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greater speed than usual, the arms are withdrawn into their pockets or stretched 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} 7 . 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 When the thread is withdrawn, it more or less entangles the 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 
Crustaceans, while they themselves fall a prey to the disc-shaped jelly-fish and 
sea-anemones. Ctenophores may continue to grow, if uninjured, almost indefinitely, 
