. Essentials of biology presented in problems. Biology. THE CRAYFISH 231 living of animals in a mutually helpful relation has been referred to as symbiosis. Of this we have already had some examples in plants as well as among animals. (See page 187.) Habitat. — Most crustaceans are adapted to live in the water; a few forms, howe^'er, are found living on land. Such are the wood lice, the pill bugs, which have the habit of rolling up into a ball to escape attack of enemies, the beach fleas, and others. The coconut crab of the tropics climbs trees in search of food, return- ing to the water at in
. Essentials of biology presented in problems. Biology. THE CRAYFISH 231 living of animals in a mutually helpful relation has been referred to as symbiosis. Of this we have already had some examples in plants as well as among animals. (See page 187.) Habitat. — Most crustaceans are adapted to live in the water; a few forms, howe^'er, are found living on land. Such are the wood lice, the pill bugs, which have the habit of rolling up into a ball to escape attack of enemies, the beach fleas, and others. The coconut crab of the tropics climbs trees in search of food, return- ing to the water at intervals to moisten the gills. / Characters of Crayfish and its Allies. — Our study of crayfish shows us that animals belonging to the same group as itself have several well-marked characteristics. The most important are the presence of a segmented limy exoskoleton, gills, jointed appendages, usually a pair to each segment of the bodj^ (except the ), stalked compound eyes, and the fact that they pass through a metamor- phosis or change of form before they reach the adult state. "We find that the Crustacea fall naturally into two classes, those in which the number of pairs of appendages varies, and those in which the number is fixed at nineteen. In this latter class are placed the crayfish, lobster, blue crab, shrimp, and most of our common crustaceans. Entomostraca. — Another sub- class of crustaceans, in which the number of appendages varies, is the group Entomostraca. They are mostly small animals, some species existing in countless numbers. One of the largest Entomostracans in- habiting fresh water is the fairy shrimp () found ap- pearing in early spring in fresh- water ponds, a Uttle translucent swimming animal from one half to three fourths of an inch in length. Another fresh-water form often seen in aquaria is the water tlca (daphnia). From the economic standpoint, probably the most im- portant crustaceans that we shall study are the copepods.
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectbiology, bookyear1911
