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. [Illustration: 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