2So CRUSTACEA. 



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. The abundance of chitin 

 may, to some extent, explain the absence of cilia in Crus- 

 taceans 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 ventral ganglia are concentrated. Sensory organs are 

 generally 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 orarchenteron — the original gastrula 

 cavity — and are especially large in the higher Crustaceans or 

 Malacostraca. 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, and of a true ccelome there is little 

 trace. In the blood ha;mocyanin is the commonest respira- 

 tory pigment. In the body or skin lipochrome pigments, 

 such as those which change from bluish green to red as the 

 lobster is boiled, frequently occur. Of modes of respiration, 

 there are many grades, — by the general surface, by currents 

 of water in and out of the posterior part of the food canal, 

 by thin plates on the legs, by well-formed gills. We miss 

 the numerous excretory nephridia of Annelids ; the green- 

 glands of lobsters, &c., probably represent a pair ; the shell- 

 glands of Phyllopods and Copepods and some other struc- 

 tures seem to be in part at least excretory. It is possible 

 that shell making is an organised method of getting rid of 

 some waste products. There are many peculiarities con- 

 nected with reproduction ; thus parthenogenesis for pro- 

 longed periods is common among " water fleas " ; herma- 

 phroditism occurs in barnacles, acorn shells, &c. ; the 

 hermaphrodites are sometimes accompanied by pigmy 

 " complemental " males ; the two sexes are often very 



