and other viscera are surrounded by a 

 'body-cavity, which is in free communication with the blood- 

 vessels and itself contains blood. As will be pointed out more 

 particularly hereafter, this cavity is to be looked upon as an 

 immense blood-sinus, and not as a true ccelome. 



There* are well-developed respiratory organs, in the form of 

 gilh l contained in a harrow branchial chamber, bounded internally 

 by the proper wall of the thorax (Fig. 404, ep) y externally by the 

 gill-cover or pleura! region of the carapace (I'd). Each gill con- 

 sists of a stem giving off niTiiit ^us branchial filaments, so that 

 the whole organ is plume-like. 'i'L filaments are hollow, and 

 communicate wit-h [;,\,, parallel canals In the stem an external, 

 the filial vein, and ah internal, the cj/'c/'cnt branchial 



rein. Th. gil- is to be considered as an out-pushing of the 

 body-will, and contains the same layers a thin layer of 

 chitin externally, then a single layer of epithelial cells, and 

 beneath this connective tissue, hollowed out for the blood 



ben< 



