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 called Decapoda (Jig. 219.) i the other tribe without 

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



220 



ArgonauU Argo, fern. 



The various forms of the extinct Belemnitidce {Jig. 218.) consti- 

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

 terised by a less complex, but internal, chambered shell {Jig. 22 1 .), 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 Tetithtdce ; 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, b, 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 (Jig. 220, c, 1) are dilated at the ex- 



• cccxcvn. t cccxcvni. p. 39. 



