CHAPTER III. 



CUTTLE FISHES, 



THE cephalopods or cuttles are the most specialised of molluscs. 

 They form a class distinct from the others in structure and 

 habits, and their shells, when present, are quite unlike those with 

 which we have principally to deal. But they are undoubtedly 

 mollusca, and, as such, demand attention before we are introduced 

 to their humbler relatives. 



They are grouped into two orders : the Dibranchiata, which have 

 two gills; and the Tetrabranchiata, which have four. The four-gilled 

 division are not represented in British waters. The two-gilled 

 cephalopods, which are represented by a dozen species, are 

 divided into 



1. Octopoda having eight arms. 



2. Decapoda having ten arms. 



The Octopoda are divided into six families, one only of which is 

 represented around our coasts, this being the Octopodidae, of which 

 the two British genera are 



1. Octopus having two rows of suckers. 



2. Eledone having one row of suckers. 



The Decapoda are divided into thirteen families, of which three 

 are recognised as British, these being 



1. Loliginidae having the shell narrow, pointed in front and as 



long as the back. 



2. Sepiolidae having the shell narrow and half as long as the body. 



3. Sepiidae having the shell with a thin chitinous margin, oval, 



thick in front, the posterior ventral end being concave and 

 ending in a spine. 



The Loliginidae are represented by two genera : 



1. Loligo having the shell keeled on the ventral side, eyes with 



closed cornea. 



2. Ommastrephes having the shell ending in a hollow cone, eyes 



with open cornea. 



25 



