Caught a small animal shaped like a wood-louse (Cymothoa) having nine rings apparent on the back, and I think seven legs on each side, also, a tail-like fin on each side, which, when closed under its belly, formed a sort of shield for the lower part of the abdomen. Antennae, transparent with pale brown tips, and a few pale brown spots in them, colour pale blue down centre of the back, dark prussian blue on each side. It had the power of rolling itself up nearly double; in the same manner as a wood-louse, but not quite so close; eyes distinct and prominent. It lived a long time out of water, and appeared to me exactly like an animal I caught on the 21st November 1837, in south latitude 24 degrees 19 minutes; east longitude 107 degrees 8 minutes.
We also this day caught a Janthina. They have a little valve for the purpose of taking in air, with which to expand their float. These animals go in company. They emit when touched a brilliant scarlet dye. A similar animal caught on the 20th November 1837, in south latitude 25 degrees 12 minutes; east longitude 106 degrees 49 minutes, emitted a violet-coloured dye. The emission of this evidently depends upon their being irritated, as I found by many experiments.
The method in which this animal fills its float is curious, it throws it back, and gradually lifts the lip of the valve out of water, until the valve stands vertical, it then closes the valve tightly round a globule of air, around which it folds, by means of the most complex and delicate machinery. The valve is then bent over until it touches the edge of the float nearest the head, and when it is in this position, the portion of it which is inflated with air looks like a bladder, the air gradually is expelled into the float, and as this process takes place the bladder in the valve diminishes, and the valve becomes by degrees like a lip pushed forwards until it lies flat on the float. The valve is composed of two portions, a cup and a lip. The time occupied from first removing the valve from the float, until the inflation, and the expulsion of air into the float being completed, so that the valve begins to move again, is 61 seconds, from the mean of several experiments.
These animals have also the power of compressing the valve into a hollow tube, which they elevate above the water like a funnel, and draw down air through it.
The colouring matter which they emit has no stinging, electric or deleterious properties whatever, that I could discover. I found that when this colouring matter was mixed with water, it became of a deep blue. In those which I caught in November 1837, I may have been deceived, and the colouring matter might also possibly have been scarlet directly it was emitted. It is difficult to conceive what use this liquid can be to the fish against its foes, yet it certainly uses it as a means of defence.
To one of these shells, the fish in which was alive and well, we found attached a number of barnacles, some of which were of large size.
This sort of Janthina was very abundant; today we caught eight, and saw great numbers of them: yesterday we caught a smaller one of a different species. (Janthina exigua.)
This kind of Janthina is attached to its float by a sort of peduncle, which it has the power of elongating, so that the fish itself sinks, with its shell, and yet remains attached to the float, which continues at the surface. In one instance, I saw this peduncle elongated to a length of 0.9 inches. It may, of course, have the power of sinking itself much lower than I have seen it do. When it is in this state, the apparatus with which it fills the float remains behind the peduncle in a state of perfect quiescence.
The scarlet fluid emitted by this animal is of such a consistency that it can be drawn away from it out of the water, like a glutinous thread.
A part of the animal requires attention, it is composed of an outer cup, or circular lip, which it has the power of contracting or expanding in the same manner as the valve; and when opened out like a cup, an orifice can be seen at the bottom of it. It can also expand, and make broad the arm; and it then appears to use them as sails.