This image comes from Philip Henry Gosse’s A Year at the Shore, specifically, the month of October. (The year isn’t specified, but the book was published in 1865.)
Gosse was a naturalist, prolific author, scientific illustrator, and fellow of the Royal Society. Here’s how he describes the larger creature in this painting:
Perhaps there is scarcely any that takes a stronger hold on the fancy, certainly none is more familiar, than a little thing that he daily marks floating in the sunlit’ waves, as the ship glides swiftly by, which the sailors tell him is the Portuguese man-of-war. Perhaps a dead calm has settled over the sea; and the observer, as he leans over the bulwarks of the ship, has opportunities of scrutinizing the ocean-rover at leisure, as it lazily rises and falls on the long sluggish heavings of the glassy surface. Then he sees that the comparison of the stranger to a ship is a felicitous one; for, at a little distance it might well be mistaken for a child’s mimic ship, shining in all the gaudy painting in which it came out of the toy-shop; and he is ready to pity the forlorn urchin in tunic and knickerbockers, whose cherished boat has broken her moorings of thread, and drifted with winds and tides far, far out of reach of land.
Not unfrequently does one of the tiny vessels come so close alongside that, by means of the ship’s bucket, with a little assistance from a smart fellow, who has jumped into the “chains” with a boat-hook, it is captured, and brought on deck to be subjected to scientific examination. A dozen voices are however lifted, warning you by no means to touch it, for well the experienced seaman knows its terrific powers of defence. It does not now appear so like a ship as when it was at a distance. It is an oblong bladder of tough membrane, varying considerably in shape (and hence no two original figures agree in this respect), and also in size, from less than an inch in length to the size of a man’s hat. Once in a voyage to Mobile, when rounding the Florida Reef, I was nearly a whole day passing through a fleet of these little Portuguese men-of-war, which studded the smooth sea us for as the eye could reach, and must have extended for many miles. These were of all sizes within the limits I have mentioned. Generally, there is a conspicuous difference between the two extremities of the bladder, one end being rounded, while the other is more pointed, or is terminated by a small knob-like swelling, or beak-shaped excrescence, where there is a minute orifice. Sometimes, however, no such excrescence is visible, and the orifice cannot be detected.
The bladder is filled with air, and therefore floats almost wholly on the surface. Along the upper side, nearly from end to end, runs a thin ridge of membrane, which is capable of being erected at the will of the animal to a considerable height, fully equal at times to the entire width of the bladder, when it represents an arched fore-and-aft sail, the bladder being the hull. From the bottom of the bladder, near the thickest extremity, where there is a denser portion of the membrane, depends a crowded mass of organs, most of which take the form of very slender, highly contractile, and moveable threads, which hang down into the deep to a depth of many feet, or even occasionally, of several yards.
The colours of this curious creature are very vivid : the bladder, though in some parts transparent and colourless, and in some specimens almost entirely so, is in general painted with the richest blues and purples, mingled with green and crimson to some less extent; these all being, not as sometimes described, iridescent or changeable reflections, but positive colours, independent of the incidence of light, and, for the most part possessing great depth and fulness. The sail-like erectile membrane is transparent, tinted towards the edge with a lovely rose-pink hue, the colours arranged in a peculiar fringe-like manner.
The other creatures are given similar similes:
The nectocalyx or swimming-bell is; however, seen in a much more developed condition in the delicate forms which swarm in our harbours and about our rocky coasts in summer and early autumn, and are known by the name of Naked-eyed Medusae. A common and very characteristic example of this order is the Tongued Sarsia, in which a single swimming-bell forms the greatest portion of the whole creature. It is a tall dome of crystalline colourless flesh, thick at the summit and thinning off at the edges; about half’ an inch in height. From the interior of this dome hangs the single polypite, exactly as the clapper hangs from the top of a bell. It is long and cylindrical, abruptly attenuated at the upper part into a sort of foot-stalk, and at the extremity enlarging into four fleshy, very protrusile, lips, capable of seizing and sucking in an object much larger than the diameter of the whole polypite. I have seen a Sarsia, in confinement in a tank, lay hold of, and swallow, a newly-hatched fish, notwithstanding the activity of the latter. For hours afterwards the little green-eyed fry was visible, the engulphment being a very slow process ; the Medusa, however, never let go its hold; and gradually the tiny fish was sucked into the interior, and passed up the cavity of the polypite, becoming more and more cloudy and indistinct, as digestion in the stomach gradually dissolved its tissues.
These little creatures are endowed with very effective powers of locomotion. In the unbounded freedom of their native sea, and in the limited dimensions of a glass vase, they are alike sprightly. By rapid pump-like contractions of their nectocalyx, they dart through the water, and shoot round and round, almost with the force and swiftness of a swimming fish. The summit of the bell always goes foremost, whether the direction of the movement be vertical, horizontal, or, as is most commonly the case, oblique; and the tentacles, and the long white polypite, drag behind in trailing lines. Now and anon, the shooting is suddenly suspended, the bell hangs over and remains awhile motionless, the tentacles are allowed to depend like spiders’ webs, or are suddeniy drawn up into shrivelled puckers, become mutually entangled and intertwisted, then slowly free themselves, and hang down again. Sometimes the motionless bell itself sinks very gradually, and the tentacle-threads take the most elegant curves and arches in their descent.