. An introduction to zoology : for the use of high schools . he creatures resort to emptyunivalve shells for protection. An allied East Indian genus, the cocoa-nut crab (Birrjus latro), lives in holes in the earth, and, instead ofdepending on its gills for respiration, uses the wall of the gill-cavity asa lung. This is an instance of what is termed change of function,a principle which must be borne in mind, in comparing the structure ofanimals which are nearly allied in form, but different in habits. 13. Two other orders of Crustacea, which resemble the cray-fish in the number of the segments
. An introduction to zoology : for the use of high schools . he creatures resort to emptyunivalve shells for protection. An allied East Indian genus, the cocoa-nut crab (Birrjus latro), lives in holes in the earth, and, instead ofdepending on its gills for respiration, uses the wall of the gill-cavity asa lung. This is an instance of what is termed change of function,a principle which must be borne in mind, in comparing the structure ofanimals which are nearly allied in form, but different in habits. 13. Two other orders of Crustacea, which resemble the cray-fish in the number of the segments and the appendages, havefresh-water representatives which are very common, althoughthe majority of both are marine. These are tlie Isopoda andthe Amphipoda; but in both, only one of the eight pairs ofthoracic appendages is turned forwards toward the Isopods have the body depressed, while in the Amphipodsit is compressed. A familiar example of the former is thewater-slater, Asellus communis. (Fig. 123). It will be observed 198 HIGH SCHOOL that the four hindmost abdominal segmentsare coalesced above into a shield, from beneathwhich the last pair of legs project. The threepairs of legs in front of these serve for respira-tion, and the eggs are carried in the female on theunder surface of the thoracic segments. TerrestrialIsopods, like the common Wood-louse (Oniscus)and its allies, exhibit an interesting adaptation forbreathing air; one of the pairs of abdominal legsbeing traversed by tubes which have the samefunction as the tracheae of insects. Among the marine Isopods several are temporary par-cummunitxz.^ asites adhering to the surface of fish ; others are perman-ent parasites, which live in the gill- or body-cavity ofother Crustacea, and which consequently loose much of their resem-blance to the free Isopods. Of the fresh-water Amphipods species of a genus Gammarus (Fig. 124), are everywhere to be50 //^v^Xj-(.>^ /\ meb with. The gills are on the thoracic
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Keywords: ., bookcentury1800, bookdecade1880, booksubjectzoology, bookyear1889
