A QUEENSLAND OYSTER-BANK.
Illustrating another characteristic growth-phase of the Australian rock-oyster.
Of the Scallops many of the larger species are highly esteemed for the table. It is notable of them that they can progress through the water in a jerky, flight-like manner by the repeated flapping of their opposing shells. In many scallops the jewel-like eyes are developed in a row round the margin of the mantle.
The Bivalve Class includes certain representatives which are held in evil repute, on account of their destructive habits. Pholas and its allies bore their way into solid rock, bringing about its complete disintegration. From the dreaded Ship-worm, or Teredo, on the other hand, there would appear to be no description of wood that can withstand its ravages.
The Bivalve Molluscs are not without fresh-water representatives. The well-known Pond- and River-mussels, which form the most conspicuous examples of this group, number several hundred species. While insipid and of no account for the human commissariat, many of its members produce pearls of value. One such, obtained from the river Conway, in North Wales, is said to occupy a place in the crown of England.
The Chitons, or Multivalve Molluscs, invite brief notice. As the last-named title implies, the shell-elements in this group are relatively numerous, consisting of eight pieces, or plates, which may form contiguous transverse shelly shields that entirely cover in and protect the dorsal surface of the elongate, boat-shaped body, or may be more or less isolated from one another. In the former instance the animal bears some considerable resemblance to a gigantic limbless wood-louse, and, like that familiar terrestrial crustacean, it is capable of rolling itself into a spheroidal shape as a means of protection. None of the chitons are provided with eyes in the adult state in that region of their body—namely, the head—where they might be most naturally expected to exist. Recent scientific investigation has, however, elicited the fact that in various species the respective shell-plates are studded with minute eye-specks, the aggregate number of visual organs thus possessed by certain forms reaching to the astonishing figures of 11,000 or 12,000. The majority of the chitons are shallow-water, rock-frequenting molluscs, which may be successfully sought by turning over stones at low water. Several species are inhabitants of British seas.
CHAPTER II.
LAMP-SHELLS.
A little group of double-shelled creatures, formerly regarded as near allies of the Oysters and Mussels, are the Lamp-shells. Their scientific appellation, signifying "arm footed," relates to the two spirally convoluted arm-like structures which constitute the salient features in these animals. That of lamp-shells bears reference to the small circular perforation near the extremity, or "beak," of the united shells, which imparts to the entire structure a not altogether remote resemblance to an ancient Greek or Roman lamp with its sub-terminal wick-hole. In all essential points of their organisation the lamp-shells differ so essentially from ordinary bivalves that they are now generally recognised as representing an independent animal class, having, as a matter of fact, a closer relationship with Worms than with Molluscs. In their earlier condition certain lamp-shells are indistinguishable from larval worms; while the convoluted arms of the adult animals, thickly beset with bristles and hairs, closely resemble the arms, or "cirrhi," of many sea-worms. The superadded valves of the lamp-shells differ fundamentally from those of the bivalve molluscs in the circumstance that they are developed upon the upper and lower surfaces respectively of the enclosed animal, and not on the sides. The union between the two valves is also accomplished through the medium of interlocking calcareous teeth, in place of a horny or ligamentous hinge-joint, as obtains in an oyster or a mussel. A supplementary calcareous support, having a corresponding spiral shape, is also developed in connection with the convoluted arms. This structure varies in the contour of its minuter details in every specific form, which thus furnishes zoologists with a basis for systematic classification. The lamp-shells collectively form two natural groups or orders. In one of these the shells are hinged together, and are of conspicuously unequal dimensions. The larger shell of the two is more distinctly concave and produced into a perforated beak. It is this structure that in some species resembles the wick-hole of an antique lamp, and has given rise to the popular title by which these shells are distinguished. The second or hingeless group is further distinguished by the shells being of almost uniform shape and size. In the most interesting example of this group, known as Lingula, the two shells are thin, horny, of a green tint, and mounted on a long, flexible, worm-like stalk. Like a worm, this creature moves about in the mud, and constructs a sand-lined dwelling-tube.