AUDITOKY ORGANS OF MOLLUSC A. 357 



followed up to the cerebral ganglion ; it sometimes passes along the 

 cerebro- pedal commissure. This change in position is sometimes 

 associated with a change in the position of the cerebral ganglia 

 themselves. 



In the Lamellibranchiata and Scaphopoda the auditory vesicles 

 are attached to the pedal ganglion ; they are either close to, or at 

 some distance from it (Naiades), or are even placed more deeply in 

 the substance of the foot (Cythera). In the Gastropoda the 

 otocysts vary greatly in position ; but the primitive position — that 

 is, close to the cerebral ganglia — is the most common one ; in the 

 Heteropoda, and many Opisthobranchiata, they are always placed 

 close to these ganglia. 



The otoliths are either numerous, and consist of small crystalline 

 structures, which form an otoconia, or there is but a single spherical 

 otolith, which is derived from one cell of the rudimentary wall of 

 the auditory vesicle, and which forms a concentrically-striated con- 

 cretion. Dentalium, the lower forms of the Lamellibranchiata, and 

 Gastropoda, and also all the Pteropoda, possess otoconia. In the 

 larval stages, however, of these Mollusca, the otoconia first appear, 

 and then, later on, the spherical otolith ; this apparently disappears 

 afterwards. But when the adult form is provided with an otolith, 

 an otolith is found in the larva, and is never jDreceded by the 

 otoconia. 



We have not as yet any connected statements about the struc- 

 ture of the end-organs in the otocyst. The most important fact 

 that we do know is that part of the ciliated epithelium is repre- 

 sented by cells with fine rod-like processes, which appear to bo 

 true auditory fibres. They form the auditory organ, and are con- 

 nected with the nerves, while the ciliated cells, which are grouped 

 in tufts, form an accommodating arrangement which acts on the 

 otoliths. 



We have a permanent proof that the otocyst is developed 

 from the ectoderm in the Cephalopoda in the presence of a fine canal, 

 which, in many of them, leads from the auditory vesicles to the 

 surface of the body. In Nautilus the otocysts lie on the cephalic 

 cartilage; in the Dibranchiata they are enclosed by it. It is 

 possible, therefore, to distinguish a membranous aud a cartilaginous 

 labyrinth, analogous to the similar parts found in the case of the 

 Vertebrata. 



In the Octopoda the auditory vesicles are simple in form ; in the 

 Decapoda they are complicated by the formation of diverticula and 

 processes. At the same time they are more closely connected with 

 the cartilage, whereas, in the Octopoda, they lie somewhat freely in 

 their cavity. The otolith, which is placed in a watery fluid, varies 

 in form — it may be flattened or rounded, or may be broken up into 

 smaller acicular pieces. The terminations of the auditory nerves 

 either form the " auditory plate," which is a thickened portion of 

 the epithelium, from which the cells send hair-like processes (audi- 

 tory hairs) (Sepia); or an "auditory crest/ ; which generally takes 



