HANDLES : ANATOMY OF TTJRRITELLA COMMUNIS. 63 



From the abdominal ganglion two nerves arise, the stouter of which 



innervates the genital gland. The following nerves are given off from. 

 ' the cerebral ganglion : — one, arising from the outer side of the ganglion, 



runs to the tentacle ; close to this, and parallel with it, another very 

 1 fine nerve can be distinguished which passes to the eye. The anterior 

 : portion of the ganglion gives origin to several nerves innervating the 



snout. The innermost of these runs along the 6ide of the oesophagus, 

 " and at the junction of this structure with the buccal mass terminates 

 . in a large ganglion, situated immediately above the radula-sac. This 



buccal ganglion (b.g.) is connected to its fellow on the other side by 



■ a stout, broad commissure. From the under side of the cerebral 

 [ ganglion a very fine nerve arises, which, passing between the cerebro- 

 i pedal and pleuro - pedal connectives, and crossing over the latter, 

 [ supplies the otocyst (of.). From the pedal ganglion several nerves 



are given off which pass to the anterior and posterior regions of the foot. 

 A slender nerve arises from the outer side of the pleuro - pedal 

 [ connective, near the point where the auditory nerve crosses. 



The eyes are situated at the base of the awl-shaped tentacles. Each 



■ consists of a closed cavitj T , the inner wall of which is densely 

 ' pigmented, and contains a spherical lens. 



The otocysts (PI. VI, Fig. 4,.ot.) approximate the posterior border 

 of the pedal ganglia. Each encloses a single large circular otolith, 

 which is striated both concentrically and radially. 



In T. rosea, Quoy & Gaim., Bouvier [1] (p. 201) describes the presence 



'■ of several small rectangular otoconia in the otocyst, in addition to 



a large circular otolith ; he does not seem to attach much importance 



to this interesting condition, nor to the difference between the two 



' species T. communis, Eisso, and T. rosea, Quoy & Gaim., in this respect. 



1 The presence of otoconia is generally regarded as a primitive feature, 



I aud is characteristic of the majority of the Diotocardia and Architamio- 



glossa, whilst the single otolith is indicative of a higher degree of 



organization, and occurs in many of the Monotocardia, notably in those 



f >rms which, on other grounds, have been regarded as specialized, e.g. 



1 the Rachiglossa, Toxoglossa, and several of the Tamioglossa. 



Whether the presence of both otolith and otoconia in the otocyst of 



' T. rosea, Quoy & Gaim., and of only an otolith in T. communis, Kisso, 



' indicates that the former species is more primitive than the latter, is 



' problematical ; but it would certainly be interesting to see if these 



d.fferences exist in other species of Turritella. 1 



A similar condition obtains in forms other than the Turritellidae. 

 ' Vivipara vivipara (Linn.) possesses otoconia, whilst a single otolith 

 ! occurs in Biihynia tentacuJata, Linn. Melania amarula, Linn., and 

 ' If. tuberculata, Mull, (and its variety If. costata), have otoliths ; 

 If. filocarinata, Mous., and If. asperata, Lam., have otoconia [1] 

 \ (pp. 419, 420). 



: ' Jt is interesting to note, as previously mentioned fp. 58), that T. rosea, Quoy & 

 Gaim., is a form in which the outer lip possesses a broad, shallow sinus, aud 

 would in this respect seem to be more primitive than T. communis, Kisso, in 

 which the outer lip is entire. 

 VOL. IV.— AUGUST, 1900. 5 



