140 LECTURE XI. 



This has mucli analogy with the circulation of the blood in the 

 earth-worm, in which the blood travels from behind forwards in the 

 dorsal vessel, and descends in great part towards the ventral vascular 

 system through the pairs of anterior beaded contractile sinuses or 

 hearts *, which differ only from the two ventricles in the Arenicola 

 by their greater number. In the lateral vascular canal, which ex- 

 tends along the anterior part of the body, at the base of the feet in 

 the Arenicola, and which is formed by the anastomoses of one of the 

 branches of the cutaneous arteries, we have the analogues of the 

 lateral vessels in the leech tribe, which are wanting in most of the 

 higher Anellides. The dorsal and ventral trunks are common to all. 



The most striking physiological character of the circulation in the 

 Anellides as a class, is the continuity of their capillary system, and the 

 difficulty of determining which is the arterial, and which the venous 

 trunk of any one of the organs or parts of the body, excepting the 

 branchiae. There alone, we find that the blood received from the 

 distinct artery is sent back by as distinct a vein, which returns along 

 the same route as the artery, as it does in the limbs of the higher 

 animals. By the rapid division and general system of anastomoses 

 of the arteries and veins, it follows that almost all the parts of the 

 body are supplied by a mixture of arterial and venous blood. 



The position and general relations of the branchial organs have 

 already been incidentally pointed out ; and it seems only necessary 

 here to allude to their different forms. In the leech and earth-worm, 

 a series of jDores or stigmata on each side of the body lead to as 

 many simple sacculi (^Jig.^l. h,h), formed by an inward folding of 

 the integument. Carry the duplicature further in, divide and subdivide 

 it, and ramifications of air tubes, like the tracheal respiratory system 

 of insects, would be produced. We may perceive in the lateral air 

 sacs of the leech and earth-worm, the first step in the development of 

 the very peculiar air-breathing organs of the higher Articulata. The 

 air sacs of the abranchiate anellides, in their actual rudimentary 

 form, have their respiratory functions reduced to the lowest state, 

 and serve chiefly the office of excretory organs, preparing and 

 discharging mucus. 



The respiratory organs of the Tubicolar anellides are in the form 

 of long, and sometimes tortuous, filaments -j- which radiate from the 

 head, generally in two lateral fasciculi. When not coloured by the 

 red circulating fluid, they are often barred and variegated by bright 

 purple, green, and yellow tints, forming a rich and gorgeous or- 

 namental crown. 



* See Preps. 876, 877, 878. ] Prep. No. 990, 



