214 THE VMA!, FUNCTIONS. 



§ 2. Details of Slruclure in the different Orders 



of Anintals. 



Among the modes of nourishment by imbibition 

 exhibited by Zoophytes, a distinction has already 

 been pointed out between those simpler forms of 

 construction, in which the nutrient fluids are de- 

 rived immediately from the digestive cavities, and 

 those in which the elaborated juices are allowed to 

 pass from the latter into other cavities, where they 

 are collected and retained, and which may be re- 

 garded as reservoirs for the supply of materials to 

 each organ, in proportion as they are required for 

 its nutrition. These reservoirs often consist merely 

 of passages forming either a reticulated system of 

 canals, as we have seen in Medusae (Fig. 251), or 

 ramified prolongations of the intestine branching- 

 out in arborescent forms, as exemplified in Asterias 

 (Fig. 258). At other times, they constitute large 

 spaces, intervening between the alimentary canal 

 and the external integument. It is probable that 

 the fluid contained in these receptacles is never 

 stagnant, but is moved with more or less activity 

 according to the exigencies of the system.* Where 

 these fluids are perfectly limpid, their motions are 

 imperceptible to the eye ; but when they contain 

 particles differing in density from the fluid itself, 



* It would appear that advantage is occasionally taken of this 

 mobilitv of the fluid contents of these cavities for effectins: the 

 mechanical purpose of protruding certain flexible organs, such as 

 the tentacula of the Hydra, and the fleshy feet of the Asterias, and 

 other Echinodennata. See Vol. I. p. 184. 



