768 CLASS xm. 



nor other external parts of the auditory organ present 1 . It is only 

 within the last ten years that a knowledge has been attained of the 

 auditory organs of Pteropods and Gasteropoda ; they are two round 

 saccules filled with fluid, each containing a small otolite or nume- 

 rous calcareous concrements, and situated on the central nerve-mass 

 beneath the oesophagus, or in some behind the eyes, in which case 

 they receive a nervous branch from the cerebral ganglion 2 . 



In most molluscs two small eyes are present, in the Cephalo- 

 pods alone are they large. They never occur in greater number, 

 nor dispersed over other parts of the body, but are always situated 

 on or near the head; in some they are non-pediculate and are 

 placed at the base of the feeler, or stand on tubercles or sometimes 

 at the point of the feeler, as we lately stated. In Nautilus they 

 are seated on little pedicles at the side of the head, like hemispheres 

 flattened on the anterior surface. In the rest of the Cephalopoda 

 they lie in lateral excavations of the cartilaginous ring of the head, 

 protected in some degree by two long, thin, oval cartilaginous 

 plates in front. In the Gasteropoda, SwAMMERDAM formerly inves- 

 tigated the eyes anatomically ; in our century this investigation has 

 been repeated and extended, especially by STIEBEL, MUELLER, and 

 KROHN. Under the skin, that in this situation is destitute of pig- 

 ment, lies the eye-ball, enclosed by a proper membrane (sclerotica) , 

 which is transparent in front, and sometimes very convex (cornea). 

 The vascular choroid, coloured by a black pigment, is furnished 

 with an annular aperture (just as an iris is). A spherical lens and 

 a vitreous body occupy the interior of the eye-ball, whilst the optic 

 nerve expands within the choroid into a cup-shaped retina 5 . 



1 See SOARPA Anat. disquisitiones de Auditu et Olfactu, Ticini, 1789, fol. pp. 5, 6, 

 Tab. IV. figs. 7 9, CUVIER Mem. s. I. Molluxq. No. i, p. 41, &c. 



2 See EYDOUX and SOULEYET I'Institut. Journal ginerah, &c. Tom. vi. 1838, 

 p. 376; KROHN, FRORIEP'S neue Notizen, 1840, 1841, s. 310 312 ; V. SIEBOLD, WIEG- 

 MANN'S Archiv, 1841, and Ann. des Sc. not. 2e Serie, Tom. 19, 1843, pp. 193 211, 

 PL 2 B. These auditory vesicles appear very early in the development of the embryo, 

 but are at first simply filled with fluid, from which the otolite or the calcareous concre- 

 ments are separated at a later period only and gradually by crystalization ; see FREY 

 in ERICHSON'B Archiv, 1845, s. 217 222. 



3 SWAMMERDAM Bijbel der Natuure, i. bl. 105, 106; STIEBEL in MECKEL'S Archiv, 

 v. 1819, s. 206 210, Tab. v. ; JOH. MUELLER, MECKEL'S Archiv fur Anat. u. Physiol. 

 1829, s. 208212, Tab. vi. figs. 4 8, Annal. des Sc. nat. Tom. 22, 1831, pp. 719 ; 

 KROHN, MUELLER'S Archiv, 1837, s. 479485, and ibid. 1839, s - 33 2 ~ 337> Taf - x - 



