3454 



CCELENTERA TES 



STINGING CAPSULES. 



I. and 2. With retracted filament ; 3. Partly pro- 

 truded ; 4. Fully protruded. 



(Highly magnified.) 



The former process is termed alter- 

 nation of generations; the egg producing a 

 stock, which is one generation, the stock 

 producing a jellyfish, which is a second; 

 and these two alternating. In the latter 

 case, when a jellyfish produces a jelly- 

 fish, one generation the stock has 

 been suppressed. This is important, since 

 there is evidence that this alternation of 

 generations was, and indeed still is, widely 

 spread in both the animal and the vege- 

 table kingdoms. It is however, as a rule, 

 suppressed, as animals rise in the scale 

 of organization. The Ctenophora are a 

 highly developed group in which this al- 

 ternation of generations has been sup- 

 pressed. 



One word as to the changes necessary 

 to turn the simple Cnidarian previously 

 described into a bell or umbrella-like 

 jellyfish. The principal change is in the 

 gelatinous layer between the outer skin 

 wall and the inner stomach or digesting 

 layer. This middle layer develops into 

 an enormous mass of glassy jelly of such 

 a shape that, instead of the body being 

 long from the mouth (oral pole) to the 

 bottom of the sac (aboral pole), the 

 animal is umbrella-like, the mouth being 

 under the bell, while the top of the bell 

 corresponds with the old base by which 

 the parent polyp was attached to the 

 ground. 



Before taking the principal forms 

 assumed by the Cnidaria, and briefly de- 

 scribing their relation to one another, the 

 stinging cells and batteries claim atten- 

 tion. These cells, though all microscop- 

 ical, vary considerably in size, without 

 their structure being essentially affected. 

 The surface protoplasm of the cell is 

 modified into a tolerably firm shell, inclos- 

 ing an oval or cylindrical vesicle. Closely 

 associated with this structure is a pointed 

 process, standing up far above the level 



