Chapter XII: Marine Molluscs (3)

The last family of the _Holostomata_ is the _Naticidæ_, the shells of which are almost globular, with only a few whorls, and a small, blunt spire. The mouth is semilunar in form, and the lip sharp. The proboscis of the animal is long and retractile, and the foot large; but perhaps the most characteristic feature is the presence of large mantle lobes which hide some of the shell when the animal is crawling. In _Natica_ (fig. 155), the typical genus, the shells are somewhat thick and smooth, with a large umbilicus. As the animal crawls a large fold of the mantle is reflected back over the head, completely covering it, and apparently obstructing its view; but this is not the case, for the creature has no eyes. _Natica_ is very abundant on some sandy beaches, where it devours small bivalves and other animals; and it is frequently washed up alive by the waves. Its shell is also a favourite one with hermit crabs. Its eggs, all connected together in a spiral band, may often be seen stranded on sandy coasts. Several species of Natica are found on our shores. An allied mollusc--_Velutina lævigata_, so called on account of the velvety epidermis that clothes the shell, completely surrounds the shell by its mantle folds when creeping.

The _Siphonostomata_ form a much smaller section than the last, and its members are distinguished mainly by the presence of a true siphon, formed by the prolongation of the mantle margin, and serving to convey water into the gill chamber. In all these the shell is spiral, usually without an umbilical opening, and the margin of the mouth is prolonged into a canal or distinctly notched. The operculum is horny, and lamellar or imbricated. The animal has a retractile proboscis, and the eyes or eye-pedicels are joined to the tentacles. All the species of this division are marine.

We will first take the family _Cypræidæ_, which contains the familiar Cowries, these forming the lowest group of the division. An examination of the shells may at first seem rather puzzling, for the spire is concealed, and the whole is convoluted in such a manner as to make the mouth long and narrow, with a channel at either end. The outer lip is also thickened and bent inward, and there is no operculum.

The animal itself is particularly interesting, for, as it creeps along on its broad foot, abruptly shortened in the front, the mantle lobes bend over the top, meeting along the middle line, where they are usually fringed with little tentacle-like processes; and, as a result, the whole shell is beautifully enamelled on the outer surface. In all the Cowries the central teeth are single, and the laterals are arranged either in twos or threes.

Perhaps the commonest representative of this family is the pretty little _Cypræa_ (_Trivia_) _europæa_ (Plate V.), the shells of which are sometimes washed up in large numbers on sandy beaches. The animal lives mainly below low-water level, but it may often be found in the larger rock pools, creeping rapidly over the tangles, and may be easily secured with the aid of a net.

In the same family we have the little _Erato_ (_Marginella_) _lævis_, the white shell of which is minutely furrowed along the lips; and also _Ovulum patulum_ (_Calpurna patula_), so called on account of its fancied resemblance to a poached egg.

We have also several species of Cone shells (family _Conidæ_) on our coasts, readily recognised by their form, which is a cone, with a long, narrow aperture, partially closed by a minute operculum. As in the last family, the foot is abruptly shortened in front. The head is very prominent, with eyes situated on the tentacles. There are two gills, and the teeth are arranged in pairs.

The Conidæ are principally inhabitants of tropical seas, where some very large species exist. Two of the British representatives, both common shells, are shown in fig. 180.

Our next family (_Buccinidæ_) is so well distributed on our coasts, that it would be difficult, we imagine, to find a spot quite free from its familiar forms. It contains all those creatures commonly known as Whelks, Dog Whelks, and Dog Winkles, ranging from deep water almost to high-water mark.

In all these the shell is notched in front, or the canal is turned abruptly upward. The foot of the animal is broad, the eyes are situated either on the tentacles or at their bases, and there are two gill plumes.

All the species are carnivorous, and some are said to be very destructive to mussels and young oysters.

The Common Whelk (_Buccinum undatum_, Plate V.) lives in deep water, whence it is dredged up largely for the market. Its clusters of egg cases are washed up in large numbers on the beach, where they form one of the commonest materials among the refuse at high-water mark. It is not uncommon, also, especially after storms, to find the unhatched eggs stranded by the waves, and these are so transparent that the embryos, several in each capsule, may be seen within. The hole through which the young escape may also be seen on the inner side.

The Dog Periwinkle (_Purpura lapillus_) abounds on all our coasts and is remarkable for the production of a dull crimson or purple fluid that may be obtained from it by pressing on the operculum. This fluid turns to a brighter colour on exposure to air, and is said to have been used largely in former times as a dye. It will be seen from our figure that the spire of this shell is shorter in proportion than that of _Buccinum_; but both are alike in that the operculum is made up of layers with a nucleus on the external edge.

