TYPE PKOSOPYGIA. 



269 



hseinolyniph circulates. Upon the outer surface of each 

 inaiitle-lobe, and formed by it, is a valve of the shell, composed 

 of a certain amount of organic matter, but largely of carbonate 

 of lime. Since the mantle-lobes are dorsal and ventral in 

 position, so too are the valves of the shell, and consequently 

 their hinge-line is posterior and their mouth anterior. In 

 a number of forms, which may be grouped together as the 

 suborder Testicar dines, the shells along the hinge-line are 

 provided with interlocking teeth, a true hinge being present, 

 the peduncle in these cases perforating a backward prolonga- 

 tion or beak of the lower valve. In a few genera, however, 

 forming the suborder J^cardines, no such hinge is present, 



FIG. 119. DORSAL VALVE OF Spirifer, SHOWING ARM SKELETON (after LEUNIS). 



the peduncle passing out between the two valves of the shell. 

 Special muscles are present extending from one valve of the 

 shell to the other and are necessary both for the opening and 

 the closing of the shell, and furthermore it should be noted 

 that except for a slight difference in concavity both valves of 

 the shell are similar and symmetrical. 



It will be seen by comparing the various facts mentioned 

 here with what is said on p. 327 regarding the shell of the 

 bivalve Mollusks that the structures in the two groups are 

 very different. This difference is emphasized by the presence, 

 in the majority of the Testicardiues, of a calcareous support 

 for the coiled lophophore attached to the inner surface of the 

 dorsal valve (Fig. 119). It consists of a pair of calcareous 

 rods which project downwards and forwards, uniting to form 

 a transverse arch, and may give rise on each side to a spirally- 

 wound process upon which the coils of the lophophore rest. 



The body-wall is composed of an outer layer of ectoderm 



