OLIGOTREMA PSAMMITES. 261 



surrounded by a distinct tliin-walled sac, the pericardium, 

 but as my sections of this region were broken by grains of 

 sand carried through the prepamtioQ from the stomach on 

 the edge of the razor, I was uuable to make oat the I'ela- 

 tions of the heart clearly, and could not determine the 

 connection between the heart and neighbouring blood-spaces. 

 Definite blood-channels can hardly be said to exist. The 

 substance of the plerome is honeycombed by numerous 

 cavities containing corpuscles, and in the neighbourhood of 

 the heart a very delicate pavement epithelium could be 

 detected forming a lining to some of the larger spaces. In 

 each tentacular arm, however, there is a definite axial blood- 

 space, "which is enlarged and forms a conspicuous cavity at the 

 base of the arm (fig. 6, bl.) 



As a rule the corpuscles contained in the blood-spaces are 

 few in number, but in some places they are very numerous, 

 e. g. in the tissues at the bases of the arms, in the neighbour- 

 hood of the anus, in some of the endopleromic cavities 

 bordering on the gut, and in the two longitudinal ridges 

 which I have identified as endocarps. These blood-cor- 

 puscles may more appropriately be called amcobocy tes. There 

 appear to be two kinds — 



1. A vesicular variety which are either (a) empty, when 

 they have the form of nearly spherical vesicles (fig. o2, c, d) 

 Avitha coarsely reticulated cytoplasm and a nucleus situated on 

 one side of the cell ; or (6) with cell contents, generally of a 

 brown colour, which are sometimes arranged like a shell 

 round the periphery of the cell (fig. 32, e), or are simply an 

 aggregate of discrete granules (fig. 28, am, c). 



2. Granular basophile cells, such as those depicted in figs. 

 32, a and b, containing numerous granules which stain 

 intensely with picro-nigrosin or picro-indigo-carmine. The 

 granules can often be shown to be connected by fine threads, 

 as is shown in fig. 32, a. 



There can be little doubt that both kinds of cells are 

 nutritive in function. As the vesicular cells are abundant in 

 the blood-spaces surrounding the stomach, and their brown 



