400 Shell Life 



1869 by the Porcupine Expedition, floating at the 

 surface, 140 miles to the north-west of Ireland. 



From the brief descriptions we have given of the 

 Cephalopods that occur on our coasts, it will be seen 

 that there is considerable difference in the characters 

 of the shell. Lankester considers that these differ- 

 ences may indicate successive stages in the develop- 

 ment of the shell. The probable course of such 

 development can only be appreciated by a comparative 

 study of the shells of all the genera, exotic as well 

 as native, fossil as well as recent ; but taking the 

 Chambered Nautilus (Nautilus 2^oin2yilius) as one 

 end of the series, we can see in the shell of Sjnrula 

 its degradation. No longer able to contain the 

 animal, it has become almost entirely enveloped in 

 folds of the mantle. " These folds gradually concresce 

 to form a definite shell-sac, by the walls of which 

 are secreted additional lamina? of calcareous shell- 

 substance. These laminae invest the original shell, 

 which gradually {Spiridirostra, Belosepia) loses the 

 spiral form and becomes straight, eventually dis- 

 appearing, while the calcareous laminae alone remain 

 (Sepia). These in their turn disappear, leaving only 

 the plate or ' pen ' upon which they were deposited 

 (Loligo), which itself also, with the shell-sac, finally 

 disappears, surviving only in the early stages of 

 Octopus.'' 



