AXNDLATA. 239 



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 extends 

 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 horaologues 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, 

 and in most there are four longitudinal vessels attached to the intes- 

 tinal tube. 



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 pores or stigmata on each side of the body lead to as 

 many simple sacculi {Jjg. 104. h, h), formed by an inward folding of 

 the integument. Cany the duplicature further in, divide and sub- 

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

 system of insects, would be produced. "NTe may perceive in the 

 lateral sacs of the leech and earth-worm, the first step, morphologi- 

 cally, in the development of the very peculiar air-breathing organs of 

 the higher Articulata ; but, in their actual rudimentary form, their 

 respiratory functions are reduced to the lowest state, and they serve 

 chiefly the office of excretory organs, preparing and discharging 

 mucus. In Branchiobdella a pair of looped canals open at the begin- 

 ning of the middle third-part of the body, and a second pair at the 

 hinder end of the body, both at the median line of the under surface. 

 Close behind the orifice each of these four canals expands into a 

 round yellow-coloured sac, from which many convoluted tubes pro- 

 ceed. They are lined by a ciliated epithelium. A greater number 

 of such pairs of organs are found in other leeches at the second third- 

 part of the body, one behind another as far as the posterior extremity : 



* See Preps. 876, 877, 878. 



