THE CEPHALOPODA 317 
ganglion of Hledone and Ominutostrephes 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 
Fic. 282. 
Central nervous system and anterior part of the digestive tract of Ommatostrephes, left-side 
view. I, radula; II, ‘‘anterior” salivary gland ; ILI, 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 five 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 Hledone, in which genus it is wholly 
organic. 
The eyes in all Cephalopoda are situated on the sides of the 