The other species figured is _Nassa reticulata_, popularly known as the Dog Whelk, and characterised by a tooth-like projection of the inner lip close to the anterior canal. It is very common near low-water mark, where it may be seen crawling over the rocks on its broad foot, from which project two hornlike appendages in front and two narrow tails behind.

From the last family of the gasteropods (the _Muricidæ_) we select two common species--_Murex erinaceus_ and _Fusus antiquus_ (Plate V.). In both these the anterior canal of the shell is straight and the posterior wanting. The eyes are on the tentacles, and there are two plumed gills. Both are carnivorous species, feeding on other molluscs; and the former is said to bore through the shells of its prey with the prominent beak of its shell.

_Murex_ may be readily distinguished by the prominent longitudinal ridges of the thick shell, its rounded aperture, and by the partly closed canal running through the beak. It is known to fishermen as the Sting Winkle; the other species is called the Red Whelk in some parts, and in Scotland is known as the Buckie. Like the common whelk, it is dredged largely for the market, and is said to be far more esteemed than the former, from which it may be distinguished by the fusiform shape of the shell and the long straight canal.

We now pass to the last and highest class of the mollusca, called the _Cephalopoda_ because they have a number of arms attached to the head, round the mouth. Unlike the majority of molluscs they are bilaterally symmetrical: and are much more highly organised, in some respects even making an approach to the vertebrates. Thus they generally have an internal hard structure, either horny or calcareous in structure, representing the vertebral column, and the circulatory system consists of arteries and veins, connected by minute capillaries. The corpuscles of the blood are also similar in form to those of the vertebrates. Externally they are all naked, with the exception of the nautilus and argonaut of the warmer seas.

The arms, so characteristic of the class, are eight or ten in number, long and muscular, and provided with numerous suckers by which the animal can cling with remarkable tenacity. These suckers are situated on the inner surface of the arms, and the disc of each one displays a series of muscular fibres, all converging from the circumference towards the centre, which is occupied by a softer structure that works inwards and outwards like the piston of a pump. Thus the suckers form a system of exhausting air-pumps by which a vacuum can be produced, and the tenacity of the grip, maintained by atmospheric pressure, is so great that the arms, strong as they are, may be torn asunder by attempting to pull them from their hold; and yet the animal can release its grip with the greatest of ease by simply releasing the pistons of its pumps.

The cephalopods are further distinguished by their very large, glaring eyes, situated on the sides of the well-formed head, and by powerful jaws that work in a vertical plane, like those of the vertebrates, but somewhat resembling the beaks of certain birds. The tongue is also very large and fleshy, and in part armed with numerous hooked spines or teeth.

The class is usually divided into two orders, one characterised by the possession of two gills, and the other of four; but the British species belong to the former, known technically as the _Dibranchiata_. This order is subdivided into two sections according to the number of arms; and the divisions are called the _Octopoda_ and _Decapoda_ respectively.

The former section includes the Octopods, of which some species inhabit our seas. They all have eight arms, of unequal size, with the suckers arranged in two rows, and their round or oval bodies seldom have any fins, locomotion being effected by means of the arms, and by the sudden expulsion of water from the siphon. The shell is rudimentary, being represented merely by two short ‘styles’ within the mantle. The species vary considerably in size, some being only about an inch long when fully grown, while others measure two feet or more, and are looked upon as formidable creatures by man. Sometimes they are washed up on our beaches, but the best way to make their acquaintance is to examine the contents of the fishermen’s drag nets as they are hauled on the beach.

In the same manner we may secure various species of the Decapods or Ten-footed Cephalopods, which comprise the Calamaries, Squids, and Cuttlefishes. These, too, properly speaking, have but eight arms, the other two appendages being really tentacles, which are usually longer than the arms, and more or less retractile; they are also expanded at the ends. The decapods are also to be distinguished from the octopods by their elongated bodies, and a flattened, fin-like appendage on either side. Their eyes, also, are capable of being rotated within the orbits, while those of the octopods are fixed; and the shell consists of one or more horny ‘pens,’ or of a calcareous ‘bone,’ contained in a cavity so loosely that it drops out of its place when the cavity is opened.

The Common Calamary (_Loligo vulgaris_) may be recognised by the accompanying illustration, from which it will be observed that the body tapers behind, bearing two rhomboidal fins in the rear. The suckers are arranged in two rows on the arms, but in fours on the expanded tips of the tentacles. The animal is a good swimmer, and sometimes crawls, head downwards, on the disc surrounding the mouth, pulling itself along by means of its arms. Its shell is a horny pen, lanceolate in form, but it divides as the age of the animal advances, so that two or more may be found in the same specimen.

