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.

SOUTHERN CROSS PEARL, VALUED AT £10,000.

Consists of nine laterally united pearls corresponding in shape and size.]

QUEENSLAND PEARLS ON BLACK-LIPPED PEARL-SHELL.

The black-lipped shell produces pearls of great value.]

Although lamp-shells are represented by comparatively few species at the present day, in the older epochs they existed in enormous abundance. It is further remarkable of this group that many species are scarcely distinguishable from their fossil ancestors. Lingula, the type last referred to, is especially notable in this respect.

----