72 PATTEN. [Vol. I. 



edge, one readily sees numerous dark spots of various sizes, 

 which, upon closer inspection, prove to be highly-organized eyes. 

 They are, undoubtedly, the light-sensitive organs which the fore- 

 going experiment showed must be present in Area. The mantle 

 edge of Area, as well as all other Lamellibranchiata that I have 

 examined, is divided into three longitudinal folds, — an outer one, 

 or sJiell fold, an inner one, the velar fold, or velum, and a median 

 one, or ophthalmic fold. At the base of the furrow, separating 

 the shell fold from the ophthalmic one, is the gland secreting the 

 cuticular-like covering of the shell. The whole mantle edge of 

 Area is well supplied with patches of pigment which are espe- 

 cially abundant on the inner face of the ophthalmic fold. These 

 pigmented areas are composed of columnar, pigmented cells, 

 among which are a number of colorless cells provided with two 

 nuclei and an axial nerve-fibre. These colorless cells are usu- 

 ally surrounded by a circle of four pigmented ones, distinguished 

 from the surrounding cells by their color and sharp configura- 

 tion. These pigmented patches, with their clusters of cells, or 

 ommatidia, and with no special thickening of the cuticula, 

 belong to the simplest light-sensitive organs known. Along 

 the summit of the ophthalmic fold, the ommatidia are collected 

 into well-defined groups to form either the pseudo-lenticulate, the 

 invaginate, or the faceted eyes. 



The pseudo-lenticulate eyes, of which there are about two 

 hundred in each individual, are scattered irregularly over the 

 surface of the ophthalmic fold. They consist of groups of 

 ommatidia, over which the cuticula is thickened to form a 

 lens-like body. The latter is composed of a number of cuticular 

 rods, each one overlying the cell by which it is secreted. The 

 whole cuticular mass is richly supplied with prolongations of 

 the nerve-fibres found between the ommatidial cells, and the 

 whole network of nerve-fibre in the cuticular mass is simply an 

 extended and modified part of the retia terminalia of the simple 

 epithelial cells. 



There are about eight hundred invaginate eyes in each full- 

 grown specimen of Area Noce. The groups of ommatidia 

 which constitute these eyes are sunken beneath the surface to 

 form minute cups, the mouths of which may be reduced to 

 narrow slits. The rods of the ommateal cells form a thick cutic- 

 ular floor for each cup. 



