18 TUBIFEX RIVULORUM. 



Williams says, it is essential to the maintenance of the life of the 

 annelid : — mechanically, by preventing contact between the intes- 

 tine and the integument, and by furnishing the fulcrum on which 

 all muscular action is based ; physiologically, by furnishing the 

 pabulum out of which the true blood is being perpetually rein- 

 forced. It holds organic corpuscules in suspension, which perform 

 irregular to-and-fro oscillations, under the agency of the muscular 

 contractions of the intestine and integument, passing from 

 segment to segment, either between the internal borders of the 

 septa and the intestine, or, as Claparede states, through orifices 

 provided in the septa themselves. The character of the corpus- 

 cules varies, according to my own observations, considerably ; 

 some being opaque and others transparent, some circular and 

 others very much elongated (see Fig. 5). This variation may be 

 connected with the fact, asserted by Lankester, that the cellular 

 membrane (endothelium) of the perivisceral cavity casts off its 

 cells into the perivisceral fluid. I must not omit to mention that 

 Dr. Williams regards the perivisceral fluid as physiologically allied 

 to the chyle of the lower animals, — that, in fact, it presents the 

 same relation to the contents of the proper blood-system of vessels 

 that the chyle of the higher animals does to the true blood in 

 them. 



The blood proper, as it is termed by Dr. Williams, is in this 

 annelid a red,''^ non-corpusculated fluid, circulating in a system 

 of closed vessels, the main trunks of which are more or less 

 intimately united to the intestine, but send out branches into the 

 peripheral portions of the body (Fig. 9). A large dorsal vessel, 

 thrown into many sinuosities, carries the blood from the tail 

 towards the head of the animal, where it bifurcates, and subdivides 

 into numerous small branches, which, reuniting, go to form the 

 feeders of the main ventral vessel, by which the blood is carried 

 back again towards the tail, where it finds its way again 

 into the dorsal. " f In each segment two great branches pass off 

 from the dorsal and ventral vessels respectively. Towards the 

 posterior border a large trunk (the perivisceral) springs on each 

 side from the dorsal, and proceeding outward towards the body- 

 wall divides into numerous capillary branches, which again unite 

 to form a trunk nearly as large as the original, that on each side 

 enters the ventral vessel. The coils are especially distinct towards 

 the posterior part of the body. About the middle of each 

 segment, again, the ventral vessel on each side gives off a branch, 

 which passes upward round the intestine ; but whether it termi- 

 nates by anastomosing with its fellow of the opposite side, or by 



* In some of the Annelida this fluid is green, 

 f See Mcintosh on Structure of Tubifex. Trans. Edin. Soc, vol. 26. 



