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.
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.