SYSTEM OF NATURE. 
99 
The Pterota of Aristotle are a group of animals un¬ 
dergoing metamorphosis after arriving at their full 
growth. Their skin is incrassated and often indurated, 
affording points of attachment for the muscles, and thus 
supplying the place of an internal skeleton. Their respi¬ 
ration is performed by tracheae or air-tubes distributed 
through the entire body, and communicating with the at¬ 
mosphere by means of exterior oval apertures arranged 
along each side. Their circulation is very anomalous: 
the apparatus for this purpose consists of a series of dor¬ 
sal hearts opening into each other, and forming a great 
longitudinal channel throughout the entire length of the 
body: in this channel a colourless fluid, in which oat¬ 
shaped corpuscules are floating, ascends by regular pulsa¬ 
tions from the posterior towards the anterior extremity : 
immediately before reaching the head the main current is 
discharged right and left, into what appears a general 
reservoir of the body ; its downward and lateral progress 
may be most readily traced by watching the motion of 
the corpuscules : in this downward course a portion is 
received into the great channel by lateral valves, two of 
which are observable in every segment of the body : be¬ 
fore entering these lateral valves the blood seems perfectly 
unrestricted in its course. The skin or skeleton is trans¬ 
versely divided into segments or rings; these are normally 
thirteen in number. All the Pterota have six legs, and 
the representatives of four wings. They are furnished with 
two antennae, which, though usually employed as tactors, 
appear likewise to be the seat of some sense unknown to us. 
The Pterota are moreover distinguished among the articu¬ 
lates by the entire want of the vegetable power of repro¬ 
ducing limbs which have been accidentally lost. 
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