74 



[Proc. B.N.F.C., 



" HYDRA : " 



ITS MOVEMENTS AND REACTIONS. 



Mr. Harbison, who was next called, had provided living 

 specimens and a microscope to illustrate his remarks. He said 

 Hydra was a small animal found in ponds and sluggish rivers 

 attached to the leaves or stems of various aquatic plants. The 

 animal measured when fully extended but half an inch, when it 

 was of the thickness of an ordinary sewing needle. It might 

 contract, however, when touched violently to a small circular 

 knot. By one end it was attached probably by means of a sticky 

 secretion, and the other end carried six or eight simple tentacles. 

 In structure it was the simplest of the multicellular animals or 

 Metazoa, consisting of a two-layered tube. Between the layers 

 endoderm and ectoderm, unlike the sponges, there was no true 

 third layer or mesoderm ; there was merely an undifferentiated 

 lamina or middle jelly, the mesoglcea. On the sponges Hydra 

 showed a great advance physiologically, being much more 

 contractile and more irritable, and in being armed with stinging 

 cells. In the natural condition of the animal it would be seen to 

 sway and contract, waving its tentacles about in an indefinite 

 fashion. If placed on a watch glass for some time it would be 

 seen to show many curious and complicated movements, and 

 these movements performed by an animal of such simple 

 construction were investigated several times since first Abbe 

 Trembly drew attention to them in 1744. The usual movements 

 in the fixed state were of the nature of compass movements, first 

 generally towards the light, if lateral illumination be used, then 

 from it, and then in a line nearly at right angles to its first line 

 Its power of locomotion was considerable, but a change of site 

 was usually made only in adverse circumstances. It moved by its 

 base alone in some cases ; in others by a looping movement, 



