82 



THREAD-CELLS AND DARTS. 



PAET III. 



into a cavity extending throughout the length of the 

 body, which is the stomach ; the other end of the 



sac is narrow, and ter- 

 minates in a disk-shaped 

 sucker, by which the Hy- 

 dra fixes itself to aquatic 

 plants, or floating objects, 

 from whence it hangs 

 down, and the tentacles 

 float in the water. 



The sac or ^ body is 

 formed of two layers, an 

 inner and an outer layer, 

 of firmer texture, formed 

 of cells imbedded in a 

 kind of sarcode, and the 

 space between the two 

 layers is filled with 

 a semifluid substance, 

 mixed with solid parti- 

 cles and full of vacuoles. 

 The inner and outer layers 

 are united at the mouth, 

 and the tentacles are 

 closed tubes in communi- 

 cation with the cavity of 

 the stomach. The ex- 

 terior layer of the tenta- 

 cles is beset with wart- 

 like excrescences, formed 

 of clusters of cells, with 



Pie 109 Thread-cells and darts. A, B, c, D, -, . . , 



Thread-cells at rest ; E, F, o, H, appearance a larger One in the Centre 



filled with a liquid. In 



all of them a long spicula, or sting, often serrated at 

 the edge, is coiled up like a thread, and fixed by one 

 end to a kind of tube, like the inverted finger of a glove, 

 that the animal can dart out in an instant. 



