Before the study of shellfish, or molluscs, was conducted on the scientific principles of the present day, shells were classified as univalves, bivalves, and multivalves. The univalves were shells in one piece, such as the whelk; the bivalves those in two pieces, such as the mussel or oyster; and the multivalves those in more than two pieces, such as barnacles or chitons, barnacles, however, being no longer classed with shells.

The highest of the five types, or natural divisions, of animals are the Vertebrata, the Mollusca, and the Annulosa. The vertebrates usually have vertebrae, or jointed backbones, and from this the highest division takes its name; but the real test is the colour of the blood, which in the vertebrates is always red.

The molluscs have soft bodies and no internal skeleton, but in lieu of this the animal is usually protected by an external shell, harder than the bones of vertebrates. The annulosa, like the molluscs, have soft bodies and no internal skeletons; but the external shell is divided into joints or segments, and is usually softer than the bones of vertebrates.

Fishes belong to the vertebrate division, oysters to the mollusc, and crabs and starfish to the annulosa.

The remaining two of the five divisions are the Caelenterata, in which the general cavity of the body communicates freely with that of the digestive apparatus, and the Protozoa, which includes all animals, such as sponges, etc., not included in the above four divisions.

The shell of an oyster takes the place of the bones of a dog; and although it may seem strange for an animal to have its bones on the outside of its body, it is really no more strange than for a fruit, such as the strawberry or raspberry, to have its seeds on the outside. Lime is the principal ingredient of all bones; and the bones of vertebrate animals contain a large proportion of phosphate of lime, while the shells of molluscs, or shellfish (as they are popularly called), consist almost entirely of carbonate of lime.

When scientists began more carefully to examine the structure of shellfish, they found that those similarly constructed had shells with certain marked peculiarities. The days of conchology were then doomed; and the study of the mollusc, or malacology, took its place.

Besides those necessary for digesting food, most shellfish have organs equivalent to those of vertebrate animals, such as feet, arms, eyes, head, heart, and tongue. Although bearing the same names, these organs rarely have a similar shape to those of the vertebrates, being necessarily adapted to the different mode of living. The foot of a cockle, shaped like an animal's tongue, enables it to move slowly from place to place, as well as to burrow in a sandy beach with the comical jerks so well known to observers. The tongues are beautifully designed for their work. The long, narrow tongue of the vegetarian mollusc works like a scythe, and mows down the delicate marine grasses on which the animal feeds. The powerful tongues of those that prefer an animal diet are able to bore through the strongest shells; and woe betide the unfortunate shellfish which, having shown signs of weakness, or disease, is surrounded by its active, carnivorous brethren. The tongue, sometimes longer even than the shell itself, is covered with rows of very hard spikes, or teeth, arranged similarly to the burrs on a file. As these teeth break, or are worn out, they are replaced by others that push themselves forward when wanted. Under a microscope of moderate power, the radula, or tongue, of a shellfish, especially a limpet, is a most interesting sight, and many molluscs can be identified merely by examining the tongue under a microscope. The shape of the teeth, the number, and the arrangement of them will settle the question.

The appetites of molluscs verge on the voracious. Break up a few cockles, or other shellfish, and place them in shallow water on a calm day, and watch the result. If in the vicinity of rocks, and during a rising tide, all the better. First come the wary little shrimps to the feast. Some are creeping cautiously, and some are jumping and racing, as if afraid of not being in time. Then the carnivorous shellfish approach from all directions, foremost amongst them being the different species of Cominella. While they are lumbering along, shells appear to be actually running; but a close inspection shows that these contain active little hermit crabs, whose tender tails, having no hard covering of their own, are snugly stowed in the empty shells of defunct molluscs. Then the sand or gravel moves, and crabs appear. The shrimps, crabs, and hermit crabs run off with the smaller morsels; but the molluscs gather round the remnants and pull and haul and roll over one another until the feast is ended, when some, being satiated, contentedly burrow into the sand; while others, with their appetites only sharpened, will wander away in search of fresh prey.

In many shells, such as the Triton, or Lotorium as it is now called (Plate III.), every increase in growth can be traced in the thick lip formed by the animal when it has increased the size of its shell. Others again, such as the Struthiolaria (Plate IV., Fig. 4), only form a lip when their full size has been attained, and by this the difference between an old and young Struthiolaria can at a glance be seen. Others form a lip at each growth, and then dissolve the lip before starting again. Vertebrate fish are supposed to grow, and increase in size, till the day of their death, but shellfish do not do this. The shell becomes stronger and thicker with age, the animal having the ability to add layer after layer of nacreous, or pearly deposit, on the inside of the shell; and as the animal shrivels and lessens in size the thickness of the shell increases. And some, when they become too large, have power to dissolve the partitions in the shell, and deposit the material on the outside of the shell.

