3450 



CCELENTERA TES 



\ 



drawn into their pockets or stretched backward like a rudder. This power of free 

 locomotion necessitates some regulating organ, so that the desired direction or posi- 

 tion of the body may be maintained. Such an organ exists at the apical pole of the 

 body, and may be described as consisting of a small weight borne on springs, by 

 which the oscillations of the body or deviations from the line of movement can be 

 instantly felt. 



The ventrally placed mouth is like a large slit between the folds of the 

 umbrella, and leads into a stomach which is either tubular or flattened. The food is 

 digested in this stomach, the indigestible parts, mixed with mucus, being again 



ejected through the mouth. The upper 

 end of the stomach is in direct com- 

 munication with a funnel-shaped space 

 of variable width. From this funnel- 

 like cavity canals arise, which branch 

 and run below the outer surface, follow- 

 ing the lines of the ribs. This funnel 

 further possesses an aperture of its own, 

 opening on the exterior, in the region 

 of the apical pole. Within the funnel is 

 found a fluid substance containing par- 

 ticles of the food pulp drawn in from 

 the stomach, but consisting chiefly of 

 water, taken in voluntarily; this fluid 

 being kept in motion by ciliary action 

 through the canal system. Although 

 water is also sometimes taken in through 

 the proper apical aperture of the funnel, 

 this aperture seems principally to serve 

 for the ejection of the fluid when of no 



further use. It is then also mixed with waste matters from the body. Stinging 

 cells, such as occur m the next group, have as yet been found in only one species 

 of Ctenophore (ffaeckelia rubra), and then only in small numbers. Instead of 

 stinging cells, the Ctenophores have adhesive cells, or small hemispherical knobs 

 found on the tentacles or capturing filaments; these being provided with elastic, 

 spirally-coiled stalks, but containing no poison. These knobs are beset with sticky 

 globules, to which small animals, such as minute crustaceans easily become attached. 

 If the prey attempt to escape, the spiral thread by which the knobs are attached 

 become stretched. When the thread is withdrawn, it more or less entangles the 

 victim, and, being like the knob, provided with a great number of sticky particles, 

 renders escape impossible. These structures are very different from stinging cells, 

 which are useless to an animal after having been once employed. An adhesive cell 

 or knob can act apparently any number of times, being each time drawn back by the 

 spiral thread to its former position. 



Ctenophores feed upon all kinds of small pelagic animals, especially Crusta- 

 ceans, while they themselves fall a prey to the disc-shaped jellyfish and sea anem- 



CYDIPPE (natural size). 



