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Some of these animals have eye-specks on the edge of the mantle; but this is not a constant feature. This class of Acephala includes all the Oysters, Clams, Mussels, and the like. When named with reference to their double shells, they are called Bivalves; and with them are associated a host of less conspicuous animals, known as Ascidians, Brachiopods, and Bryozoa.
Indeed, some of our rock-deposits consist chiefly of the fragments of their remains. The Mollusks were also represented then, as now, by their three classes, Acephala, Gasteropoda, and Cephalopoda. The Acephala or Bivalves we shall find in great numbers, but of a very different pattern from the Oysters, Clams, and Mussels of recent times.
The mantle envelops the animal, and lines its single shell as it lined the double shell of the Oyster; the gills are placed on either side of it; the stomach, with the winding alimentary canal, is in the centre of the body; the heart and liver are placed in the same relation to it as in the Acephala; and though the so-called foot would seem to be a new feature, it is but a muscular expansion of the ventral side of the body.
The delicately festooned rim of this shell, supplied by the biggest mollusk in the class Acephala, measured about six meters in circumference; so it was even bigger than those fine giant clams given to King François I by the Republic of Venice, and which the Church of Saint-Sulpice in Paris has made into two gigantic holy-water fonts.
Among the Acephala are to be named a large Cardium, also found on the Californian coast; Modiolus, two species; Mytilus; Mya, two species; and Teredo palmulatus: the latter, which is brought here by the ships, is very mischievous in the harbour, and attains to the length of two feet.
Some of them have no solid covering; but the greater part are protected by a single shell, and on this account they are called Univalves, in contradistinction to the Acephala or Bivalves. These shells, though always single, differ from each other by an endless variety of form and color, from the flat simple shell of the Limpet to the elaborate spiral and brilliant hues of the Cones and Cowries.
"Branch Mollusca," Conseil said, "class Acephala, order Testacea." "Correct, my scholarly Conseil. Now then, those Testacea capable of producing pearls include rainbow abalone, turbo snails, giant clams, and saltwater scallops briefly, all those that secrete mother-of-pearl, in other words, that blue, azure, violet, or white substance lining the insides of their valves."
Take the Oyster as an example. It has, like most Acephala, a shell with two valves united by a hinge on the back, one of these valves being thick and swollen, while the other is nearly flat.
The lowest class of Mollusks has been named Acephala, to signify the absence of any distinct head; for though their whole organization is based upon the principle of bilateral symmetry, it is nevertheless very difficult to determine which is the right side and which the left in these animals, because there is so little prominence in the two ends of the body that the anterior and posterior extremities are hardly to be distinguished.
Different as is their external covering, however, if we examine the internal structure of a Gasteropod, we find the same general arrangement of parts that prevails in the Acephala, showing that both belong to the same great division of the Animal Kingdom.
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