220 PHOLADIDJE. 



a pulp, which under the microscope has the aspect of ligneous 

 debris. The Patellae operate in like manner. 



The circulation is venous, arterial and branchial, and con- 

 sequently complete. The respiratory apparatus has been 

 strangely misunderstood ; it has been described to consist of 

 four fleshy cords, portions of which Sir Everard Home pro- 

 nounced to be the testes, and others the ovaria ; these views 

 are erroneous. But we will first mention the heart and auri- 

 cles, which are placed at the base of the ovarium in the peri- 

 toneal cavity within the mantle, but in a distinct pericardium ; 

 the heart is an elongated, very pale bluish-white opake ven- 

 tricle, accompanied by two symmetrical fusiform slender auri- 

 cles that are also opake, somewhat posterior to it, which 

 appear to pour the aerated blood into it by lateral valvular 

 ducts. On opening the ventricle its walls did not exhibit any 

 particular muscularity : we were not successful in detecting 

 the valves of the auricles. There is at the posterior part of 

 the auricles a white, suboval, subglobular, fine granular mass, 

 touching and partly surrounding them; we are unable to 

 state its nature ; it is not part of the ovarium, which termi- 

 nates before the pericardium commences, and in such a situa- 

 tion it cannot be the organ to animalize the ova : we are in- 

 clined to consider it a gland that distils a liquor for the use 

 of the heart and auricles. 



At the base of these organs the four cords that have created 

 such difference of opinion as to their uses come into view, but 

 they do not appear to be either the branchise, arteries, veins, 

 testes or ovaria ; still they have a sort of connection with the 

 branchise ; the two longer and larger brown lines have their 

 origin on each side the hemispherical valves, and proceed, 

 attached to each latero-dorsal range of the mantle, to the 

 posterior siphons ; they appear to be composed of red-brown 

 granular points ; within these two lines, but not until the 

 branchise commence, two others of smaller size and nearly 

 similar composition run parallel, and terminate with the larger 

 ones at the siphons; the addition of the two shorter and 

 smaller cords springing from the larger at the point where 

 the branchiae begin, appears to show a connection of these 



