CIRCULATION IN LOWER INVERTEBRATA. 
m 
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 Bryozoa , 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 
