128 BIOLOGY: GENERAL AND MEDICAL 



of ectodermal and entodermal cellular layers, and is 

 hollow, the space communicating with the general 

 body cavity. The ectodermal cells of the tentacles, 

 however, present a peculiar specialization not seen in 

 other parts of the ectoderm, namely, the possession of 

 certain " nettling cells," which are intended to aid in 

 securing the microscopic organisms, upon which the 

 animal preys, by stinging or stunning them in order that 

 they may be better grasped and introduced into the 

 body cavity. Each of these nettling cells contains a 

 beautiful mechanism consisting of a capsule in which a 

 long stinging filament is closely coiled. A second 

 small filament, trigger or cnidocil, projects from the 

 cytoplasm. When the trigger contacts with a suitable 

 object, the trap springs and the filament is suddenly 

 thrown out against it with stinging and paralyzing 

 effect. In addition to the nettling cells the ectoderm 

 seems to contain certain primitive muscle cells which 

 increase the mobility of the tentacles. 



Though the tentacles are primarily organs of pre- 

 hension they also serve as organs of locomotion; for, 

 should a change of position be advantageous, the hydra 

 bends over, seizes an object with its tentacles, lets go its 

 foothold, gradually turns over and effects a new attach- 

 ment elsewhere. 



Leaving the hydroids and passing to the higher coelen- 

 terates a functional differentiation soon separates pre- 

 hension, which is limited to the tentacles, from locomo- 

 tion which is effected by the development of muscle 

 cells in the umbrella as in medusa where the alternate 

 contraction and expansion enables the animals to swim 

 about. 



Distinct locomotory appendages appear in the seg- 

 mented worms in the form of bristles or setae attached 

 to each segment and directed backward. As the muscular 

 movements force the body of the animal forward these 

 appendages catch upon irregularities of the surface upon 

 which it moves, and prevent retrogression of the ad- 



