The elasmobranch fishes . elasmobranchfish03dani Year: 1934 264 THE ELASMOBRANCH FISHES SPECIAL SENSES OF ELASMOBRANCHS IN GENERAL The organs of special sense, olfactory, gustatory, optic, auditory, and sensory canal organs, although very different in the adult are, with the exception of the taste buds and the eye, similar in the beginning. In general, with the ex- ceptions made, these organs arise as thickened plates or placodes of ectoderm. An anterior placode gives rise to the nasal pit and a posterior placode sepa- rates into three parts (Mitrophanow, 1893). The first or anterior of these
The elasmobranch fishes . elasmobranchfish03dani Year: 1934 264 THE ELASMOBRANCH FISHES SPECIAL SENSES OF ELASMOBRANCHS IN GENERAL The organs of special sense, olfactory, gustatory, optic, auditory, and sensory canal organs, although very different in the adult are, with the exception of the taste buds and the eye, similar in the beginning. In general, with the ex- ceptions made, these organs arise as thickened plates or placodes of ectoderm. An anterior placode gives rise to the nasal pit and a posterior placode sepa- rates into three parts (Mitrophanow, 1893). The first or anterior of these gives origin to a branchial sense organ over the first gill; the second gives rise to the ear, and the third or posterior part produces the lateral line organ which, in the sharks, later extends to the tip of the tail. Olfactory Organ The olfactory organ in the adult is a blind sac, which in simpler forms like the notidanids and Chlamydoselach us is more or less terminal in posi- tion. In many other Elasmobranchs, however, its position is more ventral. The olfactory sac or pit itself varies greatly as to its shape, the nature of its lining, and the number and depth of the folds produced in it. In general it may be said to be ellip- tical in form, the long axis pointing anteromedially. The sac may be single or it may be double. In either case the lining is thrown into two series of ridges, the Schneiderian folds (fig. 231), which greatly in- crease the extent of its surface. The so-called sec- ondary folds are usually anterior and dorsal in po- sition while the primary folds are posterior and ventral. In certain forms the folds become exceedingly numerous, more than eighty primary folds being present. The sensory cells (fig. 230) have a group of hair-like processes which extend into the olfactory cup, and fibers which run back as the olfactory nerves to the glomeruli of the olfactory bulb. From the bulb, fibers extend posteriorly in the olfactory tract to the olfactory lobe of
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