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Microscopical Essays. 



difappeanng entirely. He foon after found the hydra grifea, Fig, 

 6, and law it eat, f wallow, and digeft worms much larger than 

 ■itfelf. This difcovery was foon followed by that of the hydra 

 fufca, Fig. 7, Plate XXI. 



The moll general attitudes of thefe bydrae are thofe which are 

 reprefented in Fig. 5 and 6, Plate XXL They fix the pofterior 

 extremity I? againft a plant, or other fubftance, as e f; the body 

 a b, and the arms a c, being extended in the water. There is a 

 fmall difference in the attitudes of the three kinds- which we are 

 now defcribinsr. 



The bodies of the hydra viridis, Fig. 5, and of the hydras 

 grifea, Fig. 6, diminilh from the anterior to the pofterior ex- 

 tremity by an almoft infenfible gradation. The hydra fufca does 

 not diminifh in the fame gradual manner, but from the anterior 

 extremity a to the part d, which is often two-thirds of the length 

 of their body, it is nearly of an equal fize ; from this part it be- 

 comes abruptly fmaller, and goes on from thence of a regular 

 fize to the end. The number of arms in thefe three kinds are at 

 lea ft fix, and at molt twelve or thirteen, though eighteen may 

 fometimes be found on the hydra grifea. They can contracl their 

 bodies till they are not above one-tenth of an inch in length ; 

 they can alio ftop at any intermediate degree, either of contrac- 

 tion or extenuon, from the greateft to the leaft. The fpecies 

 reprefented at Fig. 5 are generality* about half an inch long 

 when ftretched out. Thofe exhibited at Fig. 6 and 7 are about 

 three-fourths of an inch, or one inch, in length, though fome are 

 to be found at times about an inch and an half long. The arms_ 

 of the hydra viridis, Fig. 5, are feldom longer than their bodies ; 



thofe* 



