226 HYDRA LESS. 



degree of contractility is assigned to the muscle-processes 

 while the cells themselves are eminently irritable, the slightest 

 stimulus applied to them being followed by an immediate 

 contraction of the whole body. 



Imbedded in some of the large ectoderm cells are found 

 clear, oval sacs (A and B, ntc\ with very well-defined walls 

 and called nematocysts. Both in the living specimen and in 

 sections they ordinarily present the appearance shown in 

 Fig. 50, B. ntc, and Fig. 51 A, but are frequently met with 

 in the condition shown in Fig. 50 E, and Fig. 51 B: that 

 is, with a short conical tube protruding from the mouth of 

 the sac, armed near its distal end with three recurved 

 barbs besides several similar processes of smaller size, 

 and giving rise distally to a long, delicate, flexible fila- 

 ment. 



Accurate examination of the nematocysts shows that the 

 structure of these curious bodies is as follows. Each con- 

 sists of a tough sac (Fig. 51, A), one end of which is turned 

 in as a hollow pouch : the free end of the latter is continued 

 into a hollow coiled filament, and from its inner surface 

 project the barbs. The whole space between the wall of 

 the sac and the contained pouch and thread is tensely filled 

 with fluid. When pressure is brought to bear on the outside 

 of the sac the whole apparatus goes off like a harpoon-gun 

 (B), the compression of the fluid forcing out first the barbed 

 pouch and then the filament, until finally both are turned 

 inside out. 



It is by means of the nematocysts the resemblance of 

 which to the trichocysts of Paramcecium (p. 113) should be 

 noted that the Hydra is enabled to paralyze its prey. Prob- 

 ably some specific poison is formed and ejected into the 

 wound with the thread : in the larger members of the group 

 to which Hydra belongs, such as jelly-fishes, the nematocysts 



