HABITS AND STRUCTURE OP ARENIOOLA MARINA. 19 



( v.). The cavity of the ventricle is small and broken up by 

 a spongy mass of cells. The ventricular walls are muscular, 

 and contract from above downwards, forcing the blood into 

 the ventral vessel. (We have sometimes seen an apparent 

 reversal of the hearths action.) The spongy cardiac body 

 arises by ingrowths from the wall of the ventricle, chiefly in 

 the middle and ventral regions. It gradually encroaches on 

 the blood space, so as to reduce it considerably (PI. 5, fig. 36, 

 Card. B.) in an old specimen. The cardiac body in a young 

 specimen (fig. 38) is much smaller, and extends obliquely across 

 the heart, its general direction being downwards and back- 

 wards. The cells of the cardiac body in an old specimen 

 which we have examined are loosely arranged, so as to cause 

 the formation of a large number of intercellular spaces, some 

 of which are of considerable size, and which are in life filled 

 with blood (Figs. 36 — 38, B. S.). Between the cells there are 

 numerous fibres, which are probably muscular. The cells are 

 apparently of two kinds, which, however, merge into each 

 other : (1) cells whose protoplasm has a very vacuolated appear- 

 ance, and which contain few or no granules ( Vac. C.) ; (2) cells 

 which contain a large number of yellowish granules in the 

 protoplasm (G. C). These latter cells are possibly glandular, 

 and correspond to those found in the cardiac body of other 

 Polychsets. The function of the cardiac body may be, as 

 Schaeppi (1894) suggests, to prevent regurgitation of the blood 

 from the ventral vessel into the heart when the diastole com- 

 mences. The " cardiac body " of Polychsets, as hitherto 

 described, is an unpaired structure lying in the dorsal vessel. 

 That of Arenicola, however, is paired and in no way con- 

 nected with the dorsal vessel. Hence a strict homology is 

 scarcely probable. 



Blood. — As Professor Lankester was the first to point out, 

 the blood of Arenicola is strongly impregnated with haemo- 

 globin, but there has been no thorough investigation of the 

 constituents of the plasma. Kriikenberg (1882) , it is true, made 

 some experiments which led him to believe that there weie no 

 coagulable albumens in the blood of his specimens ; but as they 



