GENERAL INTRODUCTORY 9 
less well developed, and placed near the base of the 
“horns,” or on the summit of a special pair. The 
Bivalves, being headless, have usually no eyes; but 
some possess them during their larval existence, and 
they persist in the adult Mytilide and Pieria 
(=Avicula). Secondary visual organs are developed 
in certain forms, and more usually occur in some 
part of the margin of the mantle or siphons (see 
infra, p. 133). In the case of one or two molluscs 
eyes are developed over the back. 
That molluscs can hear is inferred, rather than/ 
known, from the presence of “otocysts,” small, 
cavities filled with fluid in which grains of shelly| 
material float. These otocysts are situated close to’ 
the pedal ganglia, and supplied by nerves from the 
cephalic ganglia (Plate III., Fig. 3, 0). 
From their discrimination of food some Mollusca 
appear to be capable of tasting, and they certainly 
can smell. The seat of the olfactory sense is believed 
to vary, and in some to reside in a tentacle, while in 
others it can be traced to a special organ called the 
“osphradium,” which in marine snails is situated 
close to the gills, and resembles them somewhat in 
appearance. 
For procuring their food all classes of the Mollusca, 
except the Pelecypoda, are furnished with one or a 
pair of horny mandibles or jaws (Plate III., Figs. 4-8). 
and the special feeding organ, the radula (Plate III., 
Figs. 11-22). 
