A Year at the Shore

by Philip Henry Gosse

The manner in which the strong pulsating movements of the Medusæ are performed depends on the position and action of certain bands of muscular tissue. Four of these radiate from the centre of the dome to the margin. This course is not a straight but a curved one. When, therefore, these bands are simultaneously and forcibly contracted in length, they are drawn from a curved into a straight line, and the cavity, which was bell-shaped, becomes more conical, and its capacity is considerably diminished; a portion of the water which it before held is therefore driven out at the mouth, and by its reaction forces the animal forward with a jerk in the opposite direction. Besides these radiating muscles, there are circular bands which pass round the margin and the interior walls of the dome. These by their contraction diminish the volume of the cavity, and aid the action described above.

FORBES’S ÆQUOREA.

The tiny Sarsia has but four tentacles, which spring from as many equidistant points on the margin of the bell. But in the genus Æquorea, of the same family, these organs are far more numerous, two species which I first discovered at Ilfracombe having, the one thirty-six, the other about two hundred tentacles. The former of these, which I have honoured with the name of the late Edward Forbes,[129] differs much in general appearance from the little Sarsia, being a cake-shaped segment of a globe, about three or four inches in diameter, and an inch and a half in thickness. The roof of the interior is low and nearly flat, or indeed dropping slightly in the centre.

Plate 29.
P. H. GOSSE, del. LEIGHTON, BROS.
FORBESIAN ÆQUOREA.

The polypite is peculiar, and would scarcely be recognised as of the same nature with the lively bottle-shaped organs of the Portuguese man-of-war, or the long nimble tongue of the Sarsia. It forms a very wide circle on the flat roof of the bell, whence the four large triangular lips descend, which are cut into a minutely divided fringe of filaments, that wave loosely in the water. There are about seventy slender vessels which radiate from the polypite circle along the roof to the margin, where they join the circular marginal vessel. I have said that the tentacles are about thirty-six in all; that is, about half as many as the radiating vessels, though the relation of number is not exact. They hang down in the usual form, to the unassisted eye appearing as excessively slender whitish or flesh-coloured threads, capable of great elongation, or of contraction into ovate spiral masses, hardly perceptible. But by the aid of the microscope we discern that each tentacle consists of a lengthened fleshy tube, on which are set at pretty regular intervals thickened semi-rings, or knobs, very much like the knobs on the horns of an antelope in appearance. They do not quite encircle the tentacle, and thus one side for the entire length is smooth and straight. These thickened swellings are collections of thread-capsules (cnidæ), which are packed as close as they can lie in them, and give to the tentacle that power of adhering by a touch to any animal whose tissues are penetrable, and of benumbing and destroying its vitality, in the manner which I have already, on more than one occasion, alluded to.

I have not yet described the colours of this Æquorea; they are, however, exceedingly lovely and beautiful. The crystalline translucency of most of these Medusæ, when they are colourless, and of the colourless parts of such as have bright hues, is exquisite in its glass-like purity; in this example the whole of the peripheral portion of the dome is of this hyaline character; but the lower part, which lies just above the flattened roof, is of a lively azure blue, seen to great perfection, when the animal is relieved by a dark background: the colour is gradually lost at about a quarter of an inch from its bottom. Then the radiating vessels are of a bright rose-colour, drawn in lines along the colourless surface of the roof; and the marginal vessel is of the same hue, as are also the four triangular lips of the polypite, with their ciliary fringes. These, as they depend, often extending below the level of the margin, waved about in various directions by the motion of the sea, or by the animal’s own movements, add greatly to its elegance.

LUMINOSITY.

That strange and at times magnificent and imposing phenomenon, the luminosity of the sea, is certainly due in part to some of the Medusæ. Members of perhaps all the classes of marine invertebrate animals are at one time or other engaged in the illumination, and no doubt the most wide-spread production of spontaneous light, and the most effective, is due to creatures which are individually unrecognisable by the eye. When the ship ploughing through the tropical sea, turns luminous furrows on each side of her prow, and leaves a long wake of curdling light astern, or when the steamer dashes the water of our own estuaries into cascades of fire and showers of coruscating sparks, it is doubtless to the microscopic Infusoria, Annelida, and Entomostraca, that we are mainly indebted for the charming spectacle. Still, many of the Medusæ are conspicuously luminous under certain conditions, generally displaying the phenomenon at the moment of irritation; the light being evolved, not apparently by any proper organs, but either by the whole of the marginal ring, or by the (often coloured) swellings that are seated at the base of the tentacles.