CEPnALOrODA. 



605 



aroaqnafic ami marine, and they arc mostly nocturnal and gregarious 

 in their habits. 



Tiie Dibranchiate order may be subdivided into two tribes ; the 

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

 tacles, hence caWed Decapoda (fff. 219.) ; the other tribe without 

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



220 



Argonauta Argo, faem. 



The various forms of the extinct Belemnitid(e {Jig. 218.) consti- 

 tuted one family in the Decapod tribe. The little Spirnla, 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 Teutkidce ; and in 

 which one genus ( OnychoteutJds) 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 arras {Jig. 220, c, 1) are dilated at the ex- 



CCCXCVII. 



t CCCXCVIII. p. 39. 



