NUTRITION IN POLYPI. 69 



undulation of the contents of the stomach, back- 

 wards and forwards, throughout the whole tube ; 

 apparently produced by the contraction and dilata- 

 tion of its different portions. The undigested mate- 

 rials being collected together and rejected by the 

 mouth,* the remaining fluid is seen to contain 

 opaque granules of various sizes, some of which are 

 observed to penetrate through the sides of the 

 stomach, and enter into the cellular structure 

 which composes the flesh of the animal. Some 

 portion of this opaque fluid is distributed to the 

 tentacula ; into the tubular cavities of which it may 

 be seen entering by passages of communication 

 with the stomach. By watching attentively the 

 motions of the granules, it will be perceived that 

 they pass backwards and forwards through these 

 passages, like ebbing and flowing tides. 



All these phenomena may be observed with 

 greater distinctness when the food of the animal 

 contains colouring matter, capable of imparting its 

 hue to the granules, and allowing of its progress 

 being traced into the vesicles which compose the 

 substance of the body. The vesicles that are 

 nearest to the cavity of the stomach are those 

 which are first tinged, and which therefore first im- 

 bibe the nutritious juices : the others are coloured 

 successively, in an order determined by their 

 distance from the surface of the stomach. Trembley 

 ascertained that a living hydra introduced into the 



* Corda found that although the larger masses of undigested 

 materials are rejected by the mouth, yet that an aperture exists at 

 the opposite end of the cavity, for the extrusion of the smaller and 

 more divided particles. This orifice had long ago been rioticed by 

 Baker, 



