CENTRAL ORGAN AND PERIPHERAL NERVES (PHYSIOLOGICAL). 39 



ectoderm are Joined to the neighboring nerve-centers by such fibers. Their 

 position in the epidermis indicates that this is a part of the sensory ap- 

 paratus, and all doubts about this being true disappear when one recog- 

 nizes how frequently these cells are in relation with structures adapted to 

 the reception of special impressions. Long, stiff hairs, swinging brushes, 

 projecting horns, seem easily able to communicate tactile impressions, while 

 one may find analogous cells arranged to form the walls of a cavity, in 

 which a pebble, an otolith swinging inside the cavity, represents the sensory 

 apparatus for maintaining the equilibrium. Lentiform parts of the ecto- 

 derm lie in other places in front of such cells, and are well adapted to 

 transmit rays of light or heat to these cells in a peculiar manner. It would 

 scarcely be possible to describe all the manifold arrangements which func- 

 tion as sensory mechanisms in the invertebrates, but it must be emphasized 

 that between the simple epithelial cell of the ectoderm and the highly 

 differentiated apparatus are found all transitional forms, and that in the 

 most highly developed this same type, the epithelial cell with a filament 

 estending inward to the nervous system, reappears. There is one place 

 where one may find a large number of intermediate forms in a limited space, 

 ranging from a simple epithelial cell connected with the nerves up to the 

 more complicated sense-hillocks. It is the skin of a transparent snail, the 

 pterotrachea. The connection of epithelial cells with nerves leading to the 

 central organ, in the angle-worm, has been well-described by Lenhossek 

 during the last few years. Eesearches of my own and those of Ketzius have 

 fully confirmed his reports. From numerous cells of the integument are 

 seen delicate fibrils arising, which extend to nerve-centers and there ter- 

 minate by division. Lenhossek has formulated an hypothesis which has 

 proved to be of great worth as a working-basis, and bids fair to simplify and 

 extend our knowledge of the peripheral sensory nervous system. According 

 to him, all sensory nerves, in the invertebrates as well as in vertebrates, arise 

 from such cells that are originally in the integument. The cells recede 

 deeper and deeper, leaving behind a long and often branched filament in 

 the skin. In the vertebrates they extend as far as the vertebral column, 

 forming the spinal ganglia. Whether the cells lie immediately in the 

 surface-epithelium or are connected with it by their processes, the sensory 

 nerves, they invariably send one filament back into the central organ. 

 Eetzius has described such transition cell-forms in mollusks, the peripheral 

 filaments being of different lengths, where the ganglion-cells, corresponding 

 to epithelial cells, are often found not in the skin itself, but under it at 

 various levels. In Pig 15 is represented, after drawings of Eetzius, a scheme 

 aiding one to connect the foregoing with the development of the sensory 

 nervous system. 



It is not only in the lower animals that the sensory end-cells are met 



