354 PHYLUM ARTHROPODA 



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 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 imagine how a develop- 

 ment of cuticular chitin would tend to produce a flexibly jointed limb 

 out of an unjointed 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 includes animals so diverse as crabs, 

 lobsters, shrimps, " beach-fleas," *' wood-lice," barnacles, 

 acorn-shells, and " water-fleas," it is difficult to state general 

 characteristics, other than those facts of structure which 

 we have already summarised. 



Admitting the parasitism 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 char- 

 acteristics we notice the two pairs of antennae, the presence 

 of carbonate of lime in the cuticle, and the nature of the 

 respiratory organs — these, with few exceptions, being 

 adapted for breathing in water. While these characters 

 remain constant throughout the group, there is an almost 

 infinite variety in detail. In regard to the segmentation of 

 the body, we notice that, apart from the general tendency 

 to reduction which is so marked in many parasitic forms, 

 the higher forms as compared with the lower show marked 

 specialisation. In the primitive Phyllopods the body con- 

 sists of a large but varying number of segments, remarkably 

 uniform in structure. The higher Crustacea, on the other 

 hand, are characterised by their relatively few but constant 

 segments, which exhibit marked division of labour ; a 



