STRUCTURE AND PAYSIOLOGY OF THE MOLLUSCA. 17 
simple structure, and perhaps only possesses sensibility of light 
without the power of distinct vision. The larval bivalves have 
also a pair of eyes in the normal position (Fig. 30) near the 
mouth ; but their development is not continued, and the adults 
are either eyeless, or possess merely rudimentary organs of 
vision, in the form of black dots (ocelli) along the margin of the 
mantle.* These supposed eyes have been detected in a great 
( fk y tg pi 
UD xD <p 
Fig. 10. Pecten varius.} 
many bivalves, but they are most conspicuous in the scallop, 
which has received the name of argus from Poli on this account 
(Fig. 10). 
In the tunicaries similar oce//i are placed between the tentacles 
which surround the orifices. 
Sense of Hearing. In the highest cephalopods, this organ 
consists of two cayities in the rudimentary cranium which pro- 
tects the brain ; a small calcareous body or otolithe is suspended 
cs 
Fig. 11. Tentacle of a Nudibranch.t 
in each, as in the yestibular cavities of fishes. Similar auditory 
capsules occur near the base of the tentacles in the gasteropoda, 
and they have been detected, by the vibration of the otolithes, 
in many biyalyes and brachiopods. With the exception of 
* “ Bach possesses a cornea, lens, choroid, and nerve: they are, without doubt, 
organs of vision.”? (Garner.) The same conclusion is arrived at by Duvernoy ina 
paper in the Annales des Sciences Natureiles for 1852. 
| ecten varius, L., ‘rom a specimen dredged by Mr. Bowerbank, off Tenby 
m, the pallial curtains; br, the branchie. 
¢ Fig. 11, Tentacle of Eolis coronata, Forbes, from Alder and Iancock. 
