254 



GENERAL ZOOLOGY 



Fig. 127, Hy'dra, see p. 263). When any small animal strikes 

 against the minute bristle (Fig. 127, A, 3) that arises from the 

 external surface of one of these cells, the 

 capsule breaks, and a slender, hollow 

 thread or filament (Fig. 127, B, 4) shoots 

 out; reversing itself from the base, like 

 the pharynx of the sand worm, and pierces 

 the skin of the creature. In the capsule 

 there is some fluid which is discharged 

 through the filament and poured into the 

 wound. The effect of the fluid is to be- 

 numb the victim. 



A sea-anemoHe resting in the attitude 

 of the one shown in Fig. 128 could not 

 swallow anything, even after the victim 

 had been overcome. The reason is that 

 the minute cilia which cover the tentacles 

 wave outward at all times. As long as 

 particles of wasfe and other undesirable 



FIG. 127. Nettle-Cells substances fall within range of the cilia, 

 of Hydra. Much,! ,-,1 -, > 



enlarged. (After they are swept out of the way ; but when- 



ever small animals come within range of 

 the cilia, and are paralyzed by the net- 

 tling filaments, the tentacles bend over 

 with their tips pointing toward the 

 mouth. The normal action of the cilia 



Schneider) 



A, nettle-cell, with un- 

 discharged nettling 

 capsule; B, nettle- 

 cell, with discharged 

 nettling capsule : 



1, nettling capsule ; 



2, nucleus of nettle- now sweeps the helpless creatures toward 

 cell: 3, bristle of ,! , ,1 A -n ,-, 

 nettle-cell ; 4, hollow the sea-anemone s mouth. All over the 



filament of nettling thick, grooved lip there are more cilia. 

 The cilia at the opposite ends of the 



mouth lie in a deep, wide groove called the sipTionoglyphe 

 (Fig. 129, 3). Here the cilia always wave inward. At the 

 sides of the mouth the cilia wave outward except when food 

 is dropped on them by the cilia of the tentacles ; then they 



