THE ANATOMY. VASCULAR SYSTEM 



75 



Fig. 19. 



the plexus and convey it to the ventral vessel. The integrity of the cominissural vessels 



is, however, preserved ; in the higher Oligochaota the integrity of these vessels is lost 



(except in the embryo, see woodcut) ; they become largely dissolved into the network 



of capillaries. In the lower Oligochaeta it is only the actual body- wall which sometimes 



possesses a capillary network, none of the other organs of the body (as a general rule) 



have such ; the sperm-sacs, for instance, are fed by specially elongated perienteric loops 



belonging to the segments in which they occur. The 



nephridia in Rhynchdmis are peculiar among the lower 



Oligochaeta, in that they have similar loops attached to 



them and following their windings ; and in Tubifex 



NASSE has described vessels in the nerve-cord. With these 



exceptions there is no peripheral vascular system other 



than the integumental network until we reach the true 



earthworms ; and here it is not always at the same pitch 



of development ; in the smaller forms, such as Ocnerodrilus 



the nephridia seem to be quite unprovided with vascular 



networks. 



Epidermal capillaries. It has been known for a long 

 time that in the clitellar region the terminal branches of 

 the vascular system push their way in among the cells, 

 forming loops, which, however, stop short some little 

 way below the surface. These capillaries are figured by 

 CLAPAREDE and by others. As to the vascularity of the 

 rest of the epidermis, it was, I believe, first pointed 

 out by myself that this was the case with Megascolex 

 coeruleus (2) and some species of Perichaeta (3). The 

 extension of blood-capillaries into the epidermis of Crio- 

 drilus has been figured by ROSA (12), and quite recently 

 it has been discovered by LENHOSSEK (1) that the epidermis 

 of Lumbricus not only the clitellum is also vascular. 

 Even among the more delicate aquatic species this con- 

 dition, very likely universal in the earthworms, is not 



unknown. Limnodrilus is furnished with apparently caecal vessels, which terminate 

 between the epidermic cells. It has been stated by the SARA SINS that in Perichaeta 

 the capillaries actually reach the exterior, and open there ; but this appears on the face 

 of it to be unlikely, and at any rate needs further proof. The statement was made 

 incidentally in connexion with an alleged similar opening of blood-capillaries on to 



L 2 



EMBRYO LUMBBICUS. 

 (After Vejdovsky.) 



i. Aperture of 'head kidney.' 2. 

 Brain. 3. Septal glands. 4, 5. Dorsal 

 vessel. 6. Intestine. 7. Ventral 

 vessel. 



