Zöology; a textbook for colleges and universities . FIG. 70. A trilobite, Dalmanites, show-ing the dorsal surface. 258 ZOOLOGY quently the larger crustaceans, such as the crayfish,which are easy to examine, are commonly used to illus-. Photosraptt by W. P. Bay FIG. 71. A lady crab (Ovalipes ocellatus), about half natural in the sea along the Atlantic coast. trate an important evolutionary principle, the modifica-tion for various functions of a series of originally similarModifica- parts. It is not difficult to see that the two pairs ofantenn3e> tne mouth appendages and the feet,
Zöology; a textbook for colleges and universities . FIG. 70. A trilobite, Dalmanites, show-ing the dorsal surface. 258 ZOOLOGY quently the larger crustaceans, such as the crayfish,which are easy to examine, are commonly used to illus-. Photosraptt by W. P. Bay FIG. 71. A lady crab (Ovalipes ocellatus), about half natural in the sea along the Atlantic coast. trate an important evolutionary principle, the modifica-tion for various functions of a series of originally similarModifica- parts. It is not difficult to see that the two pairs ofantenn3e> tne mouth appendages and the feet, are builtupon the same general plan, but are greatly altered indetail to serve different purposes. The same principleis illustrated in the mammalian teeth, which are vari-ously modified for grinding and cutting, or in the handsand feet of man. The typical crustacean appendage issaid to be biramose (two-branched); that is to say, ithas a basal part and two terminal parts, like a handwith two fingers. The outer of these terminal partsis called the exopodite (outer leg part) and the inner theendopodite (inner leg part). In this respect the Crus-tacea differ from the terrestrial group of arthropods,but the legs of terrestrial Crust
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Keywords: ., bookcentury1900, bookdecade1920, booksubjectzoology, bookyear1920
