THE CCELENTERATA 



149 



until it appears as a small jelly-like blob. If left undisturbed it will 

 gradually expand again, throwing out its tentacles, which wave 

 about in the water in search of prey. These movements are not 

 the only ones exhibited, for although usually fixed it is able to move 

 from place to place. When it does so it lengthens out and bends 

 over to one side, attaching itself to the substratum by means of its 

 tentacles or mouth some way from the foot. The latter is then 

 released and brought up nearer to the tentacles, where it becomes 

 fixed, so that the animal has thus 

 moved a short distance. This may 

 be repeated several times, and so 

 slow progress is possible in a manner 

 recalling that of a looper caterpillar. 

 All sorts of intermediate conditions 

 between the extremes of extension 

 and contraction may be seen, some- 

 times limited to one or other end of 

 the body, so that the cylinder is 

 not of equal thickness throughout. 

 Individuals may sometimes be seen 

 in the process of budding, with tiny 

 daughter forms growing from them, 

 or several knob-like projections, the 

 reproductive organs, may be present 

 on the top two-thirds of the body. 



A longitudinal section shows that 

 the mouth, the sole external aper- 

 ture, leads into a hollow cavity co- 

 extensive with the body, which is 

 therefore a very minute tube. The 

 tentacles are also hollow, being but 

 long attenuated outgrowths from the 

 body wall. This central hollow is 



FIG. 47. 'Hydra, with bud and 

 gonads. 



the gut cavity or enteron, termed B., basal disc; H.,hypostome ; o., ovary ; 

 also the ccelenteron, to indicate 



that it corresponds to both the body cavity (ccelom)' and gut 

 cavity (enteron) of the higher animals, and this particular type 

 of structure is fundamentally characteristic of the whole phylum, 

 hence its name. Another common feature of the Coelenterata is 

 exemplified by Hydra in the simplicity of its structural plan. If its 

 main axis be marked out by an imaginary line drawn through the 

 mouth and the centre of the basal disc, we find that the tentacles 

 are arranged around it radially, i.e. they are related to the axis, as 

 are the radii to the centre of a circle. This is a condition we term 



