200 SUMMARY OF CURRENT RESEARCHES RELATING TO 



eyes of the Pectinida, which he prefaces by a critical and historical 

 notice of the works of his predecessors. The smaller species have more 

 eyes on the mantle than have the large. These eyes are placed on 

 stalks, the substance of which is a direct continuation of that of the 

 edge of the mantle, and is covered by epithelial cells which vary in form 

 in different regions. The stalk, which varies in length, is generally 

 cylindrical in form ; the fibrils, of which its connective substance is 

 formed, are less numerous, and the meshes are larger than in the tactile 

 filaments on the edge of tlae mantle, and the whole tissue has a more 

 homogeneous appearance. Patten was right in asserting the presence of 

 vascular lumina ; there are also muscles, but these are not formed, as 

 Patten declares, of elongated spindle-shaped cells, but they form long 

 multinuclear cords, the cellular components of which are closely packed 

 against one another ; the whole forms a contractile subepithelial tube. 

 The ciliaris of Patten does not appear to exist, that observer having 

 mistaken connective-tissue fibrils for muscular fibres. 



The epithelium of the optic stalk is flat or low, but becomes higher 

 at about the equator of the organ ; here the cells contain pigment which 

 varies in colour in various species. Hensen was quite right in objecting 

 to the term choroid being applied to the pigment-mantle, for it is the 

 vascularization and not the pigmentation which is the characteristic of 

 the choroid. The pellucida of Hensen or cornea of most authors has an 

 outer layer which is formed on one of three types; it is succeeded by 

 an inner layer which is a direct continuation of the connective substance 

 of the stalk. 



The true optic organ consists of two parts which differ from one 

 another in structure and formation ; the distal portion is the dioptric, 

 the proximal that which perceives the light. Biitschli was wrong in 

 doubting the existence of the septum which Krohn discovered to separate 

 these two regions. The dioptric apparatus is formed by the lens ; this 

 is biconvex in form, and its greater diameter is along the lateral axis of 

 the eyeballs. It consists of numerous cells, which are generally poly- 

 hedral in form ; the plasma of these is thick and granular and stains 

 very intensely, especially with eosin ; the nucleus is generally present, 

 and small ; these cells have no investing membrane, but present a 

 distinct contour after treatment with corrosive sublimate. The author 

 cannot agree with Patten in the latter's statement that the lens is 

 invested by a membrane. 



The proximal part of the eye consists of three layers — retina, 

 tapetum, and pigment-membrane. In the retina there are again three 

 layers, which can be seen in longitudinal sections, which lie in the 

 following order proximally to the septum : layer of ganglion-cells, layer 

 of rod-cells, and layer of rods ; these are described in detail, and the 

 differences between the author's results and those of preceding writers 

 carefully pointed out. The innervation of the eye is also discussed. 



An explanation of the morphological difficulty that the rods of the 

 eye of Pecten are turned away from the light, has been attempted by 

 Biitschli, who ascribes the difference from the eye of other Molluscs to 

 the different mode of development of the lens ; in Vertebrates and in 

 Pecten it is formed outside the optic vesicle, but in other Molluscs 

 within the eye. If, however, the eyes of Pecten are not, as Patten says, 

 homologous but only analogous with those of other Molluscs, the eye of 

 Pecten will remain a histological and morphological unique. 



