. Outlines of zoology. Zoology. 274 CRUSTACEA. give us trustworthy hints as to the ancestors of the higher Crustaceans ; it is likely that the Phyllopods, Apus, bear a similar relation to the whole series; the Copepods also retain some primitive characteristics ; but it is difficult, apart from mere guessing, to say anything definite as to the more remote ancestry. We naturally think of a segmented worm-type as a plausible starting- point for Crustaceans, and it is not difficult to understand how a development of cuticular chitin would tend to produce a flexibly jointed limb out of an unj
. Outlines of zoology. Zoology. 274 CRUSTACEA. give us trustworthy hints as to the ancestors of the higher Crustaceans ; it is likely that the Phyllopods, Apus, bear a similar relation to the whole series; the Copepods also retain some primitive characteristics ; but it is difficult, apart from mere guessing, to say anything definite as to the more remote ancestry. We naturally think of a segmented worm-type as a plausible starting- point for Crustaceans, and it is not difficult to understand how a development of cuticular chitin would tend to produce a flexibly jointed limb out of an unjoin ted parapodium ; how the mouth might be shunted a. little backwards, and two appendages and ganglia a little forwards ; and how division of labour would result in the differentiation of distinct regions. General Notes on Crustaceans. Of a class that in- cludes animals so diverse as crabs, lobsters, shrimps, " beach-fleas," "wood-lice," barnacles, acorn-shells, and "water- fleas," it is difficult to state general character- istics, other than those facts of structure which we have already sum- marised. Admitting the parasit- ism of many Crustaceans, and the sedentary life of barnacles and acorn-shells, we must still allow that great activity characterises the class. With this may be connected the brilliant colouring, the power of colour change, and the phosphorescence of many forms. Except in the case of a few primitive and degenerate forms, the Crustacea are all segmented. In this, in the presence of hollow jointed appendages, in the reduction of the coelom, and in their firm chitinous cuticle, the Crustacea resemble other Arthropods ; as special characteristics we. Fig. ii8. Nervous system of shore crab (Carcinus mtznas).âMter Bethe. br., The supra-oesophageal mass; £â ., gullet surrounded by gr., the gullet ring; m., the sub-cesophageal mass representing a fusion of the thoracic ganglia of the crayfish, and giving off nerves to the limbs
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