192 ORIGIN OF FUNCTIONAL EYES chap. 



ommatidia (from 10 to 80) into one large round eye, which is 

 generally elevated above the surface of the surrounding epithe- 

 lium. Sometimes these eyes themselves tend to fuse together. 

 In one specimen of Area Noae^ 133 of these faceted eyes were 

 counted in one mantle border, and 102 in the other. 



There can be little doubt that the development of these 

 functional eyes, or sensitive spots, in bivalve Mollusca, is due to 

 special needs. They appear to be entirely absent in fresh-water 

 bivalves (with the exception of Dreissensia^ which is obviously 

 a marine genus recently become fresh-water), while they are 

 most abundant in genera living between tide marks QSolen, 

 Mya^ Mactra)^ and most highly specialised in a genus that is, 

 for a bivalve, of singularly active habits (^Peeteri). Now genera 

 living in sand between tide marks, as the three above-mentioned 

 genera are in the habit of doing, and also protruding their 

 siphons, and occasionally a considerable portion of their shells, 

 out of their burrow, are manifestly very much at the mercy of 

 their watchful enemies the gulls, and anything which would 

 enable them to apprehend the approach of their enemies would 

 be greatly to their advantage. Here, perhaps, lies the explana- 

 tion of the greater elaboration of these pigmented spots in lit- 

 toral genera, as compared with those inhabiting deeper water. 

 Pecten^ again, a genus distinguished by great activity, which 

 can ' fly ' for considerable distances in the water by flapping its 

 valves together and expelling the water from the apertures at 

 either side of the hinge, may be greatly assisted by its ocelli in 

 directing its flight so as to escape its enemies. 



III. Smell 



4 



The sense of smell — touch at a distance, as Moquin-Tandon 

 has called it — is probably the most important sense which the 

 Mollusca possess, and is unquestionably far more valuable to 

 them than that of sight. Any one who has ever enjoyed the 

 fun of hauling up lobster pots will recollect that part of the 

 contents was generally a plentiful sprinkling of Buecinum^ 

 JVassa, and Natiea^ attracted by the smell of the stinking piece 

 of fish with which the trap was baited. According to Mr. J. S. 

 Gibbons,^ Bullia rliodostoma congregates in hundreds on gigantic 

 1 Quart. Journ. of Conch, i. p. 368. 



