Fig. 30.
Common Edible Oyster (Ostrea edulis).
a, labial palpi; b, gills; c, mantle; d, junction of the two folds of the mantle; e, large adductor muscle; f, the shell.
Case 156.
The Limas (Limidæ) are very like the Pectens, but the inner edge of their mantle is fringed with very long thread-like filaments. The shells are always white, generally more or less oblique, and radiately ridged. They appear to be found in most seas, and either swim about freely like the young Scallops by flapping their valves or attach themselves by a byssus, sometimes forming a sort of nest, consisting of pieces of coral and shell or small stones, in which they are completely concealed.
Cases 156–160.
The Scallops or Fan-shells (Pectinidæ) are well known for their beautiful colours, sculpture, and excellent flavour. The animal has a distinct foot, which is not, however, used as a locomotive organ, but employed in spinning a byssus of attachment when required. The young Pectens dart through the water by opening and suddenly closing their valves. The species are very numerous, world-wide in their distribution, and may be found at depths from a few to three thousand fathoms.
Eulamellibranchia. (Fig. 25, C.)
Cases 160–204.
In this order the gills have interfilamentar and interlamellar vascular junctions, and there are generally two adductor muscles. The order is very extensive, comprising nearly sixty families, of which only the more important can be referred to.
The Carditidæ and Astartidæ have strong solid shells, frequently ornamented with radiating or concentric ribbing, and usually are coated with a dark epidermis. They have the general appearance of certain Veneridæ; but the animal has no prolonged siphons, but merely a fringed opening in the mantle. One very remarkable species, Thecalia concamerata (Case 161), has an internal cup-like process within the valves, which serves as a nursing-pouch for the young.
Cases 162–163.