88 PATTEN. [Vol. I. 



The outer end of the fibre then gives rise to minute cross- 

 fibrillae, which either adhere to the wall of the neighboring sense- 

 cell, or unite with similar fibrillae from older nerve-fibres ; lastly, 

 the tuft of sense-hairs disappears, and the conversion of the 

 sense-cell into a bipolar ganglionic one is complete, (iv.) 

 Subsequently the body of the bipolar cell gives rise to numerous 

 secondary fibres, which unite with those from other cells, 

 and so convert the bipolar cell into a multipolar one, whose 

 primitive, outer end still retains its original position between the 

 epithelial cells, (v.) This process of nerve formation may 

 occur at any part of the mantle edge, and is not confined to the 

 larval stage, but takes place also in the nearly full-grown indi- 

 vidual. Here, then, is the explanation of the intercellular nerve 

 endings in the Mollusca ; and unless degeneration of the outer 

 ends of the nerves has taken place, they should always extend to 

 the cuticula. In no case do nerves from the central nervous 

 system unite directly with the sense-cells of the epidermis. All 

 the nerve-ends in the hypodermis mark approximately the places 

 where ganglionic cells originated. The latter alone are directly 

 united on the one hand with the hypodermis and on the other 

 with the central nervous system. 



The basal membrane of Molluscs is formed by the union of 

 the root-like ends of ordinary epithelial cells. I consider that 

 the latter cells and the basal membrane represent the myo- 

 epithelial cells and the underlying layer of fibres of Coelen- 

 terates. 



My observations on the structures of the compound eye have 

 led me to the conclusion that it is a modified ocellus. The 

 primitive Arthropod ocellus I regarded as a closed optic 

 vesicle, the inner wall forming the retina whose rods are there- 

 fore upright. The outer wall of the optic vesicle in most cases 

 is not visible. The hypodermis overlying the optic vesicle is 

 represented by the " vitreous-layer^' or what I have called the 

 corneal hypodermis. In the compound eye the same layer is 

 present, which I have also called the corneal hypodermis, as a 

 thin stratum of cells over the crystalline cones. The crystal- 

 line-cone cells are, therefore, not the homologue of the vitreous 

 layer of the ocelli, but of the colorless rod-bearing cells, or re- 

 tinophorae, with which they have a common function. If this 

 be true, then the crystalline cones are not dioptric organs, but the 



