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[49]

1832.                     Diodon . Bahia.

March 10th a Diodon was caught swimming in its unexpanded form near to the shore. Length about an inch: above blackish brown, beneath spotted with yellow.

[in margin: (132)]

On head four soft projections; the upper ones longer like the feelers of a snail. Eye with pupil dark blue; iris yellow mottled with black. The dorsal caudal and anal fins are so close together, that they act as one. These as well as the Pectorals, which are placed just before the branchial apertures, are in a continual state of tremulous motion, even when the animal remains still.

The animal propels its body by using these posterior fins, in same manner as a boat is sailed, 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.

When handled a considerable quantity of a fine "Carmine red" fibrous secretion was emitted from the abdomen, and stained paper, ivory &c. of a bright colour. The fish has several means of defence it can bite hard, and can squirt water to some distance from its mouth, making at the same time a curious noise with its jaws.

After being taken out of water for a short time, and then placed in again, it absorbed by the mouth (perhaps likewise by the branchial apertures) a considerable quantity of water and air, sufficient to distend its body into a perfect globe.

This process is effected by two methods; the air is swallowed and then forced into the cavity of the body; its return being prevented by a muscular contraction which is externally visible. The water however, I ob

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[18]32                    Diodon. Bahia

served entered in a stream through the mouth which was distended and motionless; hence this latter action must have been caused by the dilatation of the animal, producing suction.

When the body is thus distended the papileæ, with which it is covered become stiff, the above mentioned tentacula on the head, being excepted. The animal being so much buoyed up, the branchial openings are out of water, but a stream regularly flowed out of them which was as constantly replenished by the mouth.

After having remained in this state for a short time, the air and water would be expelled with considerable force, from the branchial apertures and the mouth. The animal at its pleasure could emit a certain portion of the water and I think it is clear that the fluid is taken in partly for the sake of regulating the specific gravity of its body.

The skin about the abdomen is much looser than that on the back and in consequence is the most distended; hence the animal swims with its back downwards.

Cuvier doubts their being able to swim when in this position; but they clearly can not only swim forward, but also move round. This they affect not like other fish, by the action of their tails, but collapsing the caudal fins they move only by their pectorals.

When placed In fresh water seemed singularly little inconvenienced.