6 PROCEEDINGS OF THE NATIONAL MUSEUM vol.75 



cords which pass to the cerebral gangha. The two outer ones pass 

 parallel for a short distance with the commissural cords and then 

 turning abruptly backward continue along the junction of the gills 

 supplying them with its numerous branches. At the posterior por- 

 tion a gigantic nerve arises which separates into three large branches. 

 The central branch supplies the inhalent siphon, the inner or mesial 

 branch the posterior portion of the mantle, and the outer branch the 

 exhalent siphon. 



The nerves emanating from the ganglion often vary with the 

 individual mussel. A certain nerve may be entirely lacking or 

 exceedingly large or very delicate. The most constant arrangement 

 however, is the one given above and shows no variation for different 

 species. ' 



EXCRETORY SYSTEM 



Plate 1, fig. 9 

 The excretory system for all five species exhibits no peculiarities. 



CIRCULATORY SYSTEM 



Plate 1, fig. 10 



The circulatory system of L. ochraceus and of the other species 

 included in this study shows no variation. From the ventricle pro- 

 ceed two aortae, one passing anteriorly and above the rectum, though 

 almost contiguous with it; the other passing posteriorly enters the 

 visceral mass below the rectum. They are the anterior and posterior 

 aortae from which numerous arteries arise. The anterior aorta, on 

 entering the visceral mass, curves outward and slightly to the right, 

 then passes downward and a little posteriorly gives off a large artery, 

 which passes into the mantle. The main branch continues its down- 

 ward anterior course and near the center of the visceral mass divides 

 into three large arteries which pass in and out among the folds of the 

 intestine. Just before the curving main branch begins its downward 

 course a large artery is given off which curves downward back of 

 the anterior adductor muscle and curves posteriorly, to supply, with 

 its numerous veins, the ventral portion of the body mass. From 

 this artery, a little before it reaches the anterior adductor muscle, 

 ascends a branch which passes into the muscle, supplies the mouth 

 and anterior portions of the body. 



The posterior aorta bifurcates just before reaching the posterior 

 adductor muscle, sending one branch to the muscle and to the region 

 of the pericardium and the rectum. The other branch passes over 

 the muscle and enters the mantle, continuing ventrally to meet the 

 anterior mantle artery. The blood supply of the mantle is continu- 

 ally oxygenated by the current of water which bathes the inner face 

 of the mantle. It is, therefore, returned directly to the auricles 

 without passing through the gills. 





