The general appearance is so different from the Stalked Barnacle that it is difficult to believe the two belong to the same group.
They are nearly all aquatic animals, by far the larger portion being marine, and they breathe with gills. They are provided with a hard calcareous or horny covering. The head is not separated from the trunk, as in insects; and they are provided with a number of jointed organs, usually classified as three pairs of jaws, three pairs of foot-jaws, and five pairs of legs in the more typical families; but in the smaller and more aberrant species the number is more variable. In their early stages they frequently pass through very extraordinary changes of form, but after assuming their adult shape they grow by casting their shells at intervals.
We will now notice a few typical examples of the different groups of these creatures.
The Brine-shrimp is a little reddish creature about half an inch long, which prefers the concentrated solution of brine-pits to sea-water. It has eleven pairs of legs, and, notwithstanding its name, the front portion of its body is considerably broader and flatter in proportion than that of a real shrimp, the other half consisting of a jointed tail.
Barnacles were formerly considered to be shell-fish, but are now usually classed with the crabs and lobsters, because, when they are young, they appear as freely swimming creatures, with one eye, two antennæ, and six pairs of jointed limbs. When they grow larger, they fix themselves to a rock or some other object by the head, and develop a shell, usually composed of several pieces. The commonest is the Acorn-barnacle, the white shell of which, measuring rather less than an inch across, swarms on rocks at the seaside. It is shaped like a limpet, but open at the top. The Goose-barnacles hang down by a stalk, and their jointed shells more resemble those of a mussel than that of a limpet, though they are composed of several pieces. Various species similar to both those mentioned are found on piers, rocks, the bottoms of ships, and even sometimes on the skin of whales.
In dark cellars in the country, under loose bark, or under pieces of wood which have been left in the fields, we often see creeping about brown creatures about half an inch long, with jointed bodies and antennæ, and short jointed legs. They are called Wood-lice, and several species roll themselves up into a ball when alarmed. These creatures feed chiefly on decaying vegetable substances; and there is a larger marine species much like them, which is common in holes and crannies in the rocks on the seashore.
There are other curious creatures, called Whale-lice and Fish-lice, which are parasitic in their habits. Some of these look like spiders, and one or two have enormously long legs; but others are of strange and almost indescribable forms, and sometimes without legs at all. One species, found on the sprat, has two long appendages at the end of its body not unlike a pair of compasses.
Photo by W. P. Dando, F.Z.S. Regent's Park.
WOOD-LOUSE.