274 GILBERT 0. BOURNE. 



the metabolic condition of the animal. In some of my series 

 of sections the concretions are small and are contained in 

 oval cellsj as shown in fig. 16, a. These cells are almost 

 identical with the pericardial cells of Venus verrucosa 

 and Cardium edule figured by Grobben (loc. cit., figs. 53 

 and 54). In another of my series the concretions are much 

 larger, and are either surrounded by a thin cell-envelope 

 with the nucleus lying to one side, as in fig. 16, h, or the 

 cell structure is no longer distinguishable. Similar conditions 

 are figured by Grobben for divers Lamellibranciis. 



The presence of these highly characteristic cells and 

 concretions not only enables us to identif}^ the above-de- 

 scribed tracts of tissue as pericardial glands, but also to trace 

 the latter beyond their apparent limits. The pericardial 

 glands appear to thin out on the auricular walls, and to stop 

 short of the heart. But an examination of the walls of the 

 heart with high powers of the microscope reveals the fact 

 that they are penetrated by the glandular tissue. The two 

 auricles, embracing between them the intestine, lie, like the 

 ventricle, in the pallial cavity, and have relatively thick 

 muscular walls, covered externally by a columnar epithelium 

 continuous with the external epithelium of the body. 



The muscle-fibres^ both of the auricles and ventricle, cross 

 one another in all directions, forming a sort of sponge-work 

 with numerous spaces. A high power of the microscope 

 shows that the tissue of the pericardial glands runs through 

 these spaces in the muscular walls of the auricles, and extends 

 into the ventricle. The characteristic cells containing olive- 

 brown concretions can be distinguished even with a low 

 power in the inter-muscular spaces of both auricles and 

 ventricle (fig. 19). Furthermore, a careful examination of 

 the columnar epithelium of the heart shows that the inner 

 ends of its component cells do not rest on a basement 

 membrane, but are prolonged internally into fine processes 

 which run in between the muscular fibres. In other words, 

 the external epithelium of the auricles and ventricle is fused 

 to and partly immersed in the subjacent muscular tissue, 



