182 SMELL, TASTE, ALLIED SENSES 



deep-seated cell-bodies extend into the central organs. 

 Finally, in the gustatory organs the taste-buds are com- 

 posed of receptive epithelial cells that are in synaptic 

 relations with nerve terminals essentially like free end- 

 ings from which axons with deep-seated cell-bodies pass 

 into the central organs. These three types of structure 

 include, so far as is known, all the vertebrate chemorecep- 

 tors. To a common stimulus, like ethyl alcohol, the ol- 

 factory type has been shown to have by far the lowest 

 threshold followed in order by the gustatory and the com- 

 mon chemical types both of which are near together in 

 this respect. 



When these three types are compared with the recep- 

 tors of other animals, it is seen that the olfactory type 

 reproduces almost exactly that found in the skins of many 

 invertebrates, and that the other two types are character- 

 istically vertebrates. The integument of animals even as 

 simply organized as sea-anemones is rich in receptive cells 

 that reproduce in almost every detail the conditions of 

 the vertebrate olfactory neurones. Not only do these 

 lowly organized forms show this structural similarity in 

 their integumentary cells, but they are known to be so 

 responsive to minute amounts of material wafted from 

 distant food through the water to them that they have 

 been for a long time past credited with olf action (Pollock, 

 1883). Thus the vertebrates' olf actory epithelium and the 

 integument of aquatic invertebrates are strikingly alike. 



It is more than probable that the vertebrates have 

 descended from ancestors whose skin was an epithelium 

 like that on the exterior of a sea-anemone and that, as 

 this skin thickened over most of the body to give the 

 necessary protection to the slowly metamorphosing ani- 



