GENERAL INTRODUCTORY g 



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 Mytilidae and Pteria 

 (=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, o). 



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). 



