Sea-slugs 303 



place (the Ccnia of the Romans) and the finder. It 

 is only about a quarter of an inch in length, and is 

 distinguished by an elevated back, a somewhat 

 angular head adorned by a pair of very slender 

 tentacles before the eyes. The back is black, but 

 this gradually fades away on either side until it 

 merges into fawn or yellow. 



There is a group of remarkable mollusks scarcely 

 represented in the British area, and which are there- 

 fore not of sufficient local import- 

 ance to have a chapter devoted to 

 them. They are in no sense slugs, 

 but as their natural position appears 

 to be just after the Nudibranchiata 

 we deal with them here. We refer 

 to the Pteropoda, formerly regarded 

 as a Natural Order, but now con- 

 sidered as a sub-order of the Opis- 

 thobranchiata. They are Gasteropods 

 in which the side expansions of the 

 foot have been developed into fins, to 

 fit them for a life at the surface of 

 the ocean. On this account they have 

 been called the Butterflies of the Sea, but havino- 

 regard for their greater activity towards night, it 

 would perhaps be less incorrect to call them Sea 

 Moths. In infancy all are provided with shells, but 

 many dispense with these ere they attain maturity ; 

 on the presence or absence of a shell in the adult the 

 primary classification of the group into two sections 

 is based. Those in which the shell, or a covering of 

 cartilage, is always present are known as Thecoso- 

 20 



Clionc (somewhat 

 enlarged) 



