Zöology; a textbook for colleges and universities . etail, itwill be useful to give a summary of the principal groups. CHAPTER THIRTY-SIX PHYLUM ARTHROPODA Class CrustaceaSubclass Trilobita THE trilobites, now entirely extinct, were formerly Thetriio-very abundant in the sea. They were highly developed ^enatn as early as Cambrian time, groupThe body was segmented,and bore very numerousjointed appendages. It ispossible that these primi-tive crustacea gave rise tosome centipedelike typewhich took to the land,developing tracheae or airtubes for breathing. Ithas been suggested that inthis manner t
Zöology; a textbook for colleges and universities . etail, itwill be useful to give a summary of the principal groups. CHAPTER THIRTY-SIX PHYLUM ARTHROPODA Class CrustaceaSubclass Trilobita THE trilobites, now entirely extinct, were formerly Thetriio-very abundant in the sea. They were highly developed ^enatn as early as Cambrian time, groupThe body was segmented,and bore very numerousjointed appendages. It ispossible that these primi-tive crustacea gave rise tosome centipedelike typewhich took to the land,developing tracheae or airtubes for breathing. Ithas been suggested that inthis manner the trilobitesmay have been remote an-cestors of the insects ; butDr. G. C. Crampton, of theMassachusetts AgriculturalCollege, has lately givengood reasons for excludingthem from the direct lineof ancestry. At the timewhen the insects were becoming dominant the trilobiteswere disappearing. Subclass Eucrustacea (or Crustacea proper) The appendages are modified in various ways, for Appendageslocomotion, for feeding, and as organs of sense. Conse- 25?. 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-
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Keywords: ., bookcentury1900, bookdecade1920, booksubjectzoology, bookyear1920
