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 aortae. 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 is a 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 

 aortae. 



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 laminae, 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 laminae 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. 



