768 CLASS XIII. 
nor other external parts of the auditory organ present’. It is only 
within the last ten years that a knowledge has been attained of the 
auditory organs of Pteropods and Casteropods ; 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’. 
In most molluses 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 StrepeL, MUELLER, and 
Kroun. 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 ads 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’. 
1 See Soarpa Anat. disquisitiones de Auditu et Olfactu, Ticini, 1789, fol. pp. 5, 6, 
Tab. Iv. figs. 7—9, Cuvier Mém. s. l. Mollusq. No. 1, p. 41, &e. 
2 See Eypoux and Soutnyer U’Jnstitut. Jowrnal générale, &c. Tom. vi. 1838, 
p- 376; Kroun, Froriep’s neue Notizen, 1840, 1841, 8. 310—312; V. SIEBOLD, WIEG- 
MANN’S Archiv, 1841, and Ann. des Sc. nat. 2e Série, Tom. 19, 1843, pp. 193—2I1, 
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 Ericuson’s Archiv, 1845, 8. 217—222. 
3 SWAMMERDAM Bijbel der Natwure, t. bl. 105, 106; STIEBEL in MECKEL’S Archi, 
v. 1819, s. 206—210, Tab, v.; Jon. MuELLER, MECKEL’s Archiv fiir Anat. uw. Physiol. 
1829, s. 208-212, Tab. vi. figs. 4—8, Annal. des Sc. nat. Tom. 22, 1831, pp. 7—19 ; 
Kroun, MvuELLER’S Archiv, 1837, s. 479—485, and ibid. 1839, s. 332—337, Taf. x. 
