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coloured spots. Immediately above the head is a folded tube, which the animal can thrust out to a considerable length. This tube (8) acts as a pipe or " siphon," by which fresh water is carried to the gills. At the hinder end of the foot we observe an oblong horny plate (0) with numerous concentric lines upon it. This plate is known as the “ operculum" (Latin for a lid), and the function which it discharges is obvious. When the animal, namely, retires into its shell, the portion of the foot which carries this horny plate is the last to be drawn in ; and as it fits accurately into the mouth of the shell, the Whelk is thus protected by the operculum against injury.
Over its back the Whelk carries a shell, which is composed of a single piece, and is therefore said to be
univalve." The shell has the general form of a cone, the broadest end of which is turned toward the head of the animal, whilst the pointed end is directed backwards. In reality, the shell (fig. 27, B) is composed of a conical tube, which is twisted in an oblique manner round a central pillar, and which therefore forms a “spiral.”. The first few turns of the shell are comparatively small, and they constitute what is termed the spire" (fig. 27, B, a). The last turn of the shell is by far the largest (fig. 27, B, b), and as it contains the greater portion of the body of the animal, it is termed the “body-whorl.” All the turns of the shell are in contact with one another, and the body-whorl opens in front by a large oval aperture, from which the animal can protrude itself, and which is known as the “ mouth" of the shell. The inner margin of the mouth is formed by the pillar round which the whole shell is coiled, and to this pillar the animal is firmly attached by means of a special muscle. Lastly, the mouth of the shell exhibits in front a well-marked notch (fig. 27, B, n), which is for the passage of the breathing-tube or “siphon” already spoken of.
Externally, the shell exhibits numerous lines or striæ running parallel with one another and with the spiral turns of the shell. There is also a series of undulations or folds which run in the direction of the length of the shell, or, in other words, from the mouth towards the apex of the spire. The outer surface of the shell is of a brownishwhite or yellowish-brown colour, whilst the mouth is white or flesh-coloured.
Turning now to the internal anatomy of the Whelk, the mouth (of the animal, not of the shell) is found to contain a singular organ which is known as the “tongue,” and a portion of which is represented in a highly magnified form in fig. 28. The tongue consists of a long strap, which carries three rows of minute serrated teeth, composed of flint. By means of proper muscles this toothed strap can be made to move backwards and forwards over a kind of cushion upon which it rests. The animal can thus apply it like a saw to any foreign substance, and as the teeth are extremely hard, holes can be bored into other shells with great readi- Fig. 28.-Porness. The mouth (fig. 29, a) is placed at the tion of the
tongue of the end of a proboscis, which can be thrust out whelk, "highto a considerable distance, and conducts by a ly, magnified
(after Woodgullet to a proper digestive cavity or stomach. ward). From the stomach proceeds a long convoluted intestine, in great part surrounded by a voluminous liver. The intestine (d d) terminates in a distinct vent (e), which is placed upon the back.
The nervous system is chiefly aggregated round the gullet (fig. 29, f). There is a distinct heart (h) consisting of two cavities or chambers. The breathing-organs are in the form of two plume - like gills (9), which are placed in a sort of chamber, formed by the folding of the mantle, on the back of the animal. The water necessary for respiration is admitted to this chamber by means of the folded tube or “siphon,” which has been mentioned as being protruded through the notch in the front of the shell.
The common Whelk is very widely distributed throughout European seas, and is one of the most abundant of British Univalves. It is found usually in tolerably deep
water, but extending from low water to depths of one hundred fathoms. It is very voracious, and exclusively
Fig. 29.- Diagrammatic section of a Whelk. a Mouth, with masticatory ap
paratus or tongue; b Salivary glands ; c Stomach; d d Intestine, surrounded by the liver, and terminating in the vent (é); g Gill; h Heart; f Nervous system,
carnivorous, living upon other shell-fish or upon any dead animal bodies. It usually bores its way into other shells by means of the toothed tongue. It is not uncommonly used by fishermen as bait, and it is also occasionally eaten. The female Whelk lays its eggs in clusters of flask-shaped, horny capsules, each capsule containing five or six eggs. These clusters (fig. 27, C) are attached to stones, shells, or other foreign bodies, and the young, after attaining a certain degree of development, escape from the capsules by means of rounded perforations in the sides of the latter.
RECAPITULATION OF ESSENTIAL CHARACTERS. — Of the
above characters the ones which the Whelk shares with the other Gasteropods, and which therefore characterise this class of shell-fish, are only the following: The head is distinctly marked out from the rest of the body; the mouth is furnished with a peculiar toothed apparatus or “tongue;" the “foot" is used for locomotive purposes, having the form (not always) of a broad, flattened, muscular disc; and the body is not enclosed in a bivalve shell. Non-essential, though very common, is the character that the soft body is protected by a shell which is “univalve,” or consists of a single piece.
This class of animals contains minute shell-fish, which are found swimming in the open ocean far from land. They derive their name of Pteropoda from the fact that the head is furnished with two wing-like fins, by means of which the animal swims (Greek, pteron, wing; podes, feet). We may select as the type of this class the well-known Hyalea tridentata of the Mediterranean and Atlantic, but we shall give merely a brief outline of the most important points in its organisation.
The animal of Hyalea tridentata (fig. 30) is enclosed in a small, yellowish-brown, semi-transparent shell, which may be regarded as composed of a back and front plate, united to one another more or less completely. The back plate is nearly flat, and is prolonged in front so as to form a sort of hood. The front plate is strongly rounded and globular. Behind, the shell is
. 30.-Hyalea tridentata, showing
the shell and the lateral fins attached which arise from the line to the sides of the head (In). where the two plates of the shell unite with one another. In front the two plates leave a small aperture, through which the animal can protrude its head at will; and at the sides there are two slits, one on each side, through which pass long appendages of the integument or “mantle."
The animal is attached to the interior of its shell by a muscle which passes from the point of the shell behind to the head in front. On each side of the head is situated a large fin (fig. 30, ff), by means of which the animal propels itself through the water. The head is also furnished with indistinct tentacles, and exhibits centrally, on its front margin, the opening of the mouth. The mouth contains a toothed “tongue," essentially similar to that of the Whelk. The mouth opens into a long and slender gullet, which conducts to a stomach, this in turn communicating with a long and slender intestine. There is a well-developed liver, and the intestine terminates in a distinct vent placed on the right side of the neck. The nervous system forms a mass situated below the gullet. There is a heart, consisting of two chambers; but the breathingorgans are quite rudimentary, and can hardly be said to constitute regular gills.
Hyalea tridentata is a native of the Mediterranean and Atlantic Ocean, and is found in the open sea, far from land, darting about by means of the vigorous flapping of the lateral fins. It appears to be of nocturnal habits, and to sink below the surface during the daytime.
RECAPITULATION OF ESSENTIAL CHARACTERS. — The head is furnished with lateral expansions of the skin, or fins, by means of which locomotion is effected. The mouth is furnished with a toothed tongue. The animal lives in the open ocean, near the surface of the water. A non-essential but common character is, that the body is protected by a symmetrical, glassy, semi-transparent shell. These characters distinguish the class of the Pteropoda as a whole.