TI1ALASSEMA AND ECHUJEUS. 431 



enclosed portions of the anal spines become widely separated ; 

 and from their connection with the cloaca that cavity is 

 dilated, and, acting like a syringe, sucks in more water. The 

 animal then closes the anus, and contracts the cloaca by push- 

 ing the water in its body forwards. This simultaneous action 

 forces the water contained in the cloaca into the respiratory 

 sacs, along which it is conveyed by their powerful vermicular 

 or peristaltic action. A slight relaxation of the muscular 

 fil ires of the sacs, and the erection of the ciliated funnels, will 

 allow the water to pass into the cavity of the body, while the 

 action of the cilia will remove it through the same channels 

 in a contrary direction. 



The vascular system consists of two longitudinal vessels, 

 one running along the ventral surface of the body, the other 

 along the unattached surface of the intestine. The intestinal 

 trunk is always full of blood in the weak or dead animal, the 

 ventral trunk always empty or collapsed. From this circum- 

 stance, from the general arrangement of the vascular system, 

 and from the position of the respiratory organs, we are 

 inclined to think, although we have not been able to verify 

 the opinion by actual observation, that the former vessel is 

 the venous, the latter the arterial trunk. The vein com- 

 mences by numerous radicles on the oesophageal portion of 

 the digestive tube, runs along the edges of the gut, collecting 

 branches as it proceeds. On the rectum, the trunk disap- 

 pears by being divided into innumerable branches, which are 

 apparently arterial, and proceed to the respiratory sacs, which, 

 as before stated, are highly vascular. The arterial or ventral 

 vessel is apparently formed by radicles from the respiratory 

 sacs (branchial veins). Its walls are thin, and are perceived 

 with difficulty on the surface of the nervous cord. In its 

 course it supplies vessels to the intestines ; and when it arrives 

 at the convolutions of the pharynx, it sends off from its right 

 side a large trunk, which, proceeding to the right oral hook, 



VOL. t. 2 V 



