266 



THE HYDRA 



it more difficult to observe, but there is apparently the same mode 

 of response as in the jellyfish, Gonionemus (Fig. 127), where a com- 

 bination of motile, mechanical, and chemical stimulations gives 

 the maximum reaction, since this most closely imitates the kind of 

 stimulus to which the animal normally responds in the capture of 



its prey. The tentacles of a resting 

 Gonionemus, for example, are moder- 

 ately sensitive to the mechanical 

 stimulus of a fine pipette when it 

 touches them or moves slightly, or to 

 the gentle ejection of meat juice 

 against them without movement of 

 the pipette; but when the mouth of 

 the pipette is drawn rapidly along a 

 tentacle as the meat juice is extruded 

 the animal springs into action, as 

 though a small fish or crustacean had 

 come within its toils. The tentacles 

 twist and turn, bending toward the 

 mouth, and the hypostome bends 

 toward the tentacles stimulated. 



The Nervous System. — An animal 

 exhibiting the complexities of be- 

 havior that have been described in the 

 Fig. 128. — Young hydra foregoing paragraphs may be pre- 

 stained to show network of g^j^^g^j ^^ possess a nervous system 

 ectodermal nerve cells with .., , „ • i- o i. 



... . ■ t 4. With some degree oi organization, feucn 



nng-hke arrangement m toot ^ ° 



and hypostome. proves to be the case with hydra 



(After Hadzi, loc. cit.) (Figs. 128, 129 and 130). In the ecto- 



derm there is a network of nerve cells, 

 with which are connected, on the one hand, at least four different 

 kinds of sensory cells, and on the other, the muscle fibrils of the 

 large ectoderm cells. In the endoderm there is a much smaller 

 number of isolated nerve cells and also a few cells that appear 

 to be sensory. There is no microscopic evidence that the nervous 

 elements of the ectoderm and endoderm of hydra are united by 

 fibers passing through the supporting lamella. The absence of 

 a nerve network in the endoderm and the independent muscular 

 action of the two layers which this implies is further evidenced by 

 experimentation; since the movements due to the longitudinal 



