THE HABITS AND STRUCTURE OF ARENICOLA MARINA. 103 



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 vacuo- 

 lated appearance, and which contain few or no granules (Vac. 0.); (2) 

 cells which contain a large number of yellowish granules in the proto- 

 plasm (67. G). These latter cells are possibly granular, and correspond 

 to those found in the cardiac body of other Polycheets. 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 commences. The "cardiac body" of Polychsets, as hitherto 

 described, is an unimpaired structure lying ia the dorsal vessel. That 

 of Are?iicola, however, is paired and in no way connected 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 haemoglobin, but there has 

 been no thorough investigation of the constituents of the plasma. 

 Krukenberg (1882), it is true, made some experiments which led him to 

 believe that there were no coagulable albumens in the blood of his 

 specimens; but, as they were in a starving condition, a fresh examina- 

 tion is very desirable. A large quantity of albumen is certainly present, 

 which, when the specimens are fixed, becomes very hard and brittle. 



We have seen small cells (4/z in diameter) in the blood-vessels of the 

 nephridia, but it is doubtful if these are the blood-corpuscles which we 

 have not been able to demonstrate. 1 



General Remarks on the Circulatory System. — No other system of 

 organs shows the true segmentation of the body of Arenicola so well as 

 this. The lines of demarcation between the somites from one end of 

 the body to the other are marked by the segmental vessels passing 

 from the ventral to the dorsal vessel and breaking up on their way in 

 the body-wall, nephridia, or gills. Throughout the gastric region, 

 however, this arrangement is somewhat disguised, owing to the loss of 

 the connection with the dorsal vessel, an alteration caused probably by 

 the necessity fur leaving this part of the alimentary canal freely 

 moveable. 



1 Since writing this we have discovered that these small cells are the 

 blood-corpuscles. 