The time it takes a shellfish to grow to its full size varies a great deal. Oysters take about five years; but the giant Tridacna, the largest bivalve in the world, has been found so enclosed in the slow-growing coral that it could hardly open its valves.

The young of most shellfish are active little things, and are usually so different from their parents as to be unrecognisable. Some swim, or frisk about, and travel even long distances in search of suitable quarters to settle in. Others float on the surface, and are driven where the winds and currents list. Some, like mussels, are distributed all over the world, others again are found, perhaps, on one rock, or on one small sandbank in a large district. Many shells are rare, because we do not know where to look for them; but if we know and can find their food, we will find the shellfish not far away. Some change their shape so much that, as they age, they have to dissolve all the partitions made in their youth in the shell. The eggs of some are scattered on the surface of the water, while the eggs of others are hatched by the mother before being turned adrift.

Marine shellfish live in all kinds of places below high water mark; and some of the semi-amphibious ones thrive even above ordinary high water mark, where for days at a time nothing but the tops of the waves could reach them. They are found on seaweed and on rocks, and on sand or mud-banks; but especially in places near rocks on marine grass banks bare at low spring tides. Some live on the surface of the water, some burrow in sand or mud, and some bore holes for themselves in the softer rocks. Some live in deep water; but the better coloured shells are found near low water mark, or in shallow water; for light is as necessary to the perfecting of colour in shells as in flowers. Shells that have grown in a harbour are more fragile than those grown in the ocean, and are usually less brilliant in colour, as harbour water is not as clean as ocean water. The colour of shells (as of insects) depends largely on environment, and is only one, and by no means the most reliable, method of deciding the species. An expert can at a glance tell whether a given shell has come from shallow or deep water, and whether from an exposed or sheltered spot. Most shellfish move about a great deal, and migrate into deeper water in summer; and on bright clear days retire into dark corners amongst, and even under, stones. On a dull day a collector is frequently more successful than on a bright, sunny day; and in spring or early summer the best hauls of live shells can be made. Nearly all shells have an epidermis, or outer skin. In some this is very apparent, as in the Lotorium olearium (Plate V., Fig. 1), or the Solenomya parkinsoni (Plate IX, Fig. 18), while in others it is nearly transparent, and hardly perceptible. To enable the true colours of a shell to be seen the epidermis must be removed.

The supposed original form of a shell was that of a volute univalve, such as the Triton (now Lotorium), or Struthiolaria. To properly enclose the animal, and make it safe from enemies, an operculum, or lid, was so formed that when the animal retired into the shell this filled up the opening. The operculum is usually like a piece of thin, rough brown horn, and where no reference is made to an operculum in this work, it must be understood that the operculum is horny. Some shells, such as the Astralium sulcatum (Plate VI., Fig. 18), and the Turbo helicinus (Plate VI., Fig. 17), have a shelly operculum; that of the latter being the well-known cat's eye.

In some shells the operculum is small, in others large, and progressing step by step we find some, such as the scallop and oyster, with one side round, and the other (really an operculum) flat and as large as the shell; until we come to the perfect type with each valve the same shape and size. Then the operculum disappears, as in the limpet, and the covering shell becomes smaller and smaller, till in the Scutum ambiguum (Plate IX., Fig. 23) the shell bears about the same proportion to the animal that the little bonnet, fashionable a few years ago, bore to the lady that wore it. The shell is built up of very thin layers of nacre, or mother of pearl, and calcareous or chalky matter, the thinner being the layers of nacre the more lustrous and iridescent is the shell.

As would be expected from its isolated position, many of the genera of New Zealand shells are not found elsewhere. The late Professor Hutton mentions nine genera in this position.

The dispersal of shells is an interesting natural phenomenon. The eggs of molluscs are so small that they can easily be carried by currents, attached to floating seaweed or floating timber, on the hulls of ships, or in the feathers or feet of our migratory birds, such as the godwit, which every year travels from New Zealand to Siberia and back. A great many of our shells are found on the Australian coasts; and a surprising number are common to both New Zealand and Queensland.

In describing the illustrations, length means extreme length, and by measuring the shell on the plate the proportionate width can be ascertained. The illustrations are, generally speaking, half the natural length of the shell depicted; and the shell photographed, although in most cases an average full-sized specimen, in some instances was smaller than the average.