560 William Patten 



of Pecten. Moreover, analogy with Pecten and Haliotis will allow us 

 to suppose a similar system of horizontal fibrillae uniting the inner and 

 outer system of nerve fibres. 



Having obtained a more complete knowledge of those complex Cle- 

 ments, or ommatidia, of which the most specialized eyes of Arca are 

 formed, we are now in a better condition to understand the structure of 

 the second form, and the origin of both types from the aggregation of 

 Clements fonnd seattered in the pigmented portions of the mantle. 



The Invaginate Eyes. 



The general characters and distribution of the invaginate eyes bave 

 already been allnded to, and it novr remains for us to give a more 

 accurate description of them. 



When seen from above, in living specimens, they appear as very 

 dark brown or black spots. They may be sharply defined, ovai, or 

 round, or they may be surrounded by a circle of lighter pigment cells, 

 which gradually pass into the surrounding, colorless epithelium. In 

 the midst of each pigment spot, one always sees from threc to six or 

 more brilliantly refractive and colorless points, ali the more striking 

 from being situated in the midst of the dark pigment. The intense black 

 color of the pigment is often reduced by the invagination of the eyes 

 below the surface, the more or less transparent lips of the cup thus form- 

 ed fojding over, and partly hiding, the sunken eye. The deeper the 

 cups, the greater is the tendency for their lips to approach each other, 

 forming a long, slit-like opening (PI. 30, fig. 47). Whether this iris- 

 like fold is capable of regulating the amount of light by contraction or 

 expansion, I am unable to say. I bave not observed any movement, 

 which would indicate such a function. Will has described an iris-like, 

 contractile opening in Pectwiculus^ which has faceted eyes similar to 

 these of Arca^ but no invaginate ones. It is possible that he had in mind 

 the invaginate eyes oi Arca^ when describing those oi Pectwiculus . Sec- 

 tions of the invaginated eyes show that they are simple, thickened por- 

 tions of the hypodermis, sunken below the surface, sometimes forming 

 wide-mouthed and shallow depressions, or deep funnel-like pits : in the 

 latter case, the axis of depression is not perpendicular to the surface, 

 but at au angle of about 50^ or 60^ with it, the deepest part being direct- 

 ed toward the shell (fig. 42) . 



The retineum is composed of broad, square-ended cells filled 

 with dark brown, or black pigment, and a smaller number of colorless 



