THE ANATOMY. VASCULAR SYSTEM 



76 



Fig. 19. 



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



is, however, preserved ; in the higher Oligoehaeta 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 Oligoehaeta 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 Rhynchdniis are peculiar among the lower 



Oligoehaeta, in that they have similar loops attached to 



them and following their windings ; and in Tuhifex 



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 shoi-t 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-capiEaries into the epidermis of Grio- 

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

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

 of Lumhricus — 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. Lininodnlus is furnished with apparently caecal vessels, which terminate 

 between the epidermic cells. It has been stated by the Sabasins 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 LUMBEICUS. 

 (After Vejdovsky.) 



I. Aperture of 'head kidney.' 2. 

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

 6. Intestine. 7. Ventral 



