668 William Patten 



statement about the style and pedicel, except that they are not concerned 

 directly with vision; but the facts enumerated point in a certain direction, 

 and, in the absence of any proof to the contrary, we must not hesitate 

 to point out what that direction is , knowing perfectly well that the 

 evidence is likely to turn in this, or that direction , with the accession 

 of new facts. 



The facts then to be considered are the following: (1) in theMolluscs, 

 one finds in the retinophorae , beneath the rods , a refiecting- surface 

 composed of refractive granules ; (2) in the compound eye of Arca , the 

 reflective granules are surrounded by a special circle of retinulae; (3) in 

 the absence of the granules, a special refiecting membrane, the argentea, 

 is developed, serving the same purpose {Pecten) ; (4) behind the latter is 

 a layer of red pigment, the light from which may pass through the 

 argentea to the rods; (5^ the argentea consists of colorless, refractive 

 plates, the angles of whose faces are so arranged as to cause the reflec- 

 tion of incident, and interference of transmitted light. If now we 

 consider, for the sake of argument, that the pedicel is a refiector, we 

 bave the following poiuts of comparison with similar structures in the 

 Mollusca: (1) A continuous argentea being absent we bave instead, at 

 the base of each retinophora, a collection of refractive plates, whose 

 structure renders it highly probable that they bave the same optical 

 properties as those of the argentea in Pecten and Cardium\ (2) the 

 plates are formed by the retinophorae under the rods, as in Arca, and 

 not by the retinulae ; (3) they are surrounded by a special circle of 

 retinulae, as in Arca ; (4) in the absence of a red tapetum, the coloring 

 matter is lodged in the plates themselves; they therefore possess the 

 combined fuuctions of argentea and tapetum ; (5) the red light from the 

 plates passes through the styles to the crystalline cones, as is shown 

 by examining with the naked eye the red light issuing from the eyes 

 of nocturnal Lepidoptera, in which the pedicels and their red color is 

 specially developed. 



The argentea is to intensify the effect of the light, and is there- 

 fore best developed in those eyes where it is necessary to obtain a 

 great quantity of light, or in nocturnal, or deep sea animals, which need 

 to economize the light. The presence of the pedicels in nocturnal 

 Lepidoptera indicates a similar function. 



The most characteristic thing about the Arthropod eye is its con- 

 vexity , which has its advantages in an economic arrangement of the 

 peripheral ends of the retinophorae. This arrangement is favored by 

 the hard, chitinous skeleton of the eye, aflfording a protection for the 



