CEPHALOPODA. 457 



each side, of which two are median and dorsal. The inner portion forms 

 three distinct lobes in the female, i. e. a right and left lateral lobe, present 

 also in the male, and an inferior or ventral lobe, represented in the male 

 by three groups of lamellae. The lateral lobes each carry twelve tentacles, 

 and are divided in the male into two portions, a more dorsal with eight 

 tentacles, and a ventral which carries four tentacles on the right side, 

 and is known as anti-spadix, but on the left is represented by the spadix, 

 a conical body terminated by imbricated lamellae, and supposed to have 

 a sexual function like the hectocotylised arm in Dibranchiata (p. 464). 

 The tentacles are long solid cylindrical bodies, retractile into muscular 

 sheaths. The sheath with its tentacle probably corresponds to a single 

 sucker of the Dibranchiate arm. The median portion of the foot forms the 

 siphon, the base of which is covered by the mantle fold. It consists in 

 Nautilus of a right and left lobe with the free edges simply apposed, but 

 in all Dibranchiata grown together so as to form a tube. The hind 

 part of the foot is perhaps represented by the valve of the siphon, absent 

 in Octopoda, which projects from the body- wall within the siphon. 



The body or visceral dome is elongated in an oblique direction 

 between the dorso-pedal and 'antero-posterior axis. It is never twisted 

 spirally. Its sides are expanded into a pair of variously shaped fins in 

 Decapoda which are only exceptionally present in Octopoda (Pinnoctopus^ 

 Cirrhoteuthis). The mantle forms a free fold round the edge of the visceral 

 dome. The dorsal margin of the fold in Nautilus is enlarged and 

 reflected over the shell : it is very shallow in Dibranchiata. The ventral 

 or posterior part of the fold is of great extent in all^ Cephalopoda and 

 incloses a branchial cavity. 



There is no shell in the Octopoda except in the female Argonauta^ 

 but a rudimentary shell-sac appears on the anterior dorsal aspect of the 

 visceral dome and then aborts. The external calcareous shell of Argonauta 

 is formed by the expanded ends of the pair of dorsal arms. It lodges 

 the animal which is not fixed to it, is single-chambered and coiled. The 

 shell-sac of Decapoda gives origin to a shell which does not lodge the 

 animal ; it either remains open as in Spirilla, or closes so that the shell 

 is internal, in other living forms, as well as in the extinct Belemnitidae 

 and perhaps in the extinct Spirulirostra. The shell of Spirula is coiled, 

 chambered, and the chambers are connected by a siphuncle (infra). That 

 of Spirulirostra is very similar ; of the Belemnitidae straight. But in 

 these extinct forms calcareous lamellae are superadded to the chambered 

 shell. The latter is known in Belemnitidae as ' phragmacone : ' the 

 lamellae as ' guard ' or ' rostrum,' and their prolongation forwards, whether 



to the hood of Nautilus ; (2) that they are the homologues of the nuchal cartilages of Dibranchiata , 

 a view based on their microscopic structure. See Von Ihering, Neues Jahrbuch f. Mineral. Geol. 

 und Palaeont. 1881, i. 



