Diodon antennatus, _Cuv._ Mém. du Mus. tom. iv. p. 131. pl. 7.
A third species of _Diodon_, brought home by Mr. Darwin, and taken by
him at Bahia, in Brazil, is either the young of the _D. antennatus_ of
Cuvier, or else new; but the only individual in the collection is quite
small, and not more than an inch in length, excluding caudal. The fleshy
filaments above the eyes, which, according to Cuvier, so peculiarly
distinguish the _D. antennatus_, are very distinct,—but I see none on
the sides. The ground colour would seem darker than he describes, so as
to render the spots and markings on the upper parts not distinguishable
from it now, if they ever existed. In spirits it appears of a nearly
uniform deep brown red. The spines, or rather papillæ, are also shorter
than represented in his figure; but this may be only the effect of
immaturity.
According to Mr. Darwin, the colours when recent were as follows:—“Above
blackish brown, beneath spotted with yellow. Eye with the pupil dark
blue; iris yellow, mottled with black.” It is added:—“On the head four
soft projections; the upper ones longer, like the feelers of a snail.”
Mr. Darwin observes, “that the dorsal, caudal, and anal fins, in this
species, are so close together that they act as one: these, as well as
the pectorals, are in a continued tremulous motion even when the fish is
otherwise motionless. The animal propels its body by using the posterior
fins in the same manner as a boat is sculled, that is, by moving them
rapidly from side to side with an oblique surface exposed to the water.
The pectoral fins have great play, which is necessary to enable the
animal to swim with its back downwards.”
Mr. Darwin made some further observations on the habits of this species,
which have already appeared in his “Journal,” to which I may refer the
reader.[47] The tendency of them is to explain the process by which the
water and air are absorbed, when the _Diodon_ distends itself into a
spherical form; and to show that the fish _can_ swim, when floating in
this state with its back downwards, which Cuvier doubted. He thinks that
the water is taken in partly for the sake of regulating its specific
gravity. He also notices a curious circumstance with respect to this
species, viz., “that it emitted from the skin of its belly, when
handled, a most beautiful carmine red and fibrous secretion, which
permanently stained ivory and paper.”