CIEEIPEDA CRUSTACEIv. 



229 



sisting of calcified chitine. The body of the animal is 

 enclosed in a sac lined with the most delicate membrane of 



FIG. 14. A, SEA-ACORNS ; B, BARNACLES (CirripeUes). 



chitine, which in one group is prolonged into a peduncle 

 and contains the ova ; the body is distinctly articulated and 

 placed with the back downwards." 



Dr. Carpenter describes the young of the Cirripedes as 

 not fixed like the adult creatures, but moving about freely, 

 and only becoming fixed in the form of the usual acorn shell 

 after undergoing several strange metamorphoses, during 

 which stages they more resemble the ordinary Crustaceans 

 than they do in their fixed state. 



The Crustacea form the third class of the Articulata, they 

 comprise animals covered wdth a hard shell or caripace, like 

 a case made up of rings, with joints allowing the pieces to 

 move upon each other, except where several of them are 

 consolidated to form the principal covering. These rings 

 generally amount to twenty-one in number, some composing 

 the claws, others the legs, &c. The shell is chiefly made 



