IOWA ACADEMY OP SCIENCES. 175 



The adductors, Figs. 1 and 3, are two in number, anterior, 

 aa, and posterior, pa. They differ slightly in size and shape, 

 and have for their only function the closing of the thell. 



There are two pairs of retractors, anterior and posterior 

 retractor pedis muscles. Figs. 1 and 3, arp and prp. They 

 serve to withdraw, or retract, the foot from an extended 

 position. 



The foot is largely made up of crossing muscle fibers, 

 extending more or less in all directions, but capable of being 

 classed as longitudinal, vertical and horizontal. They aid in 

 protrusion, by forcing the blood where most efficient, in 

 retraction and in special movements of the protruded foot. 



The pallia! muscles. Figs. 4 and 5, are distributed to the 

 inner end of the epidermal gland in the edge of the mantle and 

 to the ridge already described. They serve to withdraw the 

 edge of the mantle from between the edges of the valves when 

 the valves are tightly closed. 



BYSSAL GLAND. 



A rudiment of the byssal gland, Fig. 1, &, persists in the 

 adult animal as a single closed sack, often showing a slight 

 sagittal constriction. It is supplied with a small nerve on each 

 side, which spring from trunks that have their origin in the 

 pedal ganglia. Most of the specimens which I have examined 

 have the rudiment of the byssal gland nearer the pedal ganglia 

 than is shown in Fig. 1. 



GILLS. 



The gills, four in number, consist of a pair, an outer and an 

 inner gill, on each side of the body. The outer, Fig. 3, o g, 

 is much smaller than the inner, i g, and falls short anteriorly 

 by about a fourth of its length. Each gill is composed of two 

 lamellee. The outer lamella of the inner gill is attached to the 

 inner lamella of the outer gill on the same side, the cuter 

 lamellae of the outer gills are attached to the mantle lobes on 

 their respective sides, and the inner lamellae of the inner gills 

 are attached anteriorly to the body wall aud posteriorly to 

 each other. Fig. 5. The gills function as respiratory organs, 

 procurers of food and brood pouches. The latter function is 

 monopolized by the inner gills, which carry the embryos until 

 they are ready to function as adults. 



Fig. 6, which represents a piece of gill cut squarely across 

 the lamellse and seen obliquely from the cut surface so that the 



