xxvii GILLS 335 



The digestion of the food, and to some extent the absorp- 

 tion of the digested products, are performed by a pair of 

 large glands (Fig. 83, D. Gl ; Fig. 86, /r), lying one on each 

 side of the stomach and anterior end of the intestine. They 

 are formed of finger-like sacs or ccBca, 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 car- 

 nivorous, its food consisting largely of decaying animal 

 matter. 



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. This cavity is not 

 fined by epithelium, and 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 gil/s (Fig. 83, b), contained in a narrow branchial 

 chamber, bounded internally by the proper wall of the 

 thorax, externally by the gill-cover or pleural region of the 

 carapace. Each gill consists of a stem giving off numerous 

 branchial filaments, so that the whole organ is plume-like. 

 The filaments are hollow and communicate with two parallel 

 canals in the stem — an external, the afferent branchial vein, 

 and an internal, the efferent branchial vein. The gill is to 

 be considered as an out-pushing of the body-wall, and con- 

 tains 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 channels. 



According to their point of origin the gills are divisible 

 into three sets — first, podobranchice or foot-gills (Fig. 87, a. 



