182 THIRTY-FIFTH REPORT ON THE STATE MUSEUM. 
and expanding, not regularly and every portion at once, but 
in a peristaltic manner. In contracting and expanding the ventricle 
assumes different shapes, from nearly circular to elongate-oval. Con- 
tinuing through the middle of the ventricle can be seen a portion of 
the rectum ; at the anterior portion where the rectum enters, the ven- 
tricle is narrow, scarcely more than the diameter of the rectum. From 
the ventricle proceed two comparatively large tubes (PI. 10, fig. 1, a.ao., 
p-ao.) the anterior and posterior aorte. The anterior aorta (Pl. 10, 
fig. 1, a, ao.), leaves the ventricle immediately above the intestine, and 
enters the body in close contiguity to that organ. The posterior 
aorta passes below the rectum. The walls of the ventricle consist of 
two thin layers of epithelium, between which isa layer of muscular 
fibre, in which are delicate nerves proceeding from the posterior 
ganglia, 
The auricles consist of nearly transparent sacs, pyramidal in form, 
connecting at their apex with the ventricle. The bases are attached 
to the walls of the pericardial cavity, and are about equal in length to 
the ventricle. 
The pyramidal shape of the auricle is seen only when the ventricle 
is pushed to one side. Naturally the auricles lie close to the side of 
~ the ventricle, and when that organ is expanded are almost entirely 
concealed. The auricles contract, forcing the blood into the ventricle, 
when the ventricle contracts, the valve (PI. 9, fig. 3) connecting the 
auricles and ventricle closes, and the blood is forced through the 
aorte. 
THE GILLS. 
(Plates 7%, 10.) 
The gills are four in number, one outer and one inner gill on each 
side of the body. Each gill consists of two lamine, united along their 
ventral edge, separated along their dorsal edge. The dorsal edge is es- 
sentially straight, the anterior, ventral, and posterior edges follow the 
curvature of the margin of the shell. The length of the gill is a little 
more than two-thirds the length of the shell; the width bears about 
the same proportion to the height of the shell, the length of the gill 
being a little less than three times the width. The outer gills are 
slightly larger and capable of greater extension than the inner gills. 
The anterior portion of the gills is situated at a distance, equal to one- 
fourth the length of the shell, from the anterior margin. 
Their manner of attachment is as follows. The outer lamine of the 
outer gills are attached to the interior of the mantle a short distance 
from the dorsal portion, just below the pericardial space. They con- 
tinue attached until within a short distance of the posterior extremity. 
