Of the frejh Water Polype. ijj 
its pofterior end b again# the fide of a glafs ; its bodj% 
and moft of its arms being ftretched out forwards, and 
one of its arms a c, fixed again# the glafs at c: when 
the polype is in this attitude, it loofens its pofterior end 
b, and contracting its body, draws it up a little nearer to 
the point c, and fixes it again# the glafs at d, after which 
it repeats the fame and fixes its pofterior end at e, and fa 
on. All that is here faid of the polypes prognsflive mo¬ 
tion equally refpects the three forts. 
They defcend to the bottom of the water, and come 
up either by the fides, or upon aquatic plants, and often 
hang down from the furface thereof, by their pofterior 
end, as at b, fig. 334. and are often feen to fufpend them- 
felves by one arm only, as at c, fig. 339. They walk 
as well upon the fuperficies of the water, as upon thofe 
bodies juft mentioned ; and perform the fame motions in 
a glafs, as they do in greater waters: They pafs over 
plants or other bodies ; they go up the fides of glafs, even 
to. the furface of the water, and pafs either under or over 
it, and fometimes reft themfelves there; then they march 
to the oppoiite fide of the glafs, and fo defcend to the 
bottom. 
If you examine the extremity of a polype’s tail, while 
it is fufpended, as at b, fig. 334. (from the furface of the 
water) with a magnifying glafs, it will be found a little 
out of the water, fomewhat concave and dry, as at b and. 
c > %• 359* an d to prove that this circumftance is afo- 
folutely neceflary to fupport them on the fuperficies of the 
water, only wet the dry end with a drop of water, arid 
the polype will immediately fall to the bottom. 
When a polype defigns to pafs from the fides of a glafs, 
to the fuperficies of the water, it need only put that part 
out of the water by which it vrould be fupported, and 
give it time to dry, which is what it always does, and 
what 
