GENERAL NOTES ON CRUSTACEANS. 



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 seg- 

 mented. In this, in 

 the presence of hollow 

 jointed appendages, in 

 the reduction of the 

 ccelom, and in their firm 

 chitinous cuticle, the 

 Crustacea resemble other 

 Arthropods ; as special 

 characteristics we notice 

 the two pairs of antennae, 

 the presence of carbon- 

 ate of lime in the cuticle, 

 and the nature of the 

 respiratory organs 

 these, with few excep- 

 tions, being adapted for FIG. 165. Nervous system of shore-crab 

 breathing in water. (Cardnus <<> After Bethe. 

 While these characters 



remain Constant through- sub-cesophageal mass representing a fusion of 



, , . the thoracic ganglia of the crayfish, and 



OUt the grOUp, there IS giving off nerves to the limbs ; behind it is 



an almncf infi nif^ vari^tv a short strand representing the abdominal 



.6 Variety gang ii a O f the crayfish. l., antennules ; 



in detail. In regard 2 -, antennae ; *., eye. 



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 



