403 



Microscopical Essays. 



It has been already obferved, that it is to thefe grains that it owes 

 it's magreen-like appearance ; it is from them alio that it derives 

 it's colour ; for w hen they are feparated from the polype, they 

 aie the lame colour with it ; whereas theglarous matter is without 

 any diftinguiihing colour. The conftruclion of the polype feems 

 then to be confined to thefe glandular grains, to the vifcous mat- 

 ter, and the invifible lib res which aft upon the giarous fu I fiance. 



The flruclure of the arms of the polypes are very analogous to 

 that of their body. When they are examined by the microfcope, 

 either in a contracted or dilated ftate, their furface is fhagreened ; 

 if the arm be much contracted, it appears more fo than the body; 

 on the contrary, it appears lefs fo in proportion as they are more 

 extended, almofl quite fmooth when at their full extenfion. ; fo 

 that in the hydra viridis the appearance of the arms is continually 

 varying, and thefe variations are more fenfible towards the ex- 

 tremity of the arm than at it's origin, as at Fig. 10, Plate XXL 

 but more thinly fcattered, or farther afunder, in the parts further 

 on, as at Fig. 9, Plate XXL The extremity is often terminated 

 by a knob, the hairs which are exhibited in Fig. 9, cannot be 

 feen without a very deep magnifier, however they indicate a fur- 

 ther degree of organization in this little animal. 



All animals of this kind have a remarkable attachment to turn 

 towards the light, and this might naturally induce the inquirer to 

 look for their eyes 5 but how carefully foever this fearch has been 

 purfued, and however excellent the microfcope with which every 

 part has been examined, yet no appearance of this organ was 

 found. Notwithstanding this, they conflantly turn themfelves 

 towards the light ; fo that if that part of the glafs in which we 



placed 



