JOUSSEAUMIA. 255 



of special cells, whose nuclei are grouped about the broad bases of attachment of the 

 bars to the filaments. These nuclei are visible in section in fig. 21.r. There can be 

 no doubt that the interfilamentar junctions are formed by the agency of these cells, 

 for their position at the attached ends of the bars is invariable, and they are not to 

 be distinguished elsewhere. Moreover, by looking through numerous preparations, I 

 have been able to recognise these groups of cells at points where the interfilamentar 

 junctions are in process of formation, and have seen in optical section the processes 

 formed by the cells projecting from, but not yet bridging over the interval between 

 adjacent filaments. These chitin-forming cells do not appear to have been recognised 

 by previous observers, but they are probably included in the general and somewhat 

 vague term " sub-filamentar " tissue. Bernard (2) gives a drawing of groups of 

 stellate cells in Scioberetia, which appear to coincide in position with those which I 

 have described, but he does not attribute any special function to them, and merely 

 refers to them as components of a " substance conjonctive transparente a nombreuses 

 cellules" [Joe. cit. p. 374). It is evident from a comparison of the sections drawn in 

 figs. 20 and 21, that these junction-forming cells in Jousseaumia are differentiated 

 from the flat non-ciliated cells covering the interlamellar edges of the filaments. 



The Pericardium and Renal Organs. Owing to the minute size and the 

 contracted state of my specimens, the relations of these organs presented great 

 difficulties. The pericardium is a more or less triangular sac, relatively of consider- 

 able size, lying above and in front of the posterior adductor muscle. It is traversed 

 obliquely by the rectum, and the ventricle of the heart surrounds the latter for a 

 considerable part of its course through the pericardium. The whole of the inner 

 lining of the pericardial walls is glandular, constituting an extensive pericardial 

 gland, but the glandular epithelium does not appear to extend to the investment of 

 the ventricle and auricles. Glandular epithelia are the first to suffer from the effects 

 of long immersion in spirit, and the preservation of my specimens was not good 

 enough to allow me to make out the details of the pericardial glandular cells with 

 any certainty. The most that I am able to say is that they are rather large 

 irregularly shaped cells with oval nuclei, and coarsely granular contents which stain 

 faintly blue in picro-indigo-carmine. 



The kidneys are conspicuous from the large concentrically striated concretions 

 which they contain. These concretions are contained in a highly vacuolated proto- 

 plasmic lining of the renal sacs. The right and left renal sacs are fused together for 

 such a considerable extent in the middle line, below the floor of the posterior end of 

 the pericardium, that their paired nature is obscured, and can only be recognised by 

 an examination of the paired ducts and the paired anterior and posterior horns into 

 which the median sac is produced. Such an extensive fusion of the two kidney 

 sacs is characteristic of the more specialised forms of Lamellibranchia, particularly 

 of the Myacea, Pholadidae and Anatinacea (Pelseneer, 12), and my sections through 

 this region of the body bear a considerable resemblance to the section through the 



