170 ANATOMY AND LIFE HISTORY OF MOLLUSCA. 



type is identical with that of the Vertebrata. They have a blind 

 spot, because the optic nerve pierces the lower side of the 

 retina, and in both the layer of rods and cones forms the outer 

 layer of the organ. 



We have two if not three species of Onchidium in Port Jackson. 

 The structure of the tubercles was made the subject of a paper by 

 Dr. von Lendenfeld, in the 10th Volume (1st Series, p. 720) of 

 the Proceedings of the Linnean Society of New South Wales. 

 The result of his investigations was the announcement, that one of 

 the species (0. chameleon, Brazier) has small papillae and no eyes. 

 The other species have generally three large eyes on each tubercle, 

 and these he says are " situated laterally," by which probably he 

 means the eyes. This is the species known as 0. ddmelii, Semper. 

 As Dr. Lendenfeld's notes are very brief, I give them in full, 

 premising with him, that he generally confirms Semper's 

 observations. 



" The eyes of Onchidium ddmelii belong to Semper's Group I. ;. 

 eyes with an epithelial retina. The epithelium of the tubercles 

 is of identical structure in the species with, and the species without, 

 dorsal eyes, and formed of an outer layer of low cylinder-cells, 

 between which there are slender sensitive cells, particularly 

 abundant round the eyes or on the sides of the tubercles of the blind 

 species. The otolith-like concretions in the numerous vesicles of 

 the dorsal skin are composed of carbonate of lime, and homologous 

 to parts of the shell of other related pulmonates. The eyes 

 multiply by division ; semi-detached eyes, and such with a simple 

 spherical pigment-layer, but with two lenses are not rare. The 

 lens consists apparently of one single cell which retains its nucleus 

 and vitality, and may divide into two. A sphincter-shaped 

 circular accommodation muscle is clearly visible. 



The retina is of a much more complicated nature than Semper, 

 who had only spirit specimens at his disposal, was able to discover. 

 The radiating fibres of the nervus opticus are interspersed with 

 small ganglia cells. Below these follow cells with peculiar plano- 

 concave bodies in them, which are highly refractive. These cells 

 are broad and cylindrical. The final branches of the opticus 

 extend downward between them to a layer of multipolar cells 

 beneath. Below this layer of granular (osmic acid) ganglia cells, 

 cylindrical and very regular hexagonal cells are found ; the axis 

 of each is situated in the direction of the entering light. These 

 hexagonal cells are attached to the pigment-skin at the outer 

 limit of the retina. Pigment granules extend up the sides of the 

 partition walls of these hexagonal cells for some distance. The 

 walls themselves are thickened below, and in this way concave 

 spaces are formed, one at the bottom of each hexagonal cell. 

 These spaces are completely surrounded by pigment. 



