226 ARTHROPODS. 



More Detailed Notes on the Characteristics of 

 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 few facts of structure which 

 we have already summarised. 



When we contrast Crustaceans with Insects, we remember 

 of course that aquatic life is as general among the former as 

 it is exceptional in the latter, and that the modes of respira- 

 tion by gills and air-tubes respectively, or the characteristic 

 modes of locomotion by swimming and flying are very 

 different. 



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 some primitive and degenerate forms, all are seg- 

 mented. The typical appendage consists of a basal piece with 

 two jointed branches. The cuticle is always chitinous, and 

 often very much calcified. Perhaps the abundance of 

 chitin explains the absence of cilia in Crustaceans and 

 other Arthropods. The rigidity of the cuticle partially 

 explains the necessity of frequent moults. As the muscles 

 contract very rapidly, they illustrate the striated condition 

 with great clearness. In crabs and some others the ventra^ 

 ganglia are concentrated. Sensory organs are very well 

 developed ; both " eyes " and " ears " may occur away from 

 the head. Much of the alimentary canal, which is almost 

 always simple, consists of fore-gut and hind-gut. These are 

 anterior and posterior invaginations of skin which meet the 

 mid-gut or archenteron — the original gastrula cavity. The 

 frequent presence of a gastric mill is quite intelligible, for it 

 occurs in the fore-gut. The body-cavity is never very large, 

 being mainly filled up with muscles and organs. In the blood 

 haemocyanin is the commonest respiratory pigment. In the 

 body or skin lipochrome pigments, such as those which change 

 from bluish green to red as the lobster is boiled, frequently 



