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 the brain, as we have described 

 iov Heptanchus. 



When seen from the outside the aperture to the olfactory organ is usually 

 separated by flaps across its middle part into two divisions. One of these 

 ai)ertures is incurrent, the other is excurrent. In Heterodonfiis the excurrent 

 aperture leads backward so that the water current, instead of passing out, 

 passes backward and into the mouth. In some Elasmobranchs two flaps, instead 

 of one, may pass over the nasal pit. When this occurs, the one passes inward 

 from the ventral margin, while the other hangs down and slightly overlaps it 



Fig. 230. Types of olfactory 

 sensory cells, Mustelus lae- 

 vis. (rrom Asai.) 



