77 



that they now lie end to end and parallel with the plane 

 of the cornea. The gelatinous lens is probably secreted 

 by the supporting cells. The optic nerve breaks up 

 underneath the optic vesicle, and a network of fibres 

 extends below the retina. 



So far as observations go, the whelk relies but little 

 on its visual organs. It is difficult to understand how 

 complicated organs of this kind could be produced in the 

 mollusca, when one thinks of the feeble responses to 

 experiments. Either our experimental methods have 

 failed so far to demonstrate the utility of many of the 

 molluscan eyes, or some of our conceptions of evolu- 

 tionary processes require modification. The tendency to 

 take anthropomorphic views, however, in work both 

 structural and experimental, for invertebrate sense 

 organs is perhaps the danger to be avoided. 



The Osphradium. 



The sense organ known as the Osphradium, situated 

 usually in close proximity to the respiratory organs, 

 attains a degree of complexity in Buccinum which is 

 probably never exceeded in the Mollusca. In the 

 Lamellibranchs the osphradium is merely an area of 

 somewhat thicker epithelium, whose presence is not even 

 marked by pigment (except in Area). It is, therefore, 

 quite invisible to the naked eye. 



The lower gastroj>ods, the Diotocardia, possess an 

 osphradium which seems to be somewhat of the same type 

 so far as general structure is concerned. As one passes, 

 however, towards the more highly developed Monoto- 

 cardia, the osphradium takes the form of an axis bearing 

 on each side a large number of leaflets. It becomes, in 

 fact, bi-pectinate, and resembles superficially a gill. 



