f »2^3? ) 
moft kinds of Fijh^ and InfeUs^ wants both moveaUe Ribs, 
and Lungs, and confequentiy have no dilatable Thorax^ ytt 
that want is madp up to 'em by an Analogous MQch^nilmy 
anfwering fufEciently the NeceiTities of their Life. 
Thofe Fifties, which have no Lungs, have Gi///, which 
do the Office of Lungs, receiving and expelling alternately 
the Wa-^r,: whereby the BhodVcjJels fuffer the fame Alte- 
ration of X)imenfiona, that they do in the Lungs of more 
perfeft Animals. 
The Lungs or Air VeJJels of Infefts are yet exceedingly 
more different in Strudure, Diftribution and Scituation from 
thofe of perfed Animals, than thofe of Fifties are, and yet 
in their Ufe and A£ltion agree perfeftly with both, that is, 
receiving and excelling the Air, and varying the Dimenjions 
and Capacities of the Blood Vejfels. - Thefe having no Tho- 
rax^ or feparate Cavity for the Heart and AirVeflels, have 
the latter diftributed thro the whole Trunk of their Bodies, 
by which they communicate with t\\Q External Air thro 
fiveral Spiracula or Vent Holes, to which are faften'd fo ma- 
ny little Tracheae, or Wind-pipes, which thenc|^:fend their 
Branches to all the Mufcles, and Vifcera, and feem to ac- 
company the Blood Veffels all over the Body, as they do 
in the Lungs only of Perfed Animals. By this difpofition 
in every Injpiration, the whole Body of thefe little Animals 
is inflated, and in every expiration comprefs'd, and confe- 
quentiy the Blood Veffels mufl: fuffer a vicijjitude of Exten- 
fion and Conrraftion, and a greater motion mufl: thereby be 
imprefs'd upon the Fluids contained in 'em, than the Heart, 
which does not in thefe Creatures appear- to be Mufcular, 
feems capable 6f giving. 
The only Animal that is exempted from this necefJary 
condition of Breathings or receiving and expelling alternately 
fome fluid into and out of . the Body, is a F<etus. But this, 
while included in the Womb, has Httle more than a vege- 
tative Life, and ought fcarce to be reckon'd among the 
iaumber of Animals. For, were it not for that fmall Aare 
of 
