SANGUIFEROUS SYSTEM. 435 



dorsal, pulsating, arterial vessels, the lateral branches of 

 which terminate in corresponding ventral venous trunks, 

 which convey it backwards to the beginning of the dorsal 

 vessel, and it is chiefly in the form and structure of this 

 great muscular pulsating centre that we perceive the higher 

 development of the vascular system in the entomoid than in 

 the helminthoid classes. The internal vibratile cilia are still 

 the principal agents of the circulation and respiration in many 

 of the lower, articulata, as in most of the radiated classes, 

 especially where the muscular, pulsating, arterial centre of 

 the sanguiferous system is not yet developed, as in the en- 

 tozoa and rotifera. 



The exterior surface is aerated, and the interior cyclosis 

 is effected, by the same means, in the wide gastric cavi- 

 ties of the cystic entozoa, as in the ciliated abdominal sacs 

 of the polygastric and rotiferous animalcules. The lim- 

 pid serous fluids likewise meander through the inert canals 

 of the cestoid and trematode worms, without the aid of con- 

 tractile muscular parietes, as in the similarly ciliated and 

 inert vessels of the acalepha and most other radiated animals; 

 and from the extensively ramified condition of their alimen- 

 tary canal, the sanguiferous system is less isolated from the 

 digestive than in higher forms. In the diplostomum clavatum 



o O J- 



and diplostomum volvens, where the digestive organs are more 

 circumscribed, a distinct system of arteries and veins is per- 

 ceived to circulate most extensively through the body a thin, 

 red-coloured serous fluid, apparently absorbed from the pa- 

 rietes of the capacious alimentary sacs. A large arterial 

 trunk, giving off numerous lateral branches in its course, is 

 observed, in both these animals, to commence from the pos- 

 terior extremity of the body, and to advance along the me- 

 dian plain to near the mouth, where it is continued, with di- 

 minished calibre, into two great lateral venous trunks. These 

 great lateral veins are directed backwards along the whole 

 extent of the body, and receive numerous branches on each 

 side, many of which are observed to be continuous with the 

 capillaries of the median artery. A single anastomosing 

 canal connects the two venous trunks with each other across 

 the middle of the animal, and connects them likewise with 

 two other longitudinal veins, more central in their position, 

 and extending along the posterior half of the body. In the 



F F 2 



