THE SENSE ORGANS 



573 



are slender neurosensory cells with swollen terminations beneath the 

 cuticula, especially abundant at the tips of the tentacles. Some molluscs 

 have osphradia or "false gills," which are believed to have an olfactory 

 function. 



The olfactory sense is so keen in land molluscs that snails were used 

 as gas detectors in the trenches of the world war, the animals withdrawing 

 into their shells in response to a 

 quantity of poison gas so minute as 

 to be imperceptible to man, yet 

 dangerous when its action is pro- 

 longed. 



The olfactory organs of insects 

 and Crustacea have the form of 

 olfactory hairs or bristles on the 

 appendages, especially in the head 

 region. 



That the olfactory organs of 

 vertebrates have evolved from any 

 of the many sense organs of inverte- 

 brates has not been demonstrated. Fig- 472.— Head of chick of s^i days, 



-, . , •■Li • -i^ i_ showing development of oronasal canal. 



It IS, however, possibly significant f/,chorioid fissure ;Z, thickening for lacrimal 



that the olfactory receptors of chor- duct; «, nasal pit; on, oronasal groove. 



1 ^ 11 1 • 1 (From Kingsley's " Comparative Anatomy 



dates are neurosensory cells which, ^f Vertebrates," after Keibel.) 



like those of invertebrates, spin 



their own neurites, but persist as constituent elements of the olfactory 



epithelium. 



The olfactory epithelium of most vertebrates is of the simple columnar 

 type, in which neurosensory cells are uniformly distributed among sup- 

 porting non-nervous epithelial cells. Each receptor terminates on the 

 surface in a brush of fine hairs, and is prolonged from its basal end in a 

 neurite, which breaks up in numerous telodendria within the olfactory 

 bulb. As a special modification in some fishes and amphibia, the sensory 

 cells are aggregated into olfactory buds, separated from one another by 

 intermediate areas of indifferent non-sensory cells. In the higher verte- 

 brates, however, the sensory cells have their original uniform distribution 

 in the olfactory epithelium. 



Olfactory organs seem to be lacking in the protochordates, yet these 

 respond to chemical stimulation, so that they must be assumed to have 

 chemo-receptors. 



Amphioxus also lacks a specialized olfactory organ and does not hunt 

 for its food, but lies buried in the sand. Koelliker's pit, which was 

 formerly assumed to be an unpaired olfactory organ, is the remnant of the 

 larval neuropore, and is non-nervous. Hatschek's pit, in the roof of the 



