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 shell. 
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 pallial 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 p, 
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 
lamellae. The outer lamella of the inner gill is attached to the 
inner lamella of the outer gill on the same side, the outer 
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 and 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 lamella and seen obliquely from the cut surface so that the 
