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In the Torpedo and the Fishing-frog the breadth of the anterior part
of the body is not so great as to render locomotion by fins
impossible, but in consequence of it the upper pair (pectorals) are
placed further back and the under pair (ventrals) are placed close
to the head, while to compensate for this advancement they are reduced
in size so as to be smaller than the upper ones. In the Torpedo the
two upper fins (pectorals) are placed on the tail, and the fish uses
the broad expansion of its body to supply their place, each lateral
half of its circumference serving the office of a fin.
The head, with its several parts, as also the organs of sense,
have already come under consideration.
There is one peculiarity which distinguishes fishes from all other
sanguineous animals, namely, the possession of gills. Why they have
these organs has been set forth in the treatise on Respiration.
These gills are in most fishes covered by opercula, but in the
Selachia, owing to the skeleton being cartilaginous, there are no such
coverings. For an operculum requires fish-spine for its formation, and
in other fishes the skeleton is made of this substance, whereas in the
Selachia it is invariably formed of cartilage. Again, while the
motions of spinous fishes are rapid, those of the Selachia are
sluggish, inasmuch as they have neither fish-spine nor sinew; but an
operculum requires rapidity of motion, seeing that the office of the
gills is to minister as it were to expiration
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