Amphipoda.
The Amphipods are very numerous, and abound in the British seas. They have long, slender, and many-jointed bodies which have no carapace: the tail in some genera is more fitted for swimming, in others for leaping. The Talitrus, or Sandhopper, common on every sandy shore in Europe, is a well-known example of the leaping genus. It is very small and exceedingly active. The upper antennæ are very short, the inferior pair are large, and longer than the whole body. The anterior feet are thin and not prehensile. The first pair end in an immovable claw; the second pair have a kind of hand, and are folded beneath the body; the following feet end in a crooked nail. The appendages of the last three rings of the tail are thick and spiny, and the tail serves as a leaping organ.
The sandhoppers hide themselves between tidemarks in large communities under masses of wet sea-weeds, on which they feed. When disturbed they leap away with great agility, and bury themselves in the sand by digging with their fore-feet, and kicking the sand away with their tail-feet. They have a strong sense of smell, for if a dead fish be buried in the sand, it is devoured by these little voracious animals in a few days.
In the fin-tailed genera the gills are suspended between the bases of the thoracic legs: they swim lying on their side, and their feet are very varied in form, but always more or less furnished with spines and hairs.
There are several genera of Amphipods that are nest-building animals; all have hooks at the end of their tails, The Amphithoæ enclose themselves in a cylindrical tube open at both ends. The animal is very active, running along the branches of the sea-weeds by means of its antennæ instead of its feet, which remain within the tube. In general only the first pair of antennæ are put out to catch prey. If the animal be prevented from advancing, it immediately turns its body within the tube, and protrudes its head from the other extremity.
Isopoda.
The order of Isopoda are so called because of the sharp and equal claws of their walking feet, which are often prehensile. Their body is short and flattened, and their small head is almost always distinct from the throat. They are very numerous, and are divided into walking, swimming, and sedentary animals; the females have horny plates on some of their feet, which fold under the throat and form a pouch, in which the eggs are hatched.
The Oniscus, common Wood-louse, or Slater, is a terrestrial Isopod. It is an oval jointed creature, which rolls itself into a ball when touched. The second of its six pairs of posterior limbs perform the part of lungs: they contain hollow organs in their interior, into which the atmospheric air penetrates directly through openings in their exterior covering: so the Oniscus and its congeners, which live on land, are drowned when put into water.
In the swimming Isopods, the five first pairs of tail-limbs are false feet, and are suspended under the tail. The gills, consisting of two great oval leaves, are fixed to them by a stalk; and are dragged through the water. This group is very numerous; many live among the sea-weeds on the coasts, others perforate submerged wood in all directions, and live in the winding galleries they have formed. The Limnoria lignorum is particularly destructive in the harbours on the British coasts, and in the locks of the canals. The tortuous holes it bores are from the fifteenth to the twentieth of an inch in diameter, and about two inches deep. The female Isopod is not more than a line or two in length, the male is a third less, and of a grey or greenish brown. These minute creatures bore their holes with their mandibles, which are so sharp and strong that they can penetrate the hardest wood, and appear to feed on it, from the quantity found in their stomachs. Their bodies are covered with pinnated hairs, their antennæ are short, and their posterior end or tail is rounded.
Most of the genus Cymothea are parasitical; they can bend the sharp nail of the three first pairs of feet upon the preceding joint, so as to form hooks with which they fix themselves to the fishes on whose juices they feed.