172 ANATOMY AND LIFE HISTORY OF MOLLUSCA. 



into the muscular tissue, and do not lie immediately under the 

 skin. The concretions are not connected in any way with the 

 papillae, because they seem to be as numerous in the grooves and 

 depressions as where the skin is raised into warty tubercles. 

 Some of the cavities are empty, but probably this happened in 

 making the section. 



The eyes may truly be described as assuming every shape. 

 They are in pairs, in triplets, on stalks or rather warts, and in 

 depressions. In pi. viii., fig. 11, there is a section through one of 

 the less conspicuous warts on the side of the animal. It represents 

 a double optical organ, the smaller one being a section close to 

 the pigment cell, and the larger through a fully developed eye. 

 In the section the rods can be seen enclosed in the fibrous layer 

 of the retina. 



In some of the eyes the radiating fibres of the optic nerve are 

 interspersed with small cells. The peculiar plano-concave bodies, 

 which are highly refractive, broad and cylindrical, between which 

 the optic nerve extends downwards to a layer of cells below, 

 were rarely seen in any of the eyes examined by me. If I am 

 right in my identification of these bodies, noticed by Dr. 

 Lendenfeld, they are only to be found in the most perfectly 

 developed eyes. I did not succeed in isolating any of the rods, 

 though an examination of the matter is still in hand. In pi. vii., 

 fig. 10, I have given an illustration of two eyes wherein the rods 

 are well shown, and an enlarged figure of one of them in fig. 12 

 will give a better idea not only of the relative positions of the 

 retina and rods, but also of the peculiarities of the eyes themselves. 

 In the upper eye of fig. 11, there is a slightly pinkish somewhat 

 heart-shaped section of the lens. In the radiating fibres of the 

 retina, the ganglia cell can be seen with the included refractive 

 bodies at the point in fig. 12, marked g c. The axes of 

 the hexagonal cells are not always in the direction of the 

 entering light, but some of them are distinctly curved. At 

 point p c in the enlarged fig. 12, pigment granules can be seen 

 extending up the sides of the partition walls of the hexagonal 

 cells. It will be seen also how the walls are thickened below 

 and how there is a ganglion cell resting on the pigment cell. Two 

 very large ganglionic cells are noticeable just below the optic 

 nerve in the lower eye, fig. 11. 



At pi. ix., fig. 13, there is a section given of a dorsal eye, 

 showing the entry of the optic nerve passing through the rods 

 and spreading out above them. In this section the cell-structure 

 can be well seen. The section has been cut obliquely, and shows 

 the enlargement of the nerve below the entry in a somewhat 

 exaggerated form. The apertures in the pigment-coat below the 

 rods, are possibly channels for the entry of the vessels of 



