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 JVoae, 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 Di^eissensia, which is obviously a 

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

 abundant in genera living between tide marks (Solen, Mya, 

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

 bivalve, of singularly active habits (Pecten). 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 

 bm-row, 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 explanation of the 

 greater elaboration of these pigmented spots in littoral genera, as 

 compared with those inhabiting deeper water. Fecten, again, a 

 genus distinguished by great activity, which can ' fly ' for con- 

 siderable 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 



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 

 Buccinum, Nassa, and Natica, attracted by the smell of the 

 stinking piece of fish with which the trap was baited. Accord- 

 ing to Mr. J. S. Gibbons/ Bullia rhodostoma congregates in 



1 Quart. Journ. of Conch, i. p. 3G8. 



