664 William Patten 



aud there unite wìth the cella, tliere is a possibility of a greater 

 variety of nerve endings, wliicli we must always bear in 

 mind in making any supposition. 



There are numerous sense cells in the skin of the Mollusca, which 

 apparently terminate in a single fibre, which certainly does not ex- 

 tend along the walls of the cells. The latter pass so gradually into 

 these fibres that it is impossible to determine the limits of either. This, 

 then , appears to be an exception to the intercellular method of nerve 

 endings. Does the nerve fibre stand in direct communication with the 

 centrai nerve system? Ithink not. It seems to me that we bave exactly 

 the same condition bere as has been described by Hertwig for the 

 Coelenterates ; that is , there are. two classes of cells in the Mollusca, 

 constituting the greater part of the hypodermis; the myo -epithelial, 

 and the neuro-epithelial cells. The former consist of the ordinary 

 epithelial cells, euding in radiating, root-like fibres, the union of 

 which gives rise to the basai membrane, which is probably 

 homologous with the sub-epithelial layer of myo-epithelial 

 fibres of the Coelenterates. The sense hair cells, which 

 terminate in a single fibre, would then be homologous with 

 the neuro-epithelial cells of the Coelenterates. The iu- 

 ward prolongations of the scuse cells in the Mollusca are 

 not then nerve fibres, arising either fromthe nervous sy- 

 stem, orfrom peripheral ganglionic cells, but are simply 

 nervous prolongations of the sense cells themselves and 

 are probably united as their inner ends with a contractile 

 one which o ri ginatednear the sense celi, and which during 

 its iuward growth has drawn the nervous fibre of the latter 

 after it. The sense cells are provided with iuter-cellular nerve fibres, 

 in the same manner as the ordinary epithelial cells. 



The centrai nervous system is simply a large group of ganglionic 

 cells, which originally arose in the same manner as the peripheral 

 ganglionic cells do now ; the invagiuation of the whole surface of the 

 hypodermis to form a nervous system is a secondary process. The union 

 of the centrai nervous system with the ganglia of remote sense organs 

 takes place in exactly the same manner as the various ganglionic cells 

 of a single sense organ became united with each other. The conversiou 

 of ordinary hypodermic cells into ganglionic ones could not be bette r 

 illustrated than l)y the cells of the outer ganglionic layer in the eye of 

 Pecten (PI. 29, fig. 33). 



Therefore in the origin of any scuse organ from a group of hypo- 



