VASCULAR CIRCULATION. 235 



or tubes of any sort. This kind of diffused cir- 

 culation is also seen in the embryos of various 

 animals, at the earliest periods of their develope- 

 ment, and before any vessels are formed. 



§ 2. Vascular Circulation. 



The next step in the gradation of structures con- 

 sists in the presence of vessels, within which the 

 fluids are confined, and by which their course 

 and their velocity are regulated ; and in general 

 these vessels form a complete circuit. The first 

 rudiments of a vascular organization are those 

 observed and described by Tiedemann, in the 

 Aster ice, which are situated higher in the animal 

 scale than Medusae ; but whether any actual 

 circulation takes place in the channels consti- 

 tuted by these vessels, which communicate both 

 with the cavity of the intestine, and with the 

 respiratory organs, is not yet determined with 

 any certainty. The HolothuricB, which also 

 belong to the order of Echinodermata, are fur- 

 nished with a complex apparatus of vessels, of 

 which the exact functions are still unknown. 

 In those species of Entozoa which exhibit a 

 vascular structure, the canals appear rather to 

 be ramifications of the intestinal tube, than 

 proper vessels, for no distinct circulation can be 



