ANATOMY AND CLASSIFICATION OF THE ARENICOLIDJ]. 463 



nephridial segment bifurcate near the body-wall, each sending 

 one branch to the parietal and the other to the nephi'idial 

 vessel (see fig. 44). A further account of the relation of 

 these vessels to the nephridia is given in the section on 

 nephridia (p. 517). 



In A. marina and A. cristata there is a well-marked 

 parietal vessel, which runs from the region of the first seta 

 almost to the posterior end of the body, but the nephridial 

 vessel is small and difficult to trace. We have not been able 

 to follow these vessels in A. Claparedii, of which we have only 

 spirit specimens at our disposal. Judging from sections of 

 this species, both the parietal and nephridial vessels are very 

 feebly developed. 



The hearts are a pair of contractile bulbs connecting the 

 gastric and ventral vessels. There is a considerable differ- 

 ence in the size of this organ in the various species. In A. 

 marina and A. cristata (fig. 30) the heart is a large organ, 

 the ventricle especially being capable of great dilatation. In 

 A. Grubii and A. ecaudata it is small even at the moment 

 of diastole (see figs. 44, 45), while at the close of systole it is 

 little more than a rhomboidal enlai-gement at the junction of 

 gastric and lateral oesophageal vessels. In A. Grubii and A. 

 ecaudata, even more so than in A. marina, the vascular 

 system clearly indicates the true segmentation of the body. 

 An inspection of the drawing of the dissection of A. Grubii 

 (PI. 25, fig. 44) will show tliis point at once. 



The blood-corpuscles of all the species are very much alike. 

 They are minute nucleated, rounded, or ellipsoidal cells from 

 5ju to 10 fx in length (fig. 40), and in comparison with the 

 complexity of the vascular system and the large quantity of 

 blood are very sparsely scattered. Their mode of origin is 

 unknown. 



The He art- body. — The structure of the heart-body re- 

 quires separate consideration. Though absent in A. Clapa- 

 redii and A. cristata, this organ is well developed in the re- 

 maining species. In post-larval and young specimens of A. 

 marina the heart contains no trace of this body, but it has ap- 



