MOLLUSCA, THE SHELL-FISH 113 



nature of the ground where it has happened to 

 fix itself. Thus arises the disfigurement of the 

 shell. 



So far we have only considered two great 

 groups of the Mollusca, two which are repre- 

 sented by common shells, familiar to everybody. 

 We must not leave the subject of the Mollusca 

 without referring to their most aristocratic group, 

 the Cephalopoda. These are represented in mu- 

 seums by the shells of the Pearly Nautilus, and 

 of its not very near relative, the Paper Nautilus; 

 and they are represented on English shores by 

 the cuttle-fishes. All these agree with the Gas- 

 teropoda in the possession of a tongue-ribbon, 

 and in classification are therefore treated with 

 them under the name Glossophora. 



With the Pteropods, transparent forms found 

 swimming over the surface of the deep sea, the 

 reader is not likely to have much to do. In 

 classification they are now placed near the Sea- 

 Slugs. 



The Placophora, or Polyplacophora, wholly 

 different from our usual idea of a shell-fish, may 

 be named as creatures which the reader is quite 

 likely to meet with. Though not very common, 

 they are widely distributed over our coasts, and 

 may be found near low-tide mark clinging to 

 stones. Imagine a wood-louse without any ap- 

 parent head which has taken to clinging to the 

 rock like a- limpet, so that it cannot be removed 

 without injury, and you have a rough idea of 

 their general appearance. Chiton is the name of 

 these animals, which have received the group 

 name of Polyplacophora, carriers of many plates, 

 because their external covering consists of an 

 armour of successive shelly plates. These also 



