COELENTERA TA 



27 



a long, whip-like, spirally-coiled filament, which lies supported 



in the liquid which fills the bag (Fig. 12, A}. Surrounding 



the whole structure, but lying chiefly to 



one side of it, is the protoplasmic cell from 



which it has been formed. When the 



dart cell has reached the surface of the 



skin, there projects from it a tiny proto- 



plasmic thread known as the " trigger hair " 



(Fig. 12, t\ for when this is touched the 



dart cell immediately "explodes," flinging 



out the thread as in Fig. 11, B. The 



thread enters the skin of the victim, 



carrying with it, apparently, some poison, 



for it has a paralysing effect on such small 



prey as water- fleas, etc. Some of these 



dart cells have little recurved barbs at the 



base of the thread, but these are absent in 



the smallest darts. 



Such specialised structures as these dart 

 cells show a great advance on anything Flo 12. Hydra, Dart 

 found amongst the simple organisms of Cells or Nematocysts. 



the Protozoa. 



Close to some of these dart 



,, 



cells have been found small, 

 irregular cells, with large nuclei, 



Nerve Cells. 



very 



, Before using ; p, proto- 

 plasm of cell in which 

 the thread is formed ; t, 

 "trigger hair"; nu, 

 nucleus. B, The same 

 cell after the thread has 

 been extruded. 



resembling the nerve cells of higher forms, 

 and it is probable that in Hydra they 

 function as a very rudimentary nervous system. 

 Digestive The digestive cells, which form the inner layer 



Cells. of the body-wall, are less specialised, though they 

 are considerably larger than the skin cells ; in some ways 

 they resemble the primitive amoebiform cell. They lie closely 

 packed, side by side, except for their inner ends, which pro- 

 ject freely into the body -cavity and are very irregular in 

 shape, some ending in blunt processes, and others in whip- 

 like threads, which may at any moment be withdrawn, and a 

 blunt process projected instead. These cells have, therefore, 

 to some extent, a power of individual movement comparable 

 to the movement of some Protozoa. 



Digestion may take place in two ways. Some of the 

 digestive cells seem to be specially glandular, and they secrete 



