318 THE CRAYFISH LESS. 



duct into the small intestine (B, D. Gl). Both caeca 

 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 (>$} containing blood and having the general 

 relations of a ccelome, but very probably only representing 

 a number of enlarged blood-spaces or sinuses. 



Respiration is performed by special organs, the gills 

 (B, Gift, see p. 317), 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 those of Polygordius, but is greatly modified. A 

 portion of the dorsal vessel is enlarged to form a muscular 

 dilatation, the heart (/fr), 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 

 head and enclosed in spacious sacs which open by ducts on 

 the bases of the antennae. They consist of convoluted tubes 

 lined by epithelium, and are probably to be looked upon as 

 greatly modified nephridia. 



