HALF AN HOUR WITH SHELL-FISH. 183 



Chitons; whilst others, as the sea-slugs, have no 

 shell at all. These all belong to that order called 

 Gasteropoda, or " belly-creepers," in allusion to their 

 mode of progression being like that of the common 

 land -snail. 



We purpose, however, drawing attention first to 

 certain marine animals whose organisation is con- 

 siderably higher than those just named. They are 

 included in an order once very numerous called 

 Cephalopoda, or " head-footed," on account of the 

 long arms being arranged round the head. A good 

 deal of interest has lately been excited in this class 

 of animals, by the introduction of one of them, the 

 " Cuttle-fish," in the aquarium of the Crystal Palace. 

 Its intense ugliness and seeming ferocity has earned 

 for it the name of the " Sea Devil !" A good idea of 

 its general appearance may be obtained from Fig. 99. 

 Perhaps it will be as well, before entering farther 

 into the details of the structure of this and other 

 nearly allied animals, if we devote a short space to 

 a consideration of the general characters of the 

 family to which they belong. 



First of all we may remark that the Cephalopoda 

 have been in existence longer than any other class 

 of shell-fish, or mollusca, except the " Lamp-shells " 

 (Bracliiopoda). They date from the early Silurian age, 

 and their remains are found more or less crowding 

 the limestones of every geological period since then. 

 This early group, however, is more nearly allied 

 to the nautilus of the Indian Ocean, than to our 



