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the fact that there springs from the mouth a pair of long, flexible, fleshy processes, which carry numerous filaments on one side, and are closely coiled up into a spiral (fig. 22, D, a). These singular processes are partially supported by the shelly loop (fig. 22, B, 2) already spoken of as existing in the smaller valve of the shell. They are termed the “arms,” and it is by their instrumentality that particles of food are brought to the mouth. The animal possesses a well-developed stomach (s), and an intestine, the latter terminating blindly. There is also a large liver (h). The nervous system consists of a central mass placed near the gullet.
Terebratula flavescens inhabits the seas of Australia, and lives a sedentary life, being attached to submarine objects by means of a muscular stalk which passes through the aperture in the beak of the larger valve of the shell.
RECAPITULATION OF ESSENTIAL CHARACTERS.—Animal included in a bivalve shell. The 'valves of the shell very markedly different in size (sometimes very slightly so), and placed over the back and front of the animal. Shell attached to some submarine object by a muscular stalk (sometimes by the shell itself). Mouth furnished with a pair of long, spirally-coiled, fringed processes or “arms." These characters distinguish the class of the Brachiopoda as a whole.
Tais division includes the numerous animals commonly known as “ Bivalve shell-fish,” such as Cockles, Mussels, Oysters, Scallops, and the like. These animals derive their common name from the fact that they have the body protected by a shell, which is composed of two pieces or
" valves" (Latin, valvo, folding-doors), and which is for this reason said to be “bivalve." They derive their scientific name, on the other hand, from the fact that they breathe by means of gills (Greek, brangchia, a gill), and that these gills have a flattened, plate-like, or“ lamellar” form (Latin, 2-2 lamella, a flat plate)
As the type of this class 8-1 we shall take the great Sand Gaper (Mya arenaria); not because it is especially common, but because it exhibits particularly well many of the characters of the Bivalves. The body of the Gaper is quite soft, and may be regarded as a kind of sac or bag, formed by the skin. This bag is termed the “mantle," because it conceals and protects the internal organs.
Fig. 23. — Diagrammatic vertical and
transverse section of Mya arenaria. The bag formed by the Back, or dorsal margin of the
right and left; m m The two halves, into it. One of these open
or “lobes," of the mantle, producing
the shell ; gg The gills, two pairs on ings is placed at the hinder each side ; h 'The heart; i Intestine ; end of the body, and serves The foot. as an aperture by which water is admitted into the interior of the body. At this opening the sac is drawn out into a long tube (fig. 24, 8), which is really double, and which the animal can thrust out and draw in again. The other opening into the mantle-sac is for the purpose of allowing what is called the “foot” to be thrust out—this really being a muscular, tongue-shaped organ, by means of which the animal can shift its position (fig. 24, f).
The mantle, though forming in this way a closed bag, with no other openings into it except those just mentioned, really consists of two halves—a right and left half-and each half produces upon its outer surface one of the “valves" of the “shell” (see fig. 23, where m m are the two halves of the mantle, and ss the two valves of the shell). If, therefore, we take the animal in its living state, we do not see any portion of the body, except the long water-tubes already spoken of; but we see the double or “ bivalve” shell. As the two halves of the mantle are right and left, and as each produces one valve of the shell, it follows that the shell consists of a “right valve " and “left valve.” We may therefore compare the Gaper to a man enclosed within two great shields, one placed upon his right arm and one upon his left arm.
Leaving the shell, however, for the present, let us now examine the internal anatomy of the Gaper. In order to do this it is necessary to remove one of the valves of the shell, and we cannot do this without some violence to the animal within. The valve, namely, is attached to the mantle which produces it, and is also kept in firm connection with the opposite valve by means of two strong muscles, which are known as the “adductor muscles” (Latin, adduco, I lead towards or bring together). Hence in taking off one valve, we have to cut the mantle along the line where it is attached to the shell, and also to cut the two adductor muscles. When this is done, and the valve and mantle on one side are removed, we have the appearances presented in fig. 24. In this figure, the cut edge of the mantle is seen at m; the letter a represents the front adductor muscle, which has been cut through ; and ' is the hinder adductor muscle, which has also been divided. We know that a is the front adductor muscle, because close beside it is situated the mouth (0); and the mouth, of course, is placed on the front of the body. The mouth is surrounded by four long membranous leaflike processes or feelers (p). Immediately above the mouth, and occupying the greater portion of the centre of the figure, are two flattened membranous plates (6),
one being nearly hidden behind the other. These are the gills upon one side, and they constitute what in the case of the oyster is termed the “beard.” Two similar gills are present on the other side of the body, but these are concealed from view. To the left of the gills we see the heart (h), the last portion of the intestine (v), and the hinder adductor (a'). To the right of the gills, below the mouth, we see the tongue-shaped muscular organ which is known as the “foot” (f). Lastly, above the gills, at the hinder end of the body (the end opposite to the mouth) we see the two muscular tubes (the so-called “ siphons”) by which water is admitted to the interior of the body, and again expelled.
We may examine the internal organs of Mya a little more minutely
25). The Mya possesses no distinct head, but this end of the body Vent; 6 Gills.
. 24.-Anatomy of Mya, after one valve and one half of the mantle have been removed. m Cut edge of mantle : s8 The breathing-tubes (siphons) cut in half; a Front adductor: a' Hinder adductor; h Heart; o Mouth, surrounded by four membranous processes or feelers ; f Foot; v
can be recognised by the presence of the mouth (0) with its membranous feelers. The mouth is destitute of teeth,
Fig. 25.-Diagram of the anatomy of Mya. o Mouth : g Stomach ; i In
testine, surrounded by the liver; v Vent; b Gills; h Heart; $s Breathing-tubes (“siphons"); f Foot; n Nervous system ; ad Front
adductor muscle; pd Hinder adductor muscle. and leads through a gullet into a stomach (g). From the stomach proceeds a long winding intestine (i) which is surrounded by a well-developed liver, and which finally terminates in a distinct vent. The vent is placed at the hinder extremity of the body, and is so situated as regards the breathing-tubes (siphons) that all undigested particles of food are carried away by the outgoing current of water which has passed over the gills. The nervous system (n) consists of three little nervous masses, connected by cords. There is a distinct heart (h), which drives the pure blood, which has come from the gills, to all parts of the body. The gills are in the form of membranous plates (6), two on each side of the body, and having their surfaces covered with minute hair-like processes, which lash to and fro in constant vibrations, and sweep the water over the gills.
We are thus led to consider how the water reaches the gills, for the animal would die of suffocation unless it could constantly get a supply of fresh water. In order to understand this, it is necessary to know how the Mya