INSTITUTE OF SCIENCE, PHILADELPHIA. 507 



where it lies below it. In Tridacna, owing to the abortion of the anterior and 

 the displacement of the posterior adductor, together with a rotation which has 

 nearly reversed the animal relatively to the valves when compared with ox<^\- 

 \\&XY Teleodcsmacea,\he. heart has come to occupy a position which, though 

 morphologically the same, is actually almost reversed with respect to the urn- 

 bones. In Teredo, Perna, Avicida and some species of Ostrca, the rectum is 

 dorsal to the ventricle. In PIioLadidea the latter is traversed by the rectum in 

 the ordinary manner. In Aiionua the heart is not enclosed in a pericardium, 

 but lies free in the pallial cavity at some distance from the gut. In Eiiaroa 

 the ventricle is pyriform, free and dorsal to the gut, but in the allied Halicardia 

 it is of the normal type, and embraces the rectum. 



The fluctuation in structural relations of the Pelecypod heart are such as 

 to give it only a slight value for systematic purposes, though its archaic char- 

 acter, in Nnciila and some Arcacca, accords well enough with the evidence 

 from other sources of their letention of some prototypic features. 



The Nephridia, or Organs of Bojanus. — While so far not available 

 for classificatory purposes, mention may be made here of the nephridia, 

 which serve as execretory organs independent of the alimentary canal. These 

 are two symmetrically situated organs, lateral to the pericardium, with which 

 they communicate by ciliated canals. Externally they open near and behind 

 the genital openings, close to the cerebrovisceral commissure, or the genital 

 glands may discharge into the channel which serves to carry off" the nephridial 

 secretions. Primitively the oviducts opened directly into the cavity of the 

 nephridia, but at present this arrangement is rare. The cavities of the two 

 nephridia (except in Niiada, etc.) are frequently united, either by a special 

 channel connecting the two, or the common cavity may have no division 

 during a considerable part of its extent, as in many Anatinacea. The cavity 

 is usually much ramified, divided by bands of connective tissue covered with 

 glandular epithelium, which in many Anomalodesmacea secretes not only 

 liquid excreta, but concentric limy concretions which might easily be taken 

 for eggs on a casual inspection. The ramifications of the nephridia extend 

 frequently outside of the visceral mass, between the walls of the mantle. 

 These in the Lyonsiellidce are produced inward as a free lamina into the 

 peripedal cavity, and form a large part of the septum to which the gills are 

 attached in some species. The nephridia are most simple in the most archaic 

 types, such as NuciUa and Solemya. Most of the blood from the lacunary 

 sinuses passes through the nephridia on its way to the gills, and there can be 

 no doubt that the glands perform a renal function. The more archaic the 

 type, the more simple the form of the nephridiiim, which gradually approaches 

 the form in which the same organ appears in some Gastropods. 



The Foot. — 111 a large majority of the bivalves the foot has the familiar 

 hatchet-shape from which the class name is derived, but as an organ of loco- 

 motion, tactile use, and possibly prehension, it is modified for special uses in 



