In this section are included some of the most beautiful, as well as common of the tribe. The forms of the umbrella are often most lovely, and present an astonishing variety. As an example of the beauty which they sometimes display, we may refer to a species which resembles an exquisitely formed glass-shade, ornamented with a waved and tinted fringe. The most perfect grace of form, the transparency of the crystal, and colour as delicate as that of the flower, combine to render this frail being one of the loveliest of living things.
In another section, locomotion is effected by a modification of ciliary apparatus. We have a familiar example in the Beroe of our own seas, a most attractive little being, and a prime favourite with naturalists, who have described its habits and celebrated its beauty with enthusiasm. We shall not soon forget the delight with which we first made acquaintance with this graceful little rover. While rambling along the shore in quest of marine animals, our attention was arrested by a drop of the clearest jelly, as it seemed to be, lying on a mass of rock, from which the tide had but just receded. On transferring it to a phial of sea-water, its true nature was at once revealed to us. A globular body floated gracefully in the vessel, scarcely less transparent than the fluid which filled it. Presently it began to move up and down within its prison-house, and the paddles by means of which the beroe dances along its ocean-path were distinctly visible. These paddles are nothing more or less than cilia of a peculiar kind, ranged in eight bands upon the surface of the body. They are set in motion at the will of the animal, and their incessant strokes propel it swiftly through the water. By stopping some of its paddles, and keeping others in play, the beroe can change its course at pleasure, and so wander 'at its own sweet will,' through the trackless waste. Beauty waits upon the course of this little crystal globe. The grace and sprightliness of its movements must strike the commonest observer. As the sunlight falls upon its cilia, they are 'tinted with the most lovely iridescent colours;' and at night they flash forth phosphoric light, as though the little creature were giving a saucy challenge to the stars.
The beroe is a most active being, its habits conforming to the organisation with which it is endowed. Such an array of paddles prophesies of a mercurial temperament and an energetic character. It can, however, anchor itself and lie by when occasion offers. It is provided with two long cables, prettily set with spiral filaments or tendrils, by means of which it can make fast to any point. When not in use, it can retract them, and stow them away in two sacs or pouches within the body, where they may be seen coiled up, through the transparent walls. The mouth is a simple opening at one pole of the globular body. No arms are needed. The beroe is spared the labour and uncertainty of the chase. As it dances gaily along, streams of water, bearing nutritive particles, pass through the orifice into its stomach.
In this creature, as in many of the lower animals, there is a remarkable power of retaining vitality after the most serious injuries; nay, in portions actually severed from the body, it will continue for some time. Mr Patterson, in his excellent Introduction to Zoology, mentions that on one occasion he divided a fragment of the body of a beroe, lately taken from the shore and shattered by a storm, 'into portions so minute that one piece of skin had but two cilia attached to it, yet the vibration of these organs continued for nearly a couple of days afterwards!' But we must leave the beroe, charmer though it be.
Another member of this section—the Ciliograde acalephæ, as they are called—is the Girdle of Venus, which resembles a ribbon in form, and is sometimes five or six feet in length, covered with cilia, and brilliantly phosphorescent. This must be one of the most beautiful of the fireworks of the ocean.
The jelly-fishes of another section are furnished with one or more air-bags, which assist them in swimming, and hence bear the name of hydrostatic acalephæ. In the Portuguese man-of-war (Physalia), the bag is large, and floats conspicuously on the surface of the water. From the top of it rises a purple crest, which acts as a sail, and by its aid the little voyager scuds gaily before the wind. But should danger threaten—should some hungry, piratical monster in quest of a dinner heave in sight, or the blast grow furious—the float is at once compressed, through two minute orifices at the extremities a portion of the air escapes, and down goes the little craft to the tranquil depths, leaving the storm or the pirate behind. In one species (Cuvieria), the floats are numerous and prettily ranged round the margin of the body. Resting on these, the creature casts about its long fishing-lines, and arrests the passing prey.
One more section remains to be noticed. The jelly-fishes which belong to it have a rudimentary skeleton—a plate which supports the soft, circular body. From the lower part of the body hang numerous tentacles (cirri), amidst which the mouth is placed. Probably these multitudinous arms assist in locomotion; and, hence the name of the family, Cirrigrades. Amongst the creatures of this division we meet with some very interesting locomotive apparatus. There are some of them by no means obliged to trust to their oars alone—they have also sails. The Velella, large fleets of which visit our seas at times, has a plate (the mast) rising from its bluish disk or deck, covered with a delicate membrane (the sail) of snowy whiteness, by means of which it traverses the ocean. This sail, it has been noticed, 'is set at the same angle as the lateen-sail' of the Malays. We cannot doubt that it is admirably suited to its purpose, and the Malays may be proud of having nature as a voucher for their contrivance.
We find in another species a still more perfect rigging. In it (Rataria) the crest is supplied with muscular bands, by means of which the sail can be lowered or raised at pleasure. These adaptations of structure are full of interest. Nothing can be more admirable than the sailing-gear of these little creatures. They have to traverse the surface of the ocean amidst all diversities of weather. Paddles alone would not suffice for them. They must be enabled to take advantage of the winds. Sails, therefore, are added, and the mightiest agents in nature are commissioned to speed the little voyagers on their way.
We have already mentioned that some of the jelly-fishes possess the power of stinging. Only a few of the larger species, however, seem to be thus endowed; and the name sea-nettle is by no means applicable to the class as a whole. The poisonous fluid which produces the irritating effect on the skin, and no doubt paralyses the creatures upon which the jelly-fish feeds, is secreted by the arms. By means of its poison-bearing tentacles, the soft, gelatinous medusa is more than a match for the armed crustacean and the scale-clad fish. We take from Professor Forbes the following graphic description of one of the stinging species:—'The Cyanæa capillata of our seas is a most formidable creature, and the terror of tender-skinned bathers. With its broad, tawny, festooned, and scalloped disk, often a full foot or more across, it flaps its way through the yielding waters, and drags after it a long train of ribbon-like arms, and seemingly interminable tails, marking its course when its body is far away from us. Once tangled in its trailing "hair," the unfortunate who has recklessly ventured across the graceful monster's path too soon writhes in prickly torture. Every struggle but binds the poisonous threads more firmly round his body, and then there is no escape; for when the winder of the fatal net finds his course impeded by the terrified human wrestling in its coils, he, seeking no contest with the mightier biped, casts loose his envenomed arms, and swims away. The amputated weapons severed from their parent body vent vengeance on the cause of their destruction, and sting as fiercely as if their original proprietor itself gave the word of attack.'
We now approach the most extraordinary portion of the history of these creatures. Recent investigations have brought to light the most interesting facts respecting their reproduction and development. It is now known that the young jelly-fish passes through a series of transformations before reaching its perfect state.