174 



NORTH AMERICAN INDEX FOSSILS. 



the brachial valve as weight. The valves close by the contraction 

 of two muscles (adductors) passing from the brachial to the pedicle 

 valve. A pair of muscles extending from the brachial valve and an- 

 other pair from the pedicle valve with insertion on the pedicle, 

 enable the shell as a whole to move in many different directions. 

 In the Inarticulata the opening and closing of the valves is effected 

 by a more complicated set of muscles. (See Lingida, Fig. 221.) 



The body of the animal, lying in the pos- 

 terior portion of the shell, occupies only 

 about one third of the interior. The body 

 wall gives - off two folds or mantles, one 

 fitting closely to and building the pedicle 

 valve, the other building the brachial 

 valve. Most of the space between the 

 mantles is filled with the tentacle-bearing 

 lophophore (Fig. 220, 1); this is supported 

 by the brachidium. Those portions of 

 the lophophore which diverge arm-like 

 from the two sides of the mouth are called 

 the brachia. Through this lophophore and 

 tentacles as well as through the mantle 

 the animal gets its necessary oxygen 

 There is also a cilia-lined groove, which is 

 bounded on the outside by the line of ten- 

 tacles and on the inner side by a wavy 

 ridge, and which extends from the mouth 

 along each arm of the lophophore. 

 Through this, microscopic food particles 

 (diatoms, infusorians, etc.) are swept into 

 the mouth by the action of the cilia. The 

 mouth, a mere slit, is in the middle of the 

 lophophore; it leads into a stomach through an cesophagiis. The 

 stomach leads into an intestine \ which often ends as a blind sac. 



The body cavity or coelome extends into the lophophore and also 

 sends off four canals (pallial sinuses) into each mantle ; the outer 

 ones are much branched. Impressions of these pallial sinuses are 

 often found on the inside of the shell and on internal molds (Fig. 

 221, s). 



The function of the blood is performed mainly by the fluid fill- 

 ing the coelome and moved by the cilia lining it. A contractile 

 heart and some vessels leading from it have been observed. 



Fig. 221. Lingula elderi, 

 outline of interior of dorsal 

 valve showing muscular and 

 vascular markings ; d, di- 

 varicator muscular scars ; ad, 

 adjustor muscular scars ; pa, 

 posterior adductor scars ; aa, 

 anterior adductor scars ; xx, 

 track of advance of the mus- 

 cular scars ; s, great pallial 

 sinuses ; ps, posterior course 

 of the latter ; o, inner rami- 

 fications of the sinuses. (Af- 

 ter Whitfield.) 



