ZOOPHYTES 467 



the digestive cavity. The sudden arrest of activity on the part 

 of the unfortunate water-flea is brought about by means of minute 

 " thread -cells" or " nettling- cells ", which, as will be explained 

 elsewhere, are poisoned weapons of .considerable virulence. 

 Numerous groups or "batteries" of these are present upon the 

 tentacles, on which they confer a roughened appearance, and 

 they are also present, though less abundantly, upon other parts 

 of the body. Nettling -cells are very characteristic of the 

 Ccelenterates, a fact which has been painfully brought home to 

 many bathers in the case of some of the larger jelly-fish, while 

 even the innocent-looking sea-anemones have been known to 

 seriously inconvenience persons who happened to touch them. 

 Even, therefore, if Hydra, like many of its brethren, were con- 

 demned to remain fixed in the same spot it would not necessarily 

 be starved, but, as already hinted, it possesses considerable powers 

 of locomotion. It can, for example, slowly shuffle along upon 

 the attached end of the body, and can also execute looping move- 

 ments by alternately attaching the two ends of its body to the 

 surface of a stone or water-weed, reminding one of what happens 

 in the case of a leech (p. 432) or a looper caterpillar (p. 364). 

 Specimens may also be found floating freely in the water, mouth 

 downwards, with the foot close to the surface and held in place 

 by what is known as surface tension. 



Structure of the Body (fig. 287). — Most of the external 

 characters of Hydra have now been dealt with, and it remains 

 to consider the minute structure of the body as determined by 

 means of the microscope. Much can be learnt by examination 

 of a thin cross-section, which may instructively be compared with 

 a similar section through the body of an earth-worm. There is 

 a central space corresponding to the digestive cavity, and outside 

 this the body-wall, which is clearly made up of an outer layer 

 and a much thicker inner layer, the two being separated by a 

 thin membrane (supporting lamella). These two layers are 

 respectively known as the ectoderm (Gk. ektos, outside; derma, a 

 skin) and endoderm, (Gk. endon, within; derma, a skin), and a 

 double body-wall made up in this way is eminently characteristic 

 of Ccelenterates and Sponges, especially in embryonic stages, on 

 which account these two phyla are often grouped together as 

 the " Diploblastica" (Gk. diplous, double; blastos, germ or bud). 

 Turning now to the section of earth-worm, we see in the middle 



