CRUSTACEA. 163 



constant progress, although it be slow and gradual. We have seen 

 the external skeleton assuming a symmetrical and jointed structure 

 in the Anellides, although consisting as yet only of a simple series 

 of rings : we have seen it provided with jointed organs of locomotign 

 and exploration in the Epizoa ; though here, indeed, the advance 

 was but transitory, and both organs and faculty of spontaneous motion 

 were quickly lost. In the Cirripedes, jointed appendages to the body 

 are retained, but their rapid actions are subservient to the acquisition 

 of food, not to locomotion. 



We now arrive at a class of articidated animals in Avhich the an- 

 nular segments of the skeleton of the body are constantly provided 

 with articulated limbs or appendages ; in which all the species are 

 free and locomotive, and are provided with distinct respiratory organs. 

 These animals are still, however, aquatic ; only a part of the class 

 can support themselves and move with their jointed limbs on dry 

 land ; the highest act of locomotion is that of climbing reeds or trees, 

 which a few species of the present class are enabled to effect by long 

 prehensile claws. But the breathing organs in all the species are or- 

 ganised for aquatic respiration ; in other words, are branchiae ; and 

 it is the combination of branchiae with jointed limbs and distinct sexes 

 which constitute the essential characters of the class Crustacea. 



The name of this class refers to the modification of the external 

 tegument by which it acquires due hardness for protecting the rock- 

 dwelling marine species from the concussion of the surrounding ele- 

 ments, from the attacks of enemies, and likewise for forming the 

 levers and points of resistance in the act of sujDporting the body, and 

 moving along the firm ground. In the crab and lobster tribes the 

 external layer of the integument is hardened by the addition of earthy 

 particles consisting of the carbonate, with a small proportion of the 

 phosphate of lime. This crust is coloured by a pigmental substance, 

 diffused more or less irregularly through it ; and it is formed upon 

 and from a vascular organised membrane, or corium, which is lined 

 by the smooth serous membrane of the visceral cavities. In the 

 smaller Crustacea the tegument retains a flexible, horny, or perga- 

 meneous texture. 



Whatever be the consistence of the external integument or skeleton, 

 it is always disposed in a series of segments, either actually separate 

 and moveable on each other, or confluent in a variable extent and de- 

 gree, so as more or less to obliterate the traces of their primitive dis- 

 tinctness. The results of very laborious and extensive study and 

 comparisons of the modifications of the external crust of the diver- 

 sified forms of the present class have been generalised with much 

 success by Dr. M. Edwards, who has shown that most of the Crus- 



M 2 



