3H THE CRAYFISH 



On each side of the small intestine is a large organ, the 

 digestive gland (D. Gl) : it consists of numberless glove- 

 finger-like processes or cozca which open by a short tube or 

 duct into the small intestine (B, D. Gl}. Both coeca 

 and duct are lined with epithelium derived from the endo- 

 derm, and the whole digestive gland is to be looked upon 

 as a branched lateral outgrowth of the enteron. The 

 secretion of digestive juice is performed exclusively by the 

 epithelium of the digestive glands. 



Between the enteric canal and the body-wall are a series 

 of spaces (jB.S) containing blood and having the general 

 relations of a coelome, but very probably only representing 

 a number of enlarged blood-spaces or sinuses. 



Respiration is performed by special organs, the gills 

 (B, Gill, see p. 313) developed in the thoracic region as out- 

 growths of the body-wall, and containing the same layers 

 (cuticle, epithelium, and connective tissue) as the latter. 

 They have a brush-like form and are protected by a fold of 

 the body-wall (Brstg). 



The blood-system is constructed on the same general 

 lines as that of Polygordius but is greatly modified. A 

 portion of the dorsal vessel is enlarged to form a muscular 

 dilatation, the heart (Ht\ and the rest of the vessels, now 

 called arteries (B, St. A), instead of forming by themselves 

 a closed system, ramify extensively over the body, their ulti- 

 mate branches opening into larger cavities or sinuses between 

 the muscles. One of these cavities the pericardial sinus 

 (Pcd. S) surrounds the heart. The heart, arteries, and 

 sinuses together form a closed system through which the 

 blood is propelled in a definite direction by the contractions 

 of the heart. 



Renal excretion is performed by a pair of glandular 

 bodies, the kidneys (A, K}, situated in the front part of the 



