MOLLUSC A. 169 



The shell is divided into chambers by transverse septa, through 

 which runs a slender tube [siph uncle], which may be either cen- 

 tral, dorsal, or lateral. In the adult of the pearly nautilus 

 (Nautilus pompilius] only one fourth of the shell is occupied by 

 the animal, the remainder having been progressively evacuated as 

 it required more room. The empty chambers are said to contain 

 air. There are no important differences between the sexes. 

 Nautilus is the only existent genus. The extinct forms were nu- 

 merous in the Palaeozoic and Secondary periods. The Ammoni- 

 ticlse were almost entirely confined to the latter period. 



AmmonitidcB. Orthoceratidce. Nautilid<s. 



* Ammonites. *Orthoceras. *Clymenia. 



*Baculites. *Cyrtoceras. Nautilus. 

 *Ancyloceras. 



Order II. DIBEANCHIATA. 



ACETABULIFERA. 



Two branchiae. Arms eight or ten, with suckers. With or 

 without an external shell. Provided with an ink-bag. 



The female Argonauta, and not the male, is provided with an 

 external shell. It is secreted by the dilated dorsal pair of arms, 

 and is not divided into chambers. Spirilla has a chambered shell 

 enclosed by the mantle ; only a small portion of the body of the 

 animal occupies the last chamber. Three species are known. 

 In the extinct Liassic genus Belemnites the shell was conical and 

 internal, having a " camerated and siphoniferous structure." In 

 the remaining genera there is an internal shell in the form of a 

 dorsal plate, which is either calcareous (as in Sepia] or corneous 

 (as in Loligo}. An ink-bag, enclosed in the visceral cavity, for 

 the secretion of a black fluid, which is discharged through the 

 infundibulum, is always present, even in the extinct species. 



The " hectocotylus " is a peculiar modification of one of the 

 arms of tlie male as a sex-organ ; in performing its genetic func- 

 tion it becomes detached, but another is subsequently developed. 

 It was first described, when so detached, as a parasitic worm, 

 which Cuvier afterwards named "Hectocotylus." 



Some species of this class attain a large size ; Architeuthis dux 

 is said to have arms forty feet long. They are all predatory and 

 carnivorous animals. 



The order has been divided into two suborders Decapoda and 

 Octopoda ; and the former again divided and subdivided. The 

 extinct forms are only known in the Secondary and Tertiary epochs. 



