. Elementary textbook of economic zoology and entomology. Zoology; Insect pests. CRAYFISH, LOBSTERS, CRABS, SHRIMPS, ETC. 115 fish-lice which are especially interesting because of their parasitic habits and the greatly modified structure resulting therefrom. Some live as commensals, that is, are associated with their hosts in such a way as to derive benefit from them, without injuring them. Others are truly parasitic and live upon the blood or tissues of their host. The most common of these attach themselves to the gills of fishes, but they may also be found as external or internal parasites,
. Elementary textbook of economic zoology and entomology. Zoology; Insect pests. CRAYFISH, LOBSTERS, CRABS, SHRIMPS, ETC. 115 fish-lice which are especially interesting because of their parasitic habits and the greatly modified structure resulting therefrom. Some live as commensals, that is, are associated with their hosts in such a way as to derive benefit from them, without injuring them. Others are truly parasitic and live upon the blood or tissues of their host. The most common of these attach themselves to the gills of fishes, but they may also be found as external or internal parasites, of whales, molluscs, marine worms, starfishes and many other animals that are found in the sea. The barnacles, order Cirripedia, look but little like other Crustacea. For a long while they were classed with the Molluscs, but a study of their development showed that in the young stages they are like the crustaceans and not the molluscs. The young are free-swimming, and after going through a series of changes they become attach- ed to some firm object. Here the young barnacle undergoes further metamorphosis, and the adult form with its compact or somewhat worm-like body is de- veloped. The appendages, or legs, are long, slender and curled. FIG. 44. — A stalked barnacle, The animal is enclosed in a shell fy™ mUL (About * natural often of the shape of a truncate cone and composed of six or more plates. The open end of the shell may be closed by a lid or operculum, thus well pro- tecting the inmate. The barnacle feeds on minute organisms which it sweeps into its mouth by means of the long feathery appendages. Although barnacles may completely cover piling and other timbers in the water, they do not injure them and may even be of some service in protecting them from wood-boring or wood-destroying animals. Sometimes, however, they are serious pests on oyster beds. The goose-barnacle, or ship-. Please note that these images are extracted from scanned page images that may have been
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