340 



ZOOLOGY 



SECT. 



The dorsal valve may be taken to represent a lever of which the 

 hinge-line is the fulcrum, the cardinal process the short arm, and 

 the main portion of the valve the long arm. The muscles all arise 

 from the ventral valve, the adductors being inserted into the inner 

 face of the dorsal valve, which they depress, the divaricators into 

 the cardinal process, their action depressing it and thus elevating 

 the valve itself. In Lingula there is a very complex muscular 

 system by means of which the valves can be rubbed upon one another, 

 or moved laterally as well as opened and shut. 



In the Articulata the enteric canal is V-shaped, as in Magel- 

 lania, the intestine being straight or nearly so, and ending blindly. 



FIG. 271. Dissections of A, Cistella; B, Rhynchonella ; and C, Lingula. a. anus; Iph. 

 lophophore ; mtk. mouth. (After Schulgin and Hancock.) 



In the Inarticulata, on the other hand, the intestine is usually 

 coiled, and always ends in an anus (Fig. 271, C, a), which generally 

 opens into the mantle-cavity, but in one genus (Crania) into a 

 pouch or sinus at the posterior end of the body between the 

 valves. 



A heart is usually present, but the function of blood is per- 

 formed mainly by the coelomic fluid, which is propelled by the 

 cilia lining that cavity and circulates both in the coelome itself and 

 in the pallial sinuses, each sinus presenting in Lingula at least 

 both an outgoing and an ingoing current. 



A single pair of nephridia, resembling those of Magellania, 



