in] 



HYDRA 



57 



showing this modification are termed myo-epithelial (Gr. /x.vs = 

 muscle) : the word epithelial is used to signify the arrangement of 

 cells in a layer to form a pavement or mosaic. 



When a Hydra is severely irritated we find that not only the 

 nematocysts but often the cnidoblasts themselves are expelled from 

 the body. This points to the fact that the irritation has spread 

 from the cnidocil to the cnidoblast and thence to the neighbouring 

 ectoderm cells. The question arises how it passes from one cell to 

 another, and this question is raised in a more acute form when we 

 see the whole body of a Hydra thrown into contraction as the result 

 of one of the tentacles touching a water-flea. The prey is grasped 

 by all the tentacles and pushed into the mouth. How are the 

 movements of Hydra co-ordinated ? Kenewed research has within 

 recent years proved the existence of a number of star-shaped cells 

 lying on the jelly at the base of the 

 ectoderm (Fig. 21). Each cell has a 

 central body and a number of radia- 

 ting processes. The processes of ad- 

 jacent cells touch each other so that 

 together they form a network (Fig. 

 22). These cells receive the stimulus 

 from one ectoderm cell and transmit 

 it to others. Their function is to 

 collect, add together and distribute 

 stimuli. They are the most primitive 

 form of neurons or nerve cells and 

 their processes may be looked on as 

 the forerunners of nerve fibres. 



The most important function of 

 the endoderm cells is to digest the 

 prey which is captured by the tentacles 

 and thrust into the coelenteron. For 

 this purpose they secrete a fluid 

 which has a great power of dissolv- 

 ing protoplasm. This fluid, termed 

 digestive juice, is poured forth into 

 the coelenteron and a large portion of 



the prey is dissolved by it and passes by diffusion into the endo- 

 derm cells, from which part is transferred in a similar manner to 

 the ectoderm. Certain portions of the prey, consisting of some of 

 the proteids, resist the action of this juice. These are seized by 



_ --3 



FIG. 22. Diagram of Hydra to 

 show the arrangement of the 

 nerve-cells. (After Hadzi.) 



1. Tentacle. 2. Mouth. 



3. Foot. 4. Nerve-cell. 



