54 SUMMARY OF CURRENT RESEARCHES RELATING TO 



sunk below tlie surface, forming shallow depressions or deep funnel-like 

 pits. The eyes are each composed of the same elements as the facetted, 

 that is, a central colourless cell, probably containing an axial nerve-fibre 

 and two nuclei, together with a cuticular rod supporting a specialized part 

 of the retia terminalia (the retinidium) ; around each of these central cells, 

 or retiuophorae, are arranged a number of pigmented ones — in this case 

 more than a single circle — which also support nerve-bearing cuticular rods. 

 Each eye is described as a retineum, i. e. a collection of ommatidia in which 

 the retinidia (or rods) of both retinulfe and retinophoraa, or of the latter alone, 

 form a continuous layer, the retinulfe retaining their pigment and primitive 

 arrangement around the retinophorte. 



The pseudo-lenticulate eyes are transitional. They occur as sharply 

 defined groups of non-invagiuated ommatidia, provided with a prominent 

 lenticular thickening of the cuticula, containing nerve-fibres. A. retinal 

 cuticula is formed by the pigmented cover-cells, as well as by the retino- 

 phorae, and hence these eyes resemble more closely the invaginated forms ; 

 on the other hand, they tend to form a protuberant convex surface, instead 

 of a concave one. 



2. After a brief notice of Pectunculus, Mr. Patten proceeds to the dis- 

 cussion of the eyes of Pecten. First, in regard to the general structure 

 and function, he shows that the parts really have the function that their 

 names and composition suggest. The movements of lens and iris are 

 described. That the lens is really such was proved by observing the 

 formation of perfect inverted images in the depths of the eye. They are 

 formed by the cornea and surface of the lens, as upright and reflected 

 images. The image falls upon the percipient rods just above the tapetum. 

 By focusing between the argeutea and the place where the image formed 

 by the lens is seen with the greatest advantage, a double image is seen, less 

 distinct towards the argentea, but increasing in sharpness towards the focal 

 point of the eye, where the two images ultimately fuse to form a single 

 one. The only explanation he has to ofi"er for the origin of this second 

 image, is that it is a reflected one of the first, formed by the curved surface 

 of the argentea. 



The distribution of the eyes is then considered, and an attempt made 

 to explain the difterences, e. g. between the two sides of the mantle. In 

 those species in which the eyes are especially numerous, a number of eyes 

 occur in which the pupils are entirely covered with pigment, but the retina 

 nevertheless perfectly developed, (a) The whole external surface of the 

 eye is covered with a continuous layer of columnar einthelium, increasing 

 in height as far as the iris, where it is suddenly reduced in thickness, and 

 losing its pigment forms the cornea, (b) The corneal cells are columnar, 

 capped externally with cuticula, and interlocked with one another by 

 irregular folds, (c) The cells of iris only differ from those of the cornea 

 in being larger and filled with pigment. They serve merely to exclude the 

 lateral rays of light from the retina, (d) The stalk consists of loose con- 

 nective tissue containing enormous blood-spaces. Two groups of long 

 striated muscle cells act as erectors and depressors. Anteriorly, they are 

 replaced by numerous fine fibres forming a hyaline " psewdo-coraea " just 

 beneath the cornea. At the edge of the iris, many fibres appear to 

 terminate in an outward curve, as though attached to the epithelium at that 

 point, forming the " ciliaris." (e) The tens, which is suspended in a large 

 blood-sinus, consists of a modified group of mesoderm cells, continuous with 

 those of tlie pseudo-cornea and connective tissue capsule. Its minute struc- 

 ture, and the formation of the membrane called the suspensory ligament are 

 then discussed. 



