HYDRA. I1YDRAFORM POLYPES. 489 



good day's journey even in summer. But sometimes a more 

 expeditious mode of travelling is adopted. The head being 

 brought down and fixed as before, the foot is made to describe a 

 semicircle over it, and takes its new attachment at an equal 

 distance on the other side ; the foot being then fixed, a similar 

 movement is performed by the head ; and thus the animal ad- 

 vances by a succession of somersets. 



1052. By these and similar means, the Hydras are enabled 

 to move along solid surfaces, such as the bottom or sides of the 

 vessel in which they are contained, and the leaves or stems of 

 aquatic plants. There is another very curious position, to which 

 it frequently has recourse. By projecting the flat surface of the 

 foot above the water for a short time, it soon becomes dry, and 

 in this state serves, by its repulsive action on the water around, 

 as a kind of float, from which the animal suspends itself. In 

 this state it can move itself, by means of its tentacula, with 

 great facility ; and it is also acted on by the wind, so that it 

 can travel a considerable distance without effort. If, whilst 

 thus floating, a drop of water be made to fall upon the foot so 

 as to wet it, the hydrostatic power of the organ will be 

 destroyed, and the animal will immediately sink to the bottom. 



1053. The first subdivision of the Polypifera, termed 

 Hydroida or Hydra-form^ or Sertularian Polypes, from the 

 name of one of its most characteristic genera, includes, with the 

 simple genus just described, all those compound structures, in 

 which a number of Polypes similar to it are associated together. 

 In all these, the polypidom, or solid frame-work which gives 

 support to the softer portions of the structure, is external to the 

 living animal matter, and incloses it as in a tube. It is of a 

 texture varying from that of membrane to that of horn ; it 

 never contains stony matter to any amount ; and it is always 

 flexible. This horny sheath is formed by the consolidation of 

 the living animal membrane, which originally acts as the 

 envelope of the soft and almost fluid texture within ; and it is 

 continuous with the external layer of that, which forms the 

 Polypes themselves. This horny tube is enlarged at certain 

 points into sheaths or cells for the protection of the Polypes ; 



