. . ly the dorsal shield asfar as its junction with the body. There is, however, amethod of combining these two modifications if, after adorsal shield has been developed, both the head-shieldand the dorsal shield are bent. We were at first inclined to attribute only the firstmethod of origin to the bivalve shells of the Ostracoda,and to deduce them from some such form as Harpesungula. It would be an obvious advantage to an animalgiven to the habit of rolling up for defence, to be able to SECT. XV CLASSIFICATION OF CRUSTACEA 257 conti
. . ly the dorsal shield asfar as its junction with the body. There is, however, amethod of combining these two modifications if, after adorsal shield has been developed, both the head-shieldand the dorsal shield are bent. We were at first inclined to attribute only the firstmethod of origin to the bivalve shells of the Ostracoda,and to deduce them from some such form as Harpesungula. It would be an obvious advantage to an animalgiven to the habit of rolling up for defence, to be able to SECT. XV CLASSIFICATION OF CRUSTACEA 257 continue to feed and breathe, and yet remain rolled upand sufficiently protected against its enemies. It is clearthat this end would hardly be attained in those casesin which a large solid pygidium closed against thehead-shield. But on the other hand, it would bequite possible by the longitudinal folding of the lateralwings of the head-shield, as shown in Fig. 57 B.\Yc may well suppose that some Trilobites adoptedthis method of protecting themselves, since, besides.
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