Microscopical Essays. 403 



thofe of Fig. 6 are commonly one inch long, while thofe of Fig. 

 7 are generally about eight inches ; whence Mr. Trembley has 

 called it the long-armed polype. 



The bulk of the hydras decreafes in proportion as they extend 

 themfelves, and vice verfa. They may be made to contract them- 

 felves, either by touching them, or agitating the water in which 

 they are contained. They all contract themfelves fo much when 

 taken out of the water, as to appear only like- a little lump of 

 jelly. They can contract, or extend their arms without extending 

 or contracting the body, or the body without making any altera- 

 tion in the arms : or they can contract or dilate only fome of the 

 arms, independent of the reft: they can alfo bend their body and 

 arms in all poffible directions. Thofe reprelented at Fig. 7, let 

 their arms in general hang down, making different turns and re- 

 turns, often directing fome of them back again to the top of the 

 water. They can alfo dilate the body at different places, fome- 

 times at one part, and then again at another ; fometimes they are 

 thick fet with folds, which, if careiefsly viewed, might be taken 

 for rings. 



They have a progreffive motion, which is performed by that 

 power by which they it retch out, contract, and turn themfelves 

 every way. For fuppoie the hydra, or polype, a b, Fig. 16, 

 plate XXL to be fixed by the tail b, having the body and arms a 

 extended in the water, in order to advance, it draws itfelf toge- 

 ther, by bending itfelf io as to bring the head and arms down to 

 the fubftance on which it is to move; to do this, it fixes the 

 head, or the arms, as in Fig. 17 ; when thefe are well fixed, it 

 loofens the tail, and draws it towards the head, as in Fig. 18, 



3 C 2 which 



