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 Circulalion. 



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 

 Asteriee, which are situated higher in the animal 

 scale than Medusse ; 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 HolotlmricB , 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 



