HALIOTIS. 207 



of growth, for conveying water to the breatliing organs ; 

 and the mantle being partly slit enables the water to pass 

 into the respiratory cavity through a tubular filament pro- 

 truding from each perforation. These filaments and per- 

 forations are the same both in youth and age, except when 

 the hindmost orifice is occasionally filled up. 



Internally the Sea-ear is lined with a bright pearly nacre, 

 which is often extremely brilliant, and glows with all the 

 colours of the rainbow. The discrimination of different 

 species depends, however, on external sculpture, and is in 

 nowise effected by the beauteous adornment of the inner 

 surface. Perforations vary, according to the taste, it may 

 be, or necessities of different households, but generally 

 correspond in individuals of the same species; whenever 

 an exception occurs, it uniformly indicates that the shell is 

 inhabited by an adult, because when the moUusk arrives at 

 maturity, he generally closes one or two of the perforations 

 in advance of a fresh opening. 



Few, if any, Haliotides are found where Chitons fix their 

 dwellings, and it seems as if their geographical distribution 

 was exchanged, to a certain extent, in the two hemispheres. 

 A few specimens are found in California, but along the 

 western coast of South America, where Chitons are most 



