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CHEMICAL SENSES 



cer 



Figure 2 Later development of the head in Scyliorhinus: olfactory organ 

 (ol). From Balfour (1885). 



sampling are found, although special accessories for this purpose are fre- 

 quently encountered (Kleerekoper 1969). 



The structure of the sensory epithelium, on the folds or lamellae of the 

 inner wall of the olfactory sac, is very similar in all vertebrates (Allison 

 1953). There is no evidence that this epithelium is basically different in 

 elasmobranchs. In the fully developed olfactory epithelium of fish, three 

 main elements can be distinguished: olfactory receptor cells and their axons, 

 supporting cells, and basal cells. Jointly, these elements form a columnar 

 epithelium arising from a basal structure, the basal lamina, which is separated 

 from the underlying cartilaginous tissue by the lamina propria. 



The olfactory receptor is a bipolar, primary neuron. Its dendritic, 

 "swollen" extremity, or "olfactory vesicle" (Schultze 1863, Kallius 1905, 

 Bloom 1954, Vinnikov 1965, Graziadei 1972), directed peripherally, carries, 

 in most fish, cilia that protrude into the lumen of the olfactory sac. The 

 number of receptors varies greatly among species (Teichmann 1954). Al- 

 though the dimensions and shape of the receptor cell and its parts may 

 differ, the basic structure is similar in all vertebrates. These variations are 

 found also within species and may reflect maturation of the cells (Andres 

 1969) which, following degeneration, seem to be replaced by differentiation 



