274 



CRUSTACEA. 



give us trustworthy hints as to the ancestors of the higher Crustaceans ; 

 it is likely that the Phyllopods, e.g. 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).—Mter Bethe. 



br., The supra-oesophageal mass; £■., 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 ; behind it. is 

 a short strand representing the abdominal 

 ganglia of the crayfish. aX., antennules ; 

 a 2 ., antennae ; e. t eye. 



