152 



COMPARISON OF SENSE-ORGANS. 



COMPABISON OP THE MODES OF ARRANGEMENT OP SENSORY CELLS AND NERVE 

 FIBRES IN THE DIFFERENT ORGANS OF SPECIAL SENSE. 



All the organs of special sense contain specially modified cells or the processes of such cells. 

 the so-called nerve-epithelium cells or sense-epithelium cells which serve to receive the 

 physical impressions upon which the sensation depends, and to transmute these impressions 

 into nerve-impulses which are then conducted by nerve-fibres to a nerve-centre. The skin 

 appears at first sight to present an exception to this general rule, but the ganglion-cells on 

 the posterior roots, which send their peripheral processes in the form of nerve-fibres to 

 end by ramifying in the skin, either in a special tactile organ or between the cells of the 



Fig. 174. DIAGRAM OP THE NERVOUS SYSTEM OP THE EARTHWORM. (E. A. g., after Lenhossek and 



Retzius. ) 



m, motor cells ; .<?, sensory fibres emanating from s' t sensory cells in the epidermis ; i, median 

 intercalated cell. 



epidermis, do in fact represent the sense-epithelium cells such as we find in the olfactory 

 organ and in the retina of the eye. This becomes plain from a comparison of the arrange- 

 ments of the sensory nerves of Annelids with those of Vertebrates. Thus in the earthworm 

 (Lumbricus) the whole epidermis is pervaded by cells (fig. 174, .s-') which resemble in nearly 

 every particular the olfactory cells of Vertebrates. They are spindle-shaped cells, having two 

 processes, one unbranched and extending straight to the free surface, the other branching, 

 and with one of its branches prolonged as a nerve-fibre to the central nervous system and 

 ending in a forked termination which comes into relation with branches of the nerve-cells 

 there present. As in the olfactory organ the sense-cells are supported by columnar epithelium 

 cells which are not connected with the nerve centre. In other Annelids (e.g., Nereis) the 

 sense-cells are no longer situated between the other cells of the epidermis, but occupy a 

 deeper position and send their peripheral process, which is still unbranched, to penetrate 

 between the cells of the epidermis, and thus to reach the surface, whilst the central process 



