272 LIONEL JAMES PIOTON. 



the heart-body generally. The mechanical principle here in- 

 volved is the same as in the case of the right ventricle of 

 the mammalian heart, which contracts upon the convex 

 solid wall of the left ventricle. In the same way the dorsal 

 vessel of the worms in question contracts on systole, so as 

 practically to obliterate the lumen between its wall and the 

 solid heart-body, so that the whole of the blood which the 

 vessel contained at diastole is expelled. In the gill-vessels of 

 Sternaspis cellular rods are found which confer on them a 

 similar mechanical advantage. 



Between the cords of a piece of a fresh heart-body, flattened 

 under a cover-slip, the minute, fusiform, non-nucleated blood- 

 corpuscles are often observed with a high power. Claparede 

 states that they are of the same colour as the plasma in which 

 they float, that is, yellow. Lankester (18) says that they are 

 colourless. They appear to me a bright pink, unlike haemoglobin. 



The histological structure of the heart-body shows points of 

 great interest, but has been for the most part neglected in the 

 literature of the subject. Three layers may be distinctly re- 

 cognised (fig. 5), — an endothelium clothing each cord, a cortex, 

 and a central or medullary tissue. The outermost layer, or 

 endothelium, is directly bathed by the blood. It is composed 

 of a single layer of cells, the nuclei of which are seen at 

 intervals. The cortex is composed of cells, elongated in 

 shape, the walls of which are very well marked (figs. 2 and 5). 

 The nuclei frequently have a deeply-stained nucleolus; but, 

 except for some refringent yellow granules, there are few cell- 

 contents. In the periphery of the organ the cells are closely 

 set together, with their long axes at right angles to its surface, 

 and the nuclei are towards the outer side. Beneath the peri- 

 pheral layer the cells are irregular and nuclei are rarer. 



The development of the medullavaries in different individuals; 

 whilst in the same individual it is reduced in amount, or absent, 

 at the intersegmental constrictions. It consists of a highly 

 granulated tissue. In this, although at some points nuclei can be 

 recognised with a comparatively low power (D, Zeiss, figs. 5, 8), 

 yet the numbers of granules which occupy the structure render 



