The Pteropods, or Wing-footers, move about by means of two fin-like flaps, proceeding wing-like from the fore part of the body. They have no disk to walk upon, nor arms for the seizure of prey, like the cephalopods and gasteropods, but resemble them by the possession of a head distinct from the rest of the body, which some, like the Hyaleas and Cleodora, conceal in a thin transparent or translucent shell, in which they also hide their head and wings at the approach of danger, and immediately sink to the bottom; while others, like the blue and violet Clios, beautifully variegated with light red spots, are perfectly naked. They generally inhabit the high seas, and are but rarely drifted by storms or currents into the neighbourhood of the land. They mostly swim about freely, but sometimes also they are found clinging by their wings to floating sea-weeds. They are small creatures, but propagate so fast that the Clio borealis and Limacina arctica form the chief food of the colossal whale.
Hyalea globulosa.
While these two little pteropods, in spite of their minute proportions, deserve to rank among the most important inhabitants of the northern seas, the Mediterranean species belong mainly to the genera Hyalea, Cleodora, and Criseis—forms wholly unknown to our own fauna except as waifs. Vast shoals of these animals frequent the deeper parts of that sea, leaving their remains strewed over its bed, between depths of from one hundred to two hundred fathoms; they are short-lived creatures, and have their seasons, being met with near the surface during spring and winter, sparkling in the water like needles of glass.
"The pteropods are the winged insects of the sea," says M. Godwin-Austen, "reminding us, in their free circling movements and crepuscular habits, of the gnats and moths of the atmosphere; they shun the light, and if the sun is bright, you may look in vain for them during the life-long day—as days sometimes are at sea; a passing cloud, however, suffices to bring some Cleodoræ to the surface. It is only as day declines that their true time begins, and thence onwards the watches of the night may be kept by observing the contents of the towing-net, as the hours of a summer day may be by the floral dial. The Cleodoræ are the earliest risers; as the sun sets, Hyalæa gibbosa appears, darting about as if it had not a moment to spare, and, indeed, its period is brief, lasting only for the Mediterranean twilight. Then it is that Hyalæa trispinosa and Cleodora subula come up; Hyalæa tridentata, though it does not venture out till dusk, retires early, whilst some species, such as Cleodora pyramidata, are to be met with only during the midnight hours and the darkest nights. This tribe, like a higher one, has its few irregular spirits, who manage to keep it up the whole night through. All, however, are back to their homes below before dawn surprises them."
The lamellibranchiate Acephala, or headless molluscs with comb-like gills, are distinguished from the preceding orders of molluscs by a more simple organisation and the peculiar formation of their external coverings. They are all contained within a bivalve shell, articulated after the manner of a hinge, and to which some of their families are attached by one strong muscle (Monomyaria), others by two (Dimyaria). In this shell, which is secreted by two large flaps or folds of their skin or mantle, they generally lie concealed like a book in its binding, and bid defiance to many of their enemies. When danger menaces, the sea-snail withdraws its head and closes the entrance of its hermitage with a lid, but the bivalve shuts its folding-doors when it wishes to avoid a disagreeable intruder. A strong elastic ligament connects the two valves, and opens them wide as soon as the muscular contraction which closed them ceases to act.
While the sea-snail creeps along upon a mighty foot, the bivalve is frequently doomed to a sedentary life, and the former protrudes from its shell a well-formed head, while the latter, like many a biped, has no head at all. The lamellibranchiate Acephala have, however, been treated by nature not quite so step-motherly as might be supposed from this deficiency, for many of them have eyes, or at least ocular spots, which enable them to distinguish light from darkness; and even auditory organs have been discovered in many of them. Their circulation is performed by a heart generally symmetrical, and their respiration by means of four branchial leaflets equal in size, and symmetrically arranged on either side of the body. The mouth is a simple orifice without any teeth, bordered by membranous lips, and placed at one end of the body between the two inner leaves of the branchiæ. The digestive apparatus consists of a stomach or intestine of different lengths, a liver, and several other accessory organs. A simple nervous system brings all the parts of the body into harmonious action.
