APLYSIA. 239 



cavity, and lower part of the inner side of the right parapodium 

 (i.e., on the walls of the pallial cavity) occur, amongst the 

 ordinary epithelial cells, cells which give rise to brush-like 

 bunches of stiff but slender processes resembling cilia in 

 appearance. (Fig. 21, pz.) These are the " Pinselzellen " of 

 Flemming. The nerves supplying the Pinselzellen penetrate 

 the epithelium and end on the surface in fine fibrils. 



Eyes. 



There are two eyes, one being situated at the base of each 

 rhinophore on its anterior side. During life the eye often 

 appears to be situated on a small elevation, but in a contracted 

 condition may be completely sunk in the skin. Each eye is a 

 bluish black spot and is surrounded by a clear circular patch 

 of unpigmented skin. 



The eye consists of a closed vesicle lying just beneath 

 the surface skin, the epithelium covering it forming a trans- 

 parent cornea. Between the wall of the vesicle and the 

 epithelium lies a small amount of connective tissue, and the 

 vesicle is surrounded internally by tissue of a similar nature, 

 but more closely packed together. 



The optic vesicle is an ovoid sac whose walls are formed 

 by a single layer of cells. These cells are not, however, of equal 

 depth. Those in proximity to the cornea, i.e., on the outer 

 side of the organ, are thin and flattened, and form the false 

 cornea. Laterally and internally the cells are of much greater 

 vertical diameter, increasing in height from the false cornea 

 to the internal wall of the vesicle, where they form the retina. 

 There is abundant pigment on the wall abutting on the interior 

 of the vesicle, but not in the cells forming the false cornea. 

 The pigment zone thus forms a rather broad horseshoe, whose 

 open ends reach to the false cornea. The retinal cells are of 

 two kinds, called by Pelseneer the retinulae and retinophora. 

 The former are pigment cells, the latter the sensory calls of the 

 organ of sight. 



