THE CEPHALOPODA 



317 



ganglion of Eledone and Omrnatostrephes occupies a situation analo- 

 gous to that of the osphradial ganglion of Gastropoda and Lamelli- 

 branchia, but the epithelium overlying it is not sensory. It seems 

 probable that an osphradium is not required in the Dibranchia, in 

 consequence of the proximity of the olfactory fossa to the opening 

 of the pallial cavity. 



The statocysts or otocysts are two in number, and are always 

 closed vesicles in adult Cephalopoda ; they are essentially organs of 

 equilibration. In Nautilus they are situated at the sides of the 

 pedal centres (Fig. 279, 0), and are closely applied to the cephalic 

 cartilage. In the Dibranchia they are placed ventrally between 

 the pedal and visceral centres (Fig. 282, X), and are wholly embedded 

 in the cranial cartilage, being separated from one another only by 

 a thin partition. The cavity of each otocyst is continued, in the 



XVIII 



Flo. 282. 



Central nervous system and anterior part of the digestive tract of Oramatostrephes, left-side 

 view. I, radula ; II, "anterior " salivary gland ; III, anterior buccal ganglia and commissure ; 

 IV, cerebral ganglion ; V, section of the optic nerve ; VI, oesophagus ; VII, left pallial nerve ; 

 VIII, " posterior " salivary gland; IX, visceral ganglion and nerve; X, seat of the otocysts ; 

 XI, infundibular nerve ; XII, pedal ganglion ; XIII, stomato-gastric ganglion ; XIV, brachial 

 ganglion and beginning of the live left brachial nerves; XV, labial commissure; XVI, 

 "tongue" ; XVII, mouth. 



Dibranchia, into a small canal which is buried in the cartilage in 

 the Decapoda but not in the Octopoda. This canal, known as 

 " Kolliker's canal," ends blindly, and is the remnant of the em- 

 bryonic connection of the otocyst with the exterior (Fig. 119, D, of). 

 The internal wall of the otocysts of the Dibranchia is not simple, 

 but is raised into several well-marked ridges separated by furrows. 

 The sensory epithelium is localised at the anterior end of the organ, 

 and forms a macula acustica, and the essential part of the otocystic 

 nerve terminates in this macula and in a lateral ridge. The nerve 

 originates from the cerebral ganglion and traverses the pedal centre 

 obliquely. In Nautilus each otocyst contains numerous otoconia, 

 but in the Dibranchia there is a single otolith balanced on the 

 principal macula acustica : this otolith consists of an organic and a 

 calcareous moiety, except in Eledone, in which genus it is wholly 

 organic. 



The eyes in all Cephalopoda are situated on the sides of the 



