Eyes of Molluscs and Arthropods, 663 



trend of which is parallel to the surface, may be seen beneath, or in the 

 inner layer of the cutieula. This nervous network, the rete terminale, 

 seems to be present in greater or less perfection thronghout the whole 

 hypodermis of the mantle edge, bnt is especially well developed either 

 near the eyes or in the pigmented portions. In some cases, large and un- 

 branching nerve fibres pass directly to the outer cuticular layer, where 

 they seem to terminate in one or more minute, stiif, sense hairs (PI. 30, 

 fig. 64 n.f. and PI. 32, fig. 153 (f). These larger nerve fibres expand, a 

 short distance below the basal membrane, into small bipolar vesicles 

 containing a nueleus. These structures are young ganglionic cells formed 

 by the elongation of the slender sense hair cells in the foUowing manner: 

 the nueleated basal expansion wanders into the underlying tissue, while 

 the external end becomes reduced a fibre, still extending to the outer 

 surface of the epithelium, and terminating in one or more sense 

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

 lae, 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 conversiou of the sense cell into a bi- 

 polar ganglionic one is complete (fig. 153 e and/). 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 Po- 

 sition between the epithelial cells, and extends into the cuticular layer. 

 This process of nerve formation may occur at any part of the mantle edge, 

 and is not confined to the larvai stage, but takes place also in the nearly 

 full grown individuai. Here then is the explanation of the uni- 

 versal intercellular nerve endings in the Mollusca, and un- 

 less degeneration of the outer ends of the nerves has taken 

 place, they shouldalways extend into the cutieula. Inno 

 case do nerves from the central nervous system unite di- 

 rectly 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 

 unite d on the one band with the hypodermis, andontheother 

 with the central nervous system. These facts must give a certain 

 bias to all our suppositions concerning nerve endings in the hypoder- 

 mis, for it is evident that, unless great secondary changes have taken 

 place, the nerves must terminate between the cells, and pro- 

 bably extend to their very outer ends; while, if we suppose that the 

 nerves from the central nervous system grow to war ds the eetoderm 



