lives. The animal lives, buried in the sand, and buried head downwards, so to speak-that is to say, the mouth is turned downwards, and the end where the "siphons' or breathing-tubes are, is turned upwards (as in fig. 24). The breathing-tubes are two long muscular canals, which are so united to one another as to look like one tube, though really quite distinct internally. They can be thrust out of the shell at will, and again partially withdrawn within the shell by means of proper muscles. When the animal wishes to breathe or obtain food, it thrusts out these breathing-tubes through the sand in which it is buried till they reach the water above. Then the water is drawn in through the mouth of one of the tubes in a constant current and is carried to the gills in the interior of the body (see figs. 24 and 25, where the direction of the water-currents is indicated by arrows). Having passed over the gills, and purified the blood in its passage, the

Fig. 26.-Interior of the left valve of Mya arenaria. b Beak; a Front end of the shell; p Hinder end of the shell; ad Scar or impression of the front adductor muscle; pd Impression of the hinder adductor; ps Line where the muscles which pull the siphon in are attached; pl Line where the mantle is attached; s Spoon-shaped process carrying the "cartilage."

water next reaches the mouth, and the animal extracts from it all the floating particles of food which it may contain. Finally, the water is conducted in a reverse

direction along the intestine, taking with it all undigested food, till it reaches the second breathing-tube, from which it is expelled in an outgoing current. On the other hand, when the animal is disturbed, or when the sand in which it lives is left bare by the retreating tide, the breathingtubes are partly drawn into the shell, and these currents

cease.

We may now shortly examine the shell of Mya. As before said, the shell consists of two pieces or valves, which, as regards the animal within, are right and left.

The two valves are like each other in general form, but the left valve is slightly the smallest. The valves are hollow or concave, and they are applied to one another along their concave aspects. They do not, however, fit quite closely, but leave an opening for the breathingtubes, so that the shell is said to "gape." Each valve is furnished along its "dorsal" margin with a prominent process or "beak" (fig. 26, b), and the valves are so applied to one another that the beaks are nearly in contact and are opposite one another. Between the beaks is a mass of horny fibres which are compressed when the valves are closed, so that when the animal relaxes the adductor muscles (by which the shell is closed), the valves are elastically forced apart. These horny fibres constitute what is called the "ligament" and "cartilage," and they are carried in part by a spoon-shaped process of shell developed below the beak of the left valve (fig. 26, 8).

Externally, the shell exhibits numerous fine lines, which run concentrically round the beaks, and which mark the stages of the growth of the shell. The shell is also covered with a thin, brown, or reddish-brown membrane, which gives it its colour.

The form of the shell is an oval, broader at one end than the other. The beak is not placed quite in the middle of the shell, but somewhat to one side. This side of the shell is the broadest and shortest, and is the side on which the mouth is situated (the "anterior" or front side, fig. 26, a). The side of the shell from which the

beak turns away is the longest and narrowest, and is the side at which the vent is situated (the posterior or hinder side, fig. 26, p).

Internally, the shell exhibits several points of importance. Placed beneath the "dorsal" margin of the shell (that is, the margin on which the beaks are situated), are two depressed and smooth impressions. One of these (fig. 26, ad) is placed near the mouth at the front end of the shell, and marks the point where the front adductor muscle was attached in the living animal. The other (fig. 26, pd) is placed towards the hinder end of the shell, and indicates where the hinder adductor was attached. Running from the one of these impressions to the other is a well-marked line (fig. 26, pl), which takes a course a little within the margin of the valve, and has a deep indentation (ps) opposite to the hinder end of the shell. This line marks the place where the mantle was attached to the shell, and the indentation or bay marks the point where the muscles which pull in the siphon were attached to the shell.

It follows from the preceding that if merely shown a single valve of the Mya, and knowing nothing of the animal, we should be able to state the following points: 1. That the broadest end of the shell was the one where the mouth was situated, because the beak turns to this end, and this half of the shell is the shortest. (This would not always be true of all Bivalves.) 2. That the animal possessed two adductor muscles for closing the shell. 3. That the animal had breathing-tubes or siphons for conducting the water to the gills, and that these tubes could be partially withdrawn within the shell.

Mya arenaria is found at various points along the British coast, imbedded in sand or mud, generally on long stretches of nearly level shore which are only uncovered at spring-tides. When the tide is out, the position of the shell is indicated by a rounded or oval hole, from which the animal squirts out water when the foot is put down near it. The shell is situated about five or six inches below the surface, with the siphons pointed up

wards; and the animal has the power of shifting its position within its burrow by protruding its "foot"-this, as already said, being a tongue-shaped muscular organ (fig. 24, f), which can be thrust out between the valves of the shell. The length of the shell may be over three or four inches, with a breadth of about two inches and a half.

RECAPITULATION OF ESSENTIAL CHARACTERS.-The body is soft, and is enclosed in a skin or integument which is termed the "mantle." The mantle produces a "shell," which protects the soft body within, and which consists of two pieces or "valves," placed one upon the right side and one upon the left side of the body. There is no distinct head, and the mouth is destitute of teeth. The breathing-organs are in the form of lamellar or platelike gills, disposed on the sides of the body. These characters distinguish the class of the Lamellibranchiata as a whole.

CHAPTER XVII.

CLASS GASTEROPODA.

IN this class are included all those animals which are commonly called "Univalve Shell-fish," such as the Whelks, Snails, Periwinkles, Limpets, &c. The name of "Univalves" is applied to them because most of them possess a shell which is composed of a single piece or "valve;" and they derive the name of Gasteropoda from the fact that the lower surface of the body is generally flattened out so as to form a broad expansion or disc, which is called the "foot," and upon which the animal creeps about (Greek, gaster, belly; podes, feet). As the type of this class we may select the common Whelk (Buccinum undatum) of British seas.

If we examine a Whelk whilst living and active, we

observe that it is a slug-shaped animal, which walks, or rather creeps about, upon the lower surface of its body,

Fig. 27.-A, Sketch of a Whelk (Buccinum undatum) in motion; f Foot; h Head carrying the feelers (t) with the eyes (e) at their bases; p Proboscis ; & Respiratory siphon, or tube by which water is admitted to the gills; o Operculum. B, Shell of the Whelk: a Spire; b Body-whorl ; n Notch in the front margin of the mouth of the shell; m Outer lip of the mouth of the shell. This figure is half the natural size. C, A small cluster of the egg-capsules of the Whelk. (B and C are after Woodward.)

carrying over its back a shell composed of a single piece (fig. 27, A). The surface upon which the animal creeps forms a flattened and very muscular disc, which is termed the "foot" (fig. 27, A, ƒ). In front the animal exhibits a very distinct head (h), which carries a pair of extensible processes (t), the purpose of which is to act as "feelers," or organs of touch. At the bases of these feelers are situated the two eyes, in the form of small

E