SEA5LUG5 



UITE an extensive group of 

 marine mollusks, well repre- 

 sented on our coasts, have, like 

 the slugs of our fields and 

 '^ gardens, given up the develop- 



ment of shells or have restricted their production to 

 small shields more or less covered by the mantle. They 

 are included in the order Opisthobranchiata, which 

 is so-called because the creatures it comprises have 

 their breathing organs behind the heart {opisthen, 

 behind). Each individual combines the special 

 functions of both sexes. The foot often bears side 

 wings, which fold up over the thin shell and protect 

 it. In these cases the shell is variously modified, its 

 solidity and spiral character being reduced in pro- 

 portion to the amount of protection it acquires. The 

 comb-like gill is, in some genera, suppressed alto- 

 gether, or secondary branchia3 are substituted for it. 

 In some p'roups the liver emerges from the body- 



^ 267 



