CEPHALOPODA. 



605 



are aquatic and marine, and they are mostly nocturnal and gregarious 

 in their habits. 



The Dibranchiate order may be subdivided into two tribes ; the 

 one provided with the eight ordinary arms and the two longer ten- 

 tacles, hence c?d\ed Decopoda {fg. 219.) ; the other tribe without 

 the tentacles, and called Octopoda {Jig. 220). 



220 



Argonauta Argo, faem. 



The various forms of the extinct BelemnitidcB (Jig. 218.) consti- 

 tuted one family in the Decapod tribe. The little Spirula, charac- 

 terised by a less complex, but internal, chambered shell {Jig- 221.), is 

 the type of a second family. The cuttle-fish, characterised by its 

 internal calcareous shell, which feebly represents that of the Belem- 

 nite, exemplifies a third family of Decapods called Sepiadce, The 

 common calamary {Loligo), in which the internal shell is reduced to 

 a horny quill-shaped plate, represents the fourth and most extensive 

 family of the present tribe, which I have called Teuthidce ; and in 

 which one genus ( Onychoteuthis) had the caruncle of more or fewer 

 of its acetabula produced into horny claws.* In all the Decapods the 

 mantle supports a pair of fins {Jig. 219, h, b\ and the funnel is gene- 

 rally provided with a valve. 



In the tribe Octopoda fins are rarely developed from the mantle ; 

 but the eight ordinary arms are longer, thicker, and are united to- 

 gether by a broader web, which forms a powerful organ for swimming 

 in a retrograde direction. One family in this tribe (^Testacea) is 

 represented by the genus Argonauta, in which, in the female sex f, 

 the first or dorsal pair of arms {fig, 220, c, 1) are dilated at the ex- 



* CCCXCVII. 



t CCCXCVIII. p. 39. 



