strongly-ribbed valve of the common Scallop; and the lower valve of the parasite, instead of being flat and thin as usual, is as much fluted as the Scallop itself.

The shells of this order generally consist of two valves connected by a hinge, either simple or composed of two or more teeth which interlock; within the hinge, or external to it, we find an elastic ligament, serving the purpose of opening the shell, and acting as an antagonist to the adductor muscle. While the animal is at rest and undisturbed, the adductor muscle ceases to act and the shell remains open, freely admitting food and water; but on the approach of danger, disturbance or removal from the water, the adductor muscle contracts with such force as not only to overcome the elasticity of the ligament, but to render it difficult to force apart the valves of the shell.

Shells of a rounded form, like the common oyster, have only one adductor muscle, and are termed Monomyaria, whilst those of an elongated form possess two adductor muscles, and are thence called Dimyaria. It is evident that a simple elastic tissue would suffice to open the shell, and that a piece of India-rubber might be substituted for the ligament, when this is internal to the hinge, but it would be inefficacious when the ligament is external to the hinge, because in the latter case the ligament must necessarily possess a contractile power. The minute structure of this ligament was first investigated by my late brother,

true

whose researches were published in the second volume of the "Transactions of the Microscopical Society." The ligament placed within the hinge he found to be composed of two different kinds of tissue, the external being nearly structureless, whilst the internal is fibrous, sometimes presenting a brilliant play of colours; the fibres take a vertical direction, they are crossed by transverse striæ, as in Fig. 188, and bear a perfect resemblance to the prismatic cellular structure

FIG. 188.

of the shell deprived of its calcareous deposit; that this is its true character, is evident, and in some shells I have detected calcareous matter in the hinge ligament.

Whatever be the nature of this tissue, it must evidently be contractile, like muscle, and so powerful is its action and so extended its range, that that in some of the Lamellibranchiate animals, as the Scallop, after division of the adductor muscle, the valves gape to the extent of three inches. This ligament must assist in the progression of the animal, for we are told that the Scallop is capable of moving rather quickly, by the rapid opening and shutting of its shell, and the Cockle can do more, for by means of its foot, aided I should imagine by the opening and shutting of its shell, it can perform a series of small leaps.

In many bivalve shells there is a peculiar collection of silken filaments called the byssus, extending from the extremity of the foot, by means of which the shells are firmly anchored to rocks or stones. A familiar example of the byssus is afforded by the common edible Mussel, and a still more striking one is that of the Pinna, which is often several inches in length. In Tridacna, when young, the byssus is so strong as to be cut with difficulty by a knife; but when the shell is older and heavier, no byssus is secreted. Examined microscopically, the byssus of the Pinnæ is found to be composed of filaments of a brown colour, without any trace of structure; in some species they are so long and silky as to be manufactured into gloves.

LECTURE XIX.

SKELETON OF MOLLUSCA-GASTEROPODA.

THE animals comprising the class Gasteropoda are not only the most numerous, but the most typical of all the Mollusca; they have one remarkable peculiarity by which they can readily be distinguished; this consists in their being provided with a fleshy disc serving as a foot, upon which they creep. The foot is present in all the true Gasteropoda, as shown in Fig. 153, but there is a small class in which this organ is so modified as to form a vertical flattened fin, enabling the animal to swim rapidly through the water; to this class the term Heteropoda has been applied, and the Carinaria, represented in Fig. 207, is one of the best known examples. The back of a Gasteropod is covered with a cloak or mantle, in or upon which the Shell is secreted; the shell is usually spiral and univalve, but in the genus

Chiton, it is multivalve. In certain of the Nudibranchiate order it is either absent or represented by small calcareous spicula.

The shape of the shell of the Gasteropoda is more or less conical; in many genera, as in Patella, it is a simple flattened cone, in others the cone is elongated, and may be either straight, as in Dentalium, or in one plane, as in Planorbis, or forming a true helix, as in the common Snail. In many aquatic species the extremity of the foot is covered by a plate of horn, or of calcareous matter called the Operculum, which is considered by Mr. Gray as the rudiment of the second valve; its principal use appears to be that of closing the shell when the animal has retired within it. Most of you no doubt are familiar with the delicate membranous film by which the mouth of the common garden Snail is closed during the period of hybernation; this is a rudimentary operculum, but under certain conditions, as I shall hereafter show, it may become thickened and very opaque by a deposit of carbonate of lime in a granular form. Some of the shells are smooth, others are covered with long spines formed upon prolongations of the mantle; these are not constant either in size or number in the same species of shell, as the animal is not only capable of removing them when they have become old, but of forming new ones in other situations.

All the shells of this class of animals are remarkable for the small amount of the organic, compared with