xi PHYLUM ARTHROPODA 549 



and in the hind-gut, the epithelium secretes a layer of chitin, 

 which thus constitutes the innermost lining of those cavities. It 

 is proved by development that the mid-gut, which has no 

 chitinous lining, is the only part of the enteric canal developed 

 from the mesenteron : the gullet and gizzard arise from the 

 stomodaeum, the hind-gut from the proctodseum. Thus a very 

 small portion of the enteric epithelium is endodermal. 



In the anterior division of the gizzard the chitinous lining is 

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

 articulated framework, the " gastric mill" on which are borne a 

 median and two lateral teeth, strongly calcified and projecting 

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

 arise from the carapace, and are inserted into the gizzard : 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 gizzard. The posterior division of the gizzard 

 forms a strainer: its walls are thickened and produced into 

 numerous seta3, which extend quite across the narrow lumen and 

 prevent the passage of any but finely divided particles into the 

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

 merely a masticating and straining apparatus. On each side of 

 the anterior 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.) t 

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

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

 discharge into wide ducts opening into the mid-gut, 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 

 hqmto-pancrcas 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 coelome. 



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

 gills, contained in a narrow branchial chamber, bounded internally 

 by the proper wall of the thorax (Fig. 438, cp), externally by the 

 gill-cover or pleural region of the carapace (kd). Each gill con- 

 sists of a stem giving off numerous branchial filaments, so that 



