The elasmobranch fishes . elasmobranchfish03dani Year: 1934 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
The elasmobranch fishes . elasmobranchfish03dani Year: 1934 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.) , 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 (
Size: 1478px × 1353px
Photo credit: © Bookend / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: archive, book, drawing, historical, history, illustration, image, page, picture, print, reference, vintage
