SENSE-ORGANS OF THE IXTEGUMEXT. 



163 



cells, connected by nerves with the central nervous system, and 

 the second are the supporting cells, which lie between the 

 former and serve as a connecting and isolating material. 



In all the higher sensory organs the medium surrounding 

 the end-organ is always moist, and as this is also the case with 

 the sensory organs of the integument of Fishes, we find 

 to a certain extent similar nerve-endings in both. Thus in both 

 cases, we meet with rod-shaped sensory cells, but in the latter, the 

 nerves coming from them do not pass through ganglion cells, as 

 they always do in the higher sensory organs. This indicates a 

 lower stage of development. 



FIG. 131. A, peripheral nerve-ending, as seen in all the higher sensory nerves ; B, 

 rod-shaped end-cell of a sensory organ of the integument of a Fish or Amphi- 

 bian, or a taste-cell ; C, free, and Z>, ganglionated nerve-ending of the integu- 

 mentary sensory organs of terrestrial Vertebrates. 



.A" 1 , first, and N, second portion of the nerve-fibre in connection with the epithelial 

 end-cell, G l ; G, ganglion cell interposed between these portions ; CS, cuticular 

 process of end-cell. 

 All the figures are diagrammatic, and are based upon a figure by MerkeL 



In those animals which in the course of development give up 

 an aquatic life and come on land, the end-organs of the nerves 

 pass further inwards from the surface, undergoing at the 

 same time changes of form. 



The rod-shaped end-cell now once for all disappears, and 

 two kinds of nerve-endings are seen in the skin terminal 

 ganglion-cells and free nerve-endings. 



SENSE-ORGANS OF THE INTEGUMENT. 



I. ROD-SHAPED ORGANS OF FISHES, DIPNOI, AND AMPHIBIA. 



a. Segmental Sense-Organs. 



These organs show considerable similarity to certain structures 

 in Chaetopods and marine Rhipidoglossa (e.g. Fissurella). 



They always consist of a central mass of cells, arranged in 

 the form of a rounded and depressed pyramid, and of a peripheral 



M 2 



