Nothing can be more elegant or various than the form and arrangement of the gills in most of the nudibranchiate gasteropods. In the Glauci and Scyllææ, we see at each side of the elongated body long arms branching out into tufty filaments; in the Briarei a hundred furcated stems serve for the aëration of the blood. On the back of the Eolides the gills are arranged in rows; in the Dorides they form a wreath or garland round the posterior intestinal aperture.
The beauty of these animals corresponds with their charming mythological names, for every part of them which is not sparkling like the purest crystal shines with the liveliest colours, red, yellow, or azure. Some inhabit the coasts, where they creep along upon a well-developed foot, others live in the deep waters, where they cling to the stems of floating sea-weed with a narrow and furrowed foot, or swim upon their back, using the borders of the mantle and of the branchiæ as oars. Though chiefly living in the warmer latitudes, they are found in every sea, and many interesting species inhabit the British waters: such as the Sea-lemon (Doris tuberculata), which, when its horns and starry wreath of branchiæ are concealed, bears a curious resemblance in size, form, colour, and warty surface to the half of a citron divided longitudinally; the exquisite Eolis coronata, whose crowded clusters of branchial papillæ are radiant with crimson and cerulean tints; and the crested Antiopa, whose transparent breathing organs are tipped with silvery white.
Eolis.
Though they have no shell to cover them, the Nudibranchiata are not left defenceless to the mercy of their enemies. The transparency of their body is a cause of safety to many of them. Some conceal themselves under stones or among the branches of the madrepores, and some on contracting cast off a part of their mantle, which they leave in possession of their hungry foe, while they themselves make their escape.
Among the British Inferobranchiata we find the rare golden or orange-coloured Pleurobranchus plumula, thus named from its branchiæ projecting like a plume from between the mantle and foot in crawling; and among the Tectibranchiata the common sea-hare (Aplysia punctata), which resembles a great naked snail; its back opening with two wide lobes, which can be expanded or closed over the opening at the animal's will. When open, they expose to view on the right side the finely fringed and lobed branchiæ, seated in a deep hollow beneath a fold of the mantle. The uncomely creature glides along over the stones upon its flat fleshy foot and up the slender stems of sea-weeds by bringing the borders of the same locomotive apparatus to meet around the stem, thus tightly grasping it as if enclosed in a tube. While progressing, the fore part is poked forward as a narrow neck furnished with two pair of tentacles, one pair of which, standing erect and being formed of thin laminæ, bent round so as to bring the edges nearly into contact, look like the ears of the timid quadruped, from which the Aplysia has derived its common name. The colour is a dark-brownish purple studded with rings and spots of white. On being disturbed, the sea-hare pours out from beneath the mantle-lobes a copious fluid of the richest purple hue, which however quickly fades, and is of no value in the arts.
More than forty species of Aplysiæ are known, most of them inhabitants of the warmer seas. The acrid humour exuded by the depilatory aplysia, or Aplysia depilans, of the Mediterranean is still supposed by the Italian fishermen to occasion the loss of the hair, and was used by the ancient Romans in the composition of their venomous potions—though it is by no means poisonous. Such are the prejudices resulting from the propensity of man to associate evil qualities with an unprepossessing appearance.
Chiton squamosus.
To the Cyclobranchiate order belong the Limpets and the Chitons. The latter, which are the only multivalve shells among the Gasteropods, are spread in more than two hundred species over every shore from Iceland to the Indies, but they are particularly abundant on the coasts of Peru and Chili. Some of the smaller species inhabit our coasts, where they may be found adhering to stones near low water mark. They are coated with eight transverse shelly plates, folding over each other at their edges like the plates of ancient armour, and inserted into a tough marginal band, so as to form a complete shield to the animal. Thus encased in coat of mail, the chitons have the power of baffling the voracity of their enemies by rolling themselves up into a ball like the wood-louse or the armadillo: they are also able to cling with such tenacity to the rock that it is difficult to detach them without tearing them to pieces. The Limpets, or Patellæ, likewise attach their shield-like shell so firmly to a hard body that it requires the introduction of a knife between the shell and the stone to detach them. It has been calculated that the larger species are thus able to produce a resistance equivalent to a weight of 150 pounds, which, considering the sharp angle of the shell, is more than sufficient to defy the strength of a man to raise them. They often congregate in large numbers in one place, and an old writer compares them to nail-heads struck into the rock. More than a hundred species are known; one of which, the Patella cochlear of the Cape, is almost invariably found squatting upon the shell of another species of limpet. The finest and largest varieties abound on the shores of the Oriental seas and the coasts of the Mediterranean, but several of the smaller species are very numerous in our littoral or sub-littoral zone, where they either feast on the green sea-weeds that we find covering at ebb-tide the stones with a thin emerald layer, or upon the coarser olive-coloured algæ. Thus Patella pellucida and Patella lævis, both remarkable for longitudinal streaks of iridescent colours on an olive-shell, may generally be found feeding either on the broad fronds or on the roots and stems of the Laminariæ, or Oar-weeds. To their labours may indeed be partly attributed the annual destruction of these gigantic algæ, for, eating into the lower part of the stems, and destroying the branches of the roots, they so far weaken the base that it is unable to support the weight of the frond, and thus the plant is detached and driven on shore by the waves.