Elements of biology; a practical Elements of biology; a practical text-book correlating botany, zoology, and human physiology elementsofbiolog00hunt Year: [c1907] CRUSTACEANS 225 Classification. — All the forms of crustaceans mentioned belong to that subclass of crustaceans called the Malacostraca, the characters of the group being a definite number of segments and appendages. All having five pairs of walking appendages are called decapods. How many of the above-mentioned forms are decapods? Entomostraca. — Another subclass qf crustaceans, in which the number of appendages is not fixed, is th
Elements of biology; a practical Elements of biology; a practical text-book correlating botany, zoology, and human physiology elementsofbiolog00hunt Year: [c1907] CRUSTACEANS 225 Classification. — All the forms of crustaceans mentioned belong to that subclass of crustaceans called the Malacostraca, the characters of the group being a definite number of segments and appendages. All having five pairs of walking appendages are called decapods. How many of the above-mentioned forms are decapods? Entomostraca. — Another subclass qf crustaceans, in which the number of appendages is not fixed, is the group Entomostraca. They arc mostly small animals, some species existing in countless numbers. Such are the fairy shrimps found appearing in early spring in fresh-water ponds, little translucent swimming forms from one half to three fourths of an inch in length. Another fresh-water form often seen in aquaria is the water flea (Daphnia). From the economic stand- point, probably the most important crustaceans that we shall study are the copepods. These tiny animals are barely visible to the naked eye. They are found in almost every part of the world, from the arctic seas to those of the tropics, and in fresh as well as salt water. They are so numerous that the sea in places is colored by their bodies. So prolific are they that it is estimated that one copepod may produce in a single year four billion five hundred million offspring. These animals form a large part of the food supply of many of our most important food fishes as well as the food of many other aquatic animals. They are, then, in an indirect way, of immense economic value. Degenerate Crustaceans. — One of the most interesting forms to a zoologist is the goose bar- nacle. This crustacean is free-swimming during its early life. Later, however, after passing through several changes in form during its de- velopment, the barnacle settles down on a rock or some floating object, fastens itself along the dorsal surfac
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