1898.] MICKOSCOPICAL JOURNAL. 51 



plialic eyes yet special organs of sight are developed on 

 the free margin of the mantle-skirt ; such is the case 

 with the scallop. 



In the living state under water, just within the 

 margin of each valve may be observed a row of minute 

 points of great brilliancy, sparkling like diamonds, each 

 surrounded by a dark ring of epithelial pigment. These 

 are the eyes provided for the use of the Pecten by which 

 its active movements are directed, for this Mollusk has 

 the power of rapid swimming by opening and shutting 

 the valves of the shell. 



Each eye is a beautiful structure provided with a scle- 

 rotic coat, a transparent cornea, pupil, crystalline lens, 

 retinal body, optic nerve, in fact everything that would 

 necessarily enter into the composition of a good organ 

 of sight. 



Quite different is the make-up of this eye from some of 

 the primitive eyes of the Cephalopods in which we should 

 naturally expect to find highly developed sensory organs 

 but which in numerous instances are simply a pair of 

 hollow' chambers opening to the exterior by minute ori- 

 fices (pinhole cameras) and perfectly devoid of any re- 

 fractive structures. 



We can account for the more complete structure 

 found in the Pecten only by studying its origin and 

 development. 



The development of the eye of some Mollusks shows 

 that it is simply a modified area of the general epi- 

 dermic layer and that the sensitiveness of its cells to the 

 action of light and their relationship to the nerve-fili- 

 ments is only a specialization or intensification of a prop- 

 erty which might, as far as we can see, occur anywhere 

 on the general surface of the body. 



The primitive optic vesicle is said to arise as a pit or de- 

 pression in the epiderm and the integument around it 

 rises in the form of a ring-like upgrowth gradually con- 



