168 



COMPARATIVE ANATOMY 



CHAP. 



closed auditory vesicles (otoeysts), whose epithelial walls usually con- 

 sist of ciliated and sensory cells. The interior of the otocyst is filled 

 with fluid and contains a varying number of otoliths (1 to over 100). 

 These vary in size, form, and chemical constitution, and in the living 

 animal oscillate in the fluid in which they are suspended. 



The otoeysts are usually found on or near the pedal ganglia, rarely 

 far from it. It is, however, well established that the auditory nerve 

 does not originate in this ganglion but in the cerebral ganglion, though 

 it often runs along close to and even in contact with the fibres of 

 the cerebropedal connective. 



In most cases the otoeysts arise as invaginations of the outer 

 epithelium. An interesting discovery has recently been made, that in 



primitive Lamellibranchs 

 (Nucula, Leda, Yoldia) each 

 of the otoeysts even in the 

 adult still opens by means of 

 a long canal on the surface 

 of the foot. In such cases 

 the otoliths are particles of 

 sand or other foreign matter 

 taken in from outside. In 

 Cephalopoda, the remains of 

 the canal of invagination is 

 retained (Kolliker's canal), 

 but it ends blindly. 



The auditory organs are 

 most highly developed in 

 those Molluscs which are 

 good swimmers, especially in 

 the Cephalopoda and Hetero- 

 poda. Among these, maeulse 

 and eristse aeustiese are 

 developed. 



Heteropoda. The struc- 

 ture of the auditory organ 

 of Pterotrachea (Fig. 144), 

 which has been thoroughly 

 examined, is as follows : 



The wall of the otocyst consists in the first place of a structure- 

 less membrane surrounded by muscle and connective tissue. Inside 

 the vesicle, which is filled with fluid, a calcareous otolith, built up of 

 concentric layers, is suspended. The inner surface of the vesicle is 

 lined by an epithelium, containing three different sorts of cells : 

 auditory, ciliated, and supporting cells. The auditory cells, which 

 carry immobile sensory hairs, are found on the wall of the otocyst at 

 a point (macula acustica) diametrically opposite to the place where the 

 auditory nerve enters. At this spot there is a patch formed of 



FIG. 144. Auditory organ of Pterotrachea (after 

 Glaus). 1, Auditory nerve ; 2, structureless membrane ; 

 3 and 4, ciliated cells ; 5, otolith ; 6, auditory cells ; 7, 

 supporting or isolating cells ; 8, large central auditory 



