greater variety of form and attain a larger size. There are some species which live at all times on the land, and some come to it as occasional visiters.

Every one who has opened the "shell" of the common Crab has noticed a number of leaf-like organs, regularly arranged into two parcels, with the points of the little leaves. or plates in each parcel brought nearly together (Fig. 37). These are the gills, organs admirably adapted to the aquatic life of the animal.

Fig. 37.-p, Part of the lining membrane of the shell.-b, Gills in their natural position.-b', Gills turned back to show their vessels.

In the Lobster, the arrangement of these parts is different. In other Crustacea, the gills are formed like feathery tufts, and float freely in the water (Fig. 38); while, in those that

live partially or altogether on land, contrivances of different kinds exist to retain so much water as to enable the gills to perform their functions.

All Crustacea are produced from ova or eggs, of which the "pea" of the common Lobster is a familiar example. The young do not, however, on their liberation from the ova, present a miniature resemblance

to the species to which they belong. They are, in fact, so very unlike, that they were formerly described as animals of singular form and rare occurrence, under the name of Zoea. The annexed figure of one of these (Fig. 39) is copied from a Dutch work published in 1778. Five of

E

Fig. 39.-ZOEA (MAGNIFIED).

them only were known, when in 1822, Mr. J. V. Thomson, to his great surprise, met with Zoeas in considerable abundance in the Cove of Cork. Further research showed that these animals, which had been regarded as so rare that the capture of each was recorded as an event, were to be found in vast profusion in our bays and estuaries, and were but the young of the common crabs!

Those who have only seen the common edible crab (Cancer pagurus, Fig. 41) in its mature condition, will perhaps be surprised to learn that it existed at one time under the form represented in Fig. 40, its members being adapted for swim

n...

ming, and its body so minute that its natural size, when in that state, is shown by the speck adjoining the letter n.

All the crustacea possess the capability of reproducing extremities which are injured. Thus, if the leg of a crab be fractured, it throws off the injured limb, near to the body. The bleeding then stops, and a new limb is gradually developed. Another provision exists by which the growing animal throws off its "shell" and is supplied with one of larger dimensions. In two or three days the new covering assumes the hardness of the old one; and, until then, the animal, as if conscious of its defenceless state, avoids, as much as possible, all exposure.

Fig. 40.-Young of the
COMMON CRAB.

The mouth in some crustacea is formed for suction, in others for mastication. Some have the eyes mounted on footstalks; in others the footstalk is absent. Again, some have the gills enclosed in the body, and have ten legs; others have the gills external, and the number of the legs or appendages variable. By such characters they are divided into sections, orders, sub-orders, genera, and species.

The crabs used as food are, of course, those which are most valued and sought after. The large edible crab is that which, in the North of Ireland, is known as the crab (Fig. 41). It is distributed round all our coasts, and is generally taken by wicker-baskets, like the cage-shaped wire mousetraps, and baited with guts of fish or other garbage; but it is also taken by means of a piece of hooked iron thrust into its retreats at low water.

The smaller edible crab of British authors is the most common species round the entire coasts of Great Britain A and Ireland, lurking beneath stones or tangle, or half concealed in the moist sand. We learn from Leach that

this species "is sent to London in immense quantities, and eaten by the poor, who esteem it a great delicacy." It is

used in like manner in Paris, but is never offered for sale in the markets of the North of Ireland.

The Pea-crabs form an interesting group, from their diminutive size, and their singular habitation in those shells which, like the cockle, are composed of two pieces, or valves. One of these was celebrated in connexion with the Crab; as,

"The anchored Pinna and her cancer friend."

The Pinna, according to tradition, being warned of the approach of danger by the alacrity of the little Crab, who was the joint and friendly occupant of her mansion. Two and even three Crabs are occasionally found in one mussel or one pinna.

The hermit-crabs belong to a different order. The tail is prolonged and soft, and hence, in self-defence, the animal is obliged to occupy some univalve shell, which has been deserted by its original occupant. When thus in search of a dwelling-house, it is said to present an amusing spectacle as it inserts the tail successively into several shells until one is found to fit. From the fact of each Crab being the solitary inmate of its retreat, the common English name has no doubt been bestowed.

In the Crustacea of the next order, the tail is not only longer but is different in form, being divided into five broad flat pieces, so as to act with great effect upon the water, The common Lobster is perhaps the best known example; it is taken all round the rocky portions of the coast.