534 



HYDRA, OR FRESH-WATER POLYPE. 



arranged with great regularity around the orifice. The body 

 is prolonged at its lower end into a narrow base, which is fur- 

 nished with a suctorial Disk ; and the Hydra usually attaches 

 itself by this, while it allows its tendril-like tentacula to 



float freely in the 

 Fig. 274. water, like so many 



fishing-lines. The 

 wall of the Body is 

 composed of Cells im- 

 bedded in Sarcode- 

 substance ; and it 

 consists of two prin- 

 cipal layers, an outer 

 and more compact, 

 of which the cells 

 form a tolerably- 

 even surface, and an 

 inner that lines the 

 Stomach, into the 

 cavity of which some 

 of the cells project. 

 Between these layers 

 there is a space 

 chiefly occupied by 

 undifferentiated Sar» 

 code, having many 

 'vacuoles' or 'lacunse' 

 (which often seem to 

 communicate with 

 one another) ex- 

 cavated in its sub- 

 stance. The Arms 

 are made-up of the 

 same materials as 

 the body ; but their 

 surface is beset witlj 

 little wart-like pror 

 minences, whichj 

 when carefully ex» 

 amined, are found 

 to be composed of 

 clusters of ' Thread* 

 cells,' having a single; 

 large cell with a long 

 spiculum in the centre of each. The structure of these Thread - 

 cells or ' Urticating organs ' will be described hereafter (§ 428) ; 

 at present it will be enough to point-out that this apparatus, re- 

 peated many times on each Tentacle, is doubtless intended to give 



Hydra fusca, with a young bud at 6, and a 

 more advanced bud at c. 



