SECT. m. TENTACLES OF HYDR&. 83 



Thus armed, the tentacles are formidable weapons ; 

 they are highly contractile and wonderfully strong, tena- 

 ciously adhering to the small worms and aquatic insects 

 on which the Hydrse feed, and they are aided by the 

 roughness of their surface. They transfix their prey, 

 and are believed to infuse a liquid poison from the dart, 

 or thread-cells, into the wound, then twisting their other 

 tentacles round the victim, it is instantly conveyed to 

 the mouth, and slowly forced into the digesting cavity, 

 where it is seen through the transparent skin to move for 

 a short time, but as soon as the nutritious juice is ex- 

 tracted, the animal ejects the refuse by its mouth. In the 

 inner layer, enclosing the cavity of the stomach, there 

 are cells containing a clear liquid with coloured particles 

 floating in it, which is supposed to perform the part of 

 a liver ; and, as the Hydrse have no respiratory organs, 

 their juices are aerated through their skin. They have 

 no perceptible nerves nor nerve centres, yet they are irri- 

 table, eminently contractile, and are attracted towards 

 the light all these being probably sympathetic motions. 



Though in general stationary, the Hydra can change 

 its place ; it bends its body, stretches to a little distance, 

 and fixes its anterior extremity firmly by its tentacles ; 

 then it detaches its sucker and brings it close to its 

 mouth, fixes it, and again stretches its fore part to a 

 little distance along its path, and repeats the same 

 process, so that it moves exactly after the manner of 

 certain caterpillars. It can even move along the water 

 by attaching the expanded disk of its sucker to the 

 surface, where it soon dries on being exposed to the 

 air, and becomes a float, from whence the Hydra hangs 

 down with its tentacles extended like fishing lines, as in 

 fig. 110 ; or it can use them as oars to row itself along 

 under the surface of the water. 



On account of their simple organization, the Hydrse 

 are endowed with the most astonishing tenacity of life. 



