CIRCULATION IN LOWER INVERTEBRATA. 257 



system of channels excavated through the tissues, after passing 

 through which it finds its way again to the respiratory sur- 

 face, and thence to the heart. But after a certain duration of 

 its flow in this direction, the current stops, and then re-com- 

 mences in the contrary direction, proceeding first to the 

 respiratory organs, and then to the system in general. It 

 would seem as if in this, one of the lowest forms of animals 

 possessing a distinct circulation, the central power were not 

 yet sufficiently strong to determine the course which the fluid 

 is to take. In the group of JBryozoa, which forms a connecting 

 link between the Tunicata and Zoophytes, we lose all trace 

 of a distinct circulation, which is only represented by the 

 movement of fluid in the general cavity of the body, and in 

 the prolongations of this cavity in the arms that surround 

 the mouth (fig. 64). In the Star-fish, Sea- Urchin, and other 

 animals belonging to the class ECHINODERMATA, there seems 

 to be a regular circulation of nutritious fluid, carried on through 

 distinct vessels, but without any definite heart The only 

 trace, indeed, of anything like a propelling organ, is an en- 

 largement of one of the trunks, which pulsates with tolerable 

 regularity ; but this would not seem to have force enough to 

 propel the fluid through a complex system of vessels ; and 

 the circulation seems to be carried on chiefly by some force 

 produced in the capillaries ( 280). 



296. The circulating apparatus of higher animals is only 

 represented in Zoophytes, Medusae, and the lower Worms, by 

 ramifying prolongations of the digestive cavity, which extend 

 throughout the body, and are specially distributed to the 

 respiratory surface, so as to subject the products of digestion 

 to the aerating process. Thus, in the stony corals which are 

 formed by animals constructed upon the general plan of the 

 Sea Anemone ( 127), the gelatinous flesh that connects the 

 polypes is traversed by a network of canals that open freely 

 into the sides of their visceral cavities, of which they may be 

 regarded as extensions ; whilst in the Campanularia (fig. 72) 

 and other composite Hydraform Zoophytes, a like communi- 

 cation is established by a system of canals passing along the 

 stem and branches, and becoming continuous with the base of 

 each polype. In this system of canals, viewed under a sufficient 

 magnifying power, a granular fluid may be seen to move, the 

 direction of the flow being sometimes from the stem towards 



