112 



described, passes over the surface of the septal membrane, 

 perforates it, and its fibres unite with the distal cells. 



As to the function of the eyes in Pecten, Patten, 

 apparently in order to surmount the difficulty of an 

 animal having more than two eyes of such complexity, 

 has advanced the theory that they are organs for the 

 reception of solar energy, which is then transmitted along 

 the optic nerves to centres where it is used in the 

 building up of protoplasmic compounds, or in metabolism 

 generally. Leaving aside the physiological objections to 

 such an idea, which has been severely criticised by more 

 recent workers, it will be seen that Patten assumes the 

 structures are such as would be evolved for the purpose of 

 receiving solar energy, the rays of which are concentrated 

 by the lens. 



A lens, however, cannot increase the solar energy 

 falling upon its surface, it can only cause the rays to fall 

 upon a smaller area of the retina. In a review of Patten's 

 paper in the Q. J. M. S. for 1887, it was pointed out that 

 " a naked epidermic surface of area equal to that of the 

 lens would present a perfect instrument for the absorption 

 of solar energy." 



Observations carried out on Pectens living in the 

 tanks at the Port Erin Biological Station have shown that 

 they are very sensitive both to light and to sound waves. 

 If a dark object is moved in front of a Pecten so that the 

 shadow falls over the eyes, a rapid closing of the valves 

 immediately follows. A sudden increase in the illumina- 

 tion apparently produces no effect. The flashing of the 

 light from a bright lantern on the animals, which have 

 the valves open and the tentacles extended in the darkness 

 of the aquarium at night, causes no retraction or closure 

 of the valves. 



It is obvious that with a lens forming an image on a 



