480 



CCELENTERA TES. 



is evidence that this alternation of generations was, and indeed still is, widely 

 spread in both the animal and the vegetable kingdoms. It is however, as a rule, 



suppressed, as animals rise in the scale of 

 organisation. The Ctenophora are a highly- 

 developed group in which this alternation 

 of generations has been suppressed. 



One word as to the changes neces- 

 sary to turn the simple Cnidarian above- 

 described into a bell or umbrella - like 

 jelly-fish. The principal change is in the 

 gelatinous layer between the outer skin- 

 wall and the inner stomach or digesting 1 

 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 jDole), the animal 

 is umbrella-like, the mouth being under 

 the bell, while the top of the bell corre- 

 sponds 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 

 describing their relation to one another, 

 the stinging - cells and batteries claim 

 attention. These cells, though all micro- 

 scopical, vary considerably in size, with- 

 out their structure being essentially 

 affected. The surface protoplasm of the 

 cell is modified into a tolerably firm shell, 

 enclosing an oval or cylindrical vesicle. 

 Closely associated with this structure is 

 a pointed process, standing up far above 

 the level of the skin, known as the 

 cnidocil. Within the vesicle is found, 

 either spirally rolled or in an irregular 

 tangle, a long filament, or hollow tube, 

 which is a prolongation of the vesicle, but 

 turned outside in. This tube, which is 

 more than twenty times as long as the 

 cell, is pointed at the tip, and almost up 

 to the tip beset with two rows of fine, 

 spirally-arranged, barbed hooks. When 

 the cnidocil is touched or irritated, this 

 filament is violently shot forth, being 

 turned inside out like the finger of a glove. 



STINGING CAPSULES. 



! With retracted filament ; 3, Partly protruded ; 

 1 Fully protruded. (Highly magnified.) 



