HYDRO IDS. 



25 



anemones, the hydroids reject such 

 portions of their food as they do not 

 assimilate tlirough the month. In the 

 fresh-water hydra an oritice has been 

 observed at the lower extremity of 

 the stomach. This, however, does not 

 correspond to the alimentary canal of 

 higher organisms ; it is the analogue, 

 in the simple hydra, of the rami- 

 fying cavity wliich permits a free 

 circulation throughout the compound 

 group. 



A circulation has been observed 

 in the varieties which possess a horny 

 sheath, which is, however, very dilFer- 

 ent in some respects from the circula- 

 tion of the blood in higher organisms. 

 The somatic fluid, as it is called, is 

 loaded with granules which, upon mi- 

 croscopic examination, prove to be 

 composed of disintegrated elements 

 of food, of solid colored matter se- 

 creted by the walls of the somatic 

 cavity, of cells detached from the liv- 

 ing tissue of the animal, and of parti- 

 cles of effete matter. The fluid seems 

 to be more nearly akin to cliyme, or 

 chyle, than it is to blood. There is 

 perpetual motion in the somatic fluid ; 

 the flow will sometimes be steady for 

 a while, and then a sudden reversal 

 will take place in the direction of the 

 current, before it has reached an ex- 

 tremity. The gastric cavity is trav- 

 ersed by the stream, as well as all 

 portions of the hydrasoma. In some 

 species the cause of the flow has re- 

 vealed itself under the microscope. 

 The cavities through which the cur- 

 rent moves are seen to be clothed with 

 cilia tiny lashes whose rhythmic mo- 

 tion forever propels the fluid ; this cil- 

 iary action is no doubt greatly aided 

 by the contractility of the walls. In 

 many species the cilia, if there be any, 

 are too minute for detection ; but it is 

 a fair provisional inference that where ^"^' ^ 



-Sbrtiilarina cupressina. 

 ural Size) 



(Nat- 



