384 LIMPETS ; CHITONS. CIBRHOBRANCHIATA. 



the formation of the shell, are the Chitons ; of which some small 

 species inhabit our shores, but which attain to much greater size 

 between the tropics. Their shell (Fig. 663) is composed of a 

 number of plates arranged behind one another with great regu- 

 larity, and connected by a very complex series of ligaments and 

 muscles, which reminds the Naturalist of those which unite and 

 move the different segments in the Articulated animals. The 

 Limpets and Chitons* fix themselves to the rock, in the same 

 manner as the Haliotis ; and it is stated that Crows and other 

 birds, which endeavour to detach them for food, are sometimes 

 caught by the points of their bills, and held firmly until they 

 are drowned by the advancing tide. The Limpets are herbi- 

 vorous, feeding upon sea-weeds, which 

 they reduce with their long riband-shaped, 

 rasp-like tongues. Although we usually 

 see them attached quite motionless to 

 rocks, this is because exposure to the air 

 is unfavourable to their movement, since 

 too free an admission of it between their 

 gills would dry them up. It is when 

 covered with water that their activity 

 manifests itself. In many points of struc- 

 ture, the animals of this Order approach 

 those of Bivalve shells. The rock, in the 

 no. 673 -CHITON. Limpet, may almost be regarded as a second 



valve ; since the muscle takes a firm 



attachment to its surface, and draws down the shell upon it, in the 

 same manner that the adductor muscle of the Bivalves enables 

 the animal to inclose itself by drawing the two valves of the 

 shell together ( 1016). Moreover the eyes are very imperfect 

 or entirely absent ; and many other points of internal structure 

 prove the inferiority of these animals to the Gasteropods in 

 general. 



SECTION C. CIRRHOBRANCHIATA. 



1001. This group 'ncludes only the family DENTALIID^E, the 

 animals of which inhabit a slightly curved, tapering shell, re- 



