476 



CCELENTERA TES. 



greater speed than usual, the arms are withdrawn into their pockets or stretched 

 backward like a rudder. This power of free locomotion necessitates some 

 regulating organ, so that the desired direction or position 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 bod}'. Stinging- 

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

 of Ctenophore (Haeckelia 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 

 becomes 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 When the thread is withdrawn, it more or less entangles the 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 



L^taceans, while they themselves fall a prey to the disc-shaped jelly-fish and 



sea-anemones. Ctenophores may continue to grow, if uninjured, almost indefinitely, 



cydippe (uat. size). 



