THE ELASMOBRANCII PISHES 



265 



w^y 



from the dorsal side. In certain forms more than a single dorsal flap obtains. 

 In Scyllium a double dorsal, and in Myliohatis numerous dorsals are present. 

 In some of the rays (also in Squatina) these dorsal flaps extend downward as 

 loose extensions of skin and are of slight value in forming a tube of the cup. 

 By the meeting of dorsal and ventral flaps the normally elliptical aperture is 

 changed into a figure 8, thus producing an anteromedial and a posterolateral 

 opening to the olfactory organ. 



As the fish moves forward a current 

 is produced mechanically through the 

 olfactory cup and over the olfactory 

 membrane. The importance of such a 

 current in forms which have no direct 

 connection from the nose to the mouth, 

 that is, in which the olfactory organ is 

 solely an organ of sense taking no part 

 secondarily in respiration, is clearly 

 seen. That the sense of smell is well 

 developed in Elasmobranchs has been 

 demonstrated by the experiments of 

 Sheldon (1911). If for example the 

 nostrils of the shark be plugged with 

 cotton so as to prevent a circulation of the water over the olfactory membrane, 

 the shark will swim over food without detecting it by sight; but if the nostrils 

 now be unplugged, or even a single nostril, food will be found although it has 

 been concealed. In fact, of all the special senses the sense of smell is probably 

 of the most service. 



Fig. 231. Sagittal section of developing 

 nasal pit, Acanthias. (From Berliner.) 

 s.f., Schneiderian folds. 



DEVELOPMENT OF OLFACTORY ORGAN 



We may now notice briefly the development of the olfactory organ in the 

 Elasmobranchs. As is common for vertebrates in general this organ first ap- 

 pears as a thickened epidermal placode or plate which early pits in from the 

 outside toward the brain so as to form a shallow vesicle. By further growth 

 this vesicle sinks deeper and forms a blind sac (fig. 231) in which arise the 

 Schneiderian folds (s.f.) of the primary and secondary types. Furthermore 

 these folds have produced from their sides numerous accessory folds in which 

 are found the olfactory sense-cells above mentioned (fig. 230). The two divi- 

 sions of the olfactory nerve put these two areas of folds into communication 

 with the olfactory luill), but each division of the nerve may be connected with 

 both of the areas of the folds. 



Gustatory or Taste Organs 



Taste buds are present in the Elasmobranchs often in considerable numbers. 

 These are located in the buccal cavity and pharynx and are distributed over 

 the floor and tongue, along the sides, and over the roof of both cavities. An in- 