Belonging to the same family we have the Common Squid (_Sepiola atlantica_), also a very abundant species. Here the body is shorter and purse-like, and the fins are dorsal and rounded. It seldom exceeds four or five inches in length, and, like the Calamary, is used largely as a bait by fishermen.

Another family--the _Sepiadæ_--contains the Cuttlefish (_Sepia officinalis_), the ‘bone’ of which is such a common object on the beach. This latter is a broad, curved plate of carbonate of lime, made up of a number of regular layers, and having a cavity hollowed out at the posterior end. It is exceedingly light and porous in structure, and at one time was used largely as an antacid as well as a dentifrice. It is also proportionately large, being both as long and as broad as the body of the animal.

Cuttlefishes live principally in the shallow water close to shore, where they swim backwards by the sudden propulsion of water from their siphons; and their eggs, which look like clusters of black grapes, are frequently thrown up on the beach, generally attached to the stems and fronds of sea weeds.

As a rule the cephalopods swim slowly by the aid of their fins or by a rhythmic contraction by which water is expelled from their siphons, but when in danger the muscular contraction is so violent that they dart through the water with great speed, and even leap into the air to avoid their enemies. But they have another and much more remarkable way of escaping from their foes:--They possess a gland, the duct of which opens into the base of the funnel or siphon, that prepares an inky fluid; and when the animal is disturbed it suddenly ejects this fluid, rendering the surrounding water so cloudy that it is often enabled to retreat unobserved. The 'ink' of the _Sepia_ was used for writing in former times, and is still employed in the preparation of the artist’s pigment that bears the same name. Fishermen are well acquainted with this peculiar characteristic of the animal, for they are frequently bespattered with the contents of the ink bag of the _Sepia_ when the creature is included in the contents of their draw-nets, and have learnt to handle it cautiously until the objectionable fluid has been all discharged.

We will conclude this chapter by giving a tabular summary of the classification of the molluscs which will probably be useful to the collector of marine objects.

CLASSIFICATION OF THE MOLLUSCA

Class =LAMELLIBRANCHIATA=--Plate-gilled. Headless, usually enclosed
in bivalve shell.
Section =SIPHONIDA=--Mantle lobes more or less united to form
tubular siphons.
Families--_Pholadidæ_, _Gastrochænidæ_, _Anatinidæ_,
_Myacidæ_, _Solenidæ_, _Tellinidæ_, _Mactridæ_,
_Veneridæ_, _Cyprinidæ_, _Lucinidæ_, _Cardiadæ_, &c.
Section =ASIPHONIDA=--Mantle lobes free or nearly so. No true
siphons.
Families--_Arcadæ_, _Mytilidæ_, _Aviculidæ_, _Ostreidæ_,
&c.

Class =CEPHALOPHORA=--Head-bearing. Usually enclosed in a univalve
shell.
Section =PTEROPODA=--Wing-footed molluscs.
Section =GASTEROPODA=--Stomach-footed molluscs.
Order =Nucleobranchiata=--Viscera form a nucleus on the back.
Order =Opisthobranchiata=--Shell generally absent. Gills more
or less exposed.
Section NUDIBRANCHIATA--Naked gills.
Section TECTIBRANCHIATA--Gills covered by shell or mantle.
Order =Pulmonifera=--Lung-breathers. Terrestrial.
Order =Prosobranchiata=.
Section HOLOSTOMATA--Aperture of shell entire (sea snails).
Families--_Chitonidæ_, _Dentaliadæ_, _Patellidæ_,
_Calyptræidæ_, _Fissurellidæ_, _Haliotidæ_,
_Turbinidæ_, _Littorinidæ_, _Turritellidæ_,
_Cerithiadæ_, _Pyramidellidæ_, _Naticidæ_, &c.
Section SIPHONOSTOMATA--Possess a true siphon. Carnivorous.
Families--_Cypræidæ_, _Conidæ_, _Buccinidæ_, _Muricidæ_,
&c.

Class =CEPHALOPODA=--Sucker-bearing arms round the mouth.
Order =Dibranchiata=--Two gills.
Section OCTOPODA--Eight arms.
Families--_Argonautidæ_, _Octopodidæ_.
Section DECAPODA.
Families--_Teuthidæ_ (Calamaries, Squids), _Sepiadæ_, &c.
Order =Tetrabranchiata=--Four gills (containing _Nautilidæ_).