THE SENSE ORGANS 40I 



to chemical stimulation. Neurosensory cells occur in clusters on the oral 

 cirri and scattered over the surface of the body, and these are probably the 

 chemoreceptors. 



In cyclostomes, as in all the higher vertebrates, are paired olfactory 

 pits and nerves. But, during development, the two pits join to form 

 one median organ. The proximity of the olfactory pits and their connex- 

 ion with the hypophysis may be explained as the result of the great enlarge- 

 ment of the upper lip of the sucking mouth. The connexion of the 

 olfactory pits with the pharynx by way of the hypophysis in myxinoids 

 is not to be compared with the paired posterior nares or choanae of 

 air-breathing vertebrates. Paired narial passages are not present in 

 most fishes and make their first appearance in Dipnoi. 



The olfactory epithelium of cyclostomes is a many-layered ciliated 

 membrane, beset with neurosensory cells like those of worms. By the 

 folding of this epithehum, as in fishes, the number of hair-cells 

 is multipHed, and the sensitiveness of the organ correspondingly 

 increased. In Petromyzon the bellows-like action of the pharyngeal 

 muscles forces water in and out of the hypophysial pit as if it were a 

 pipette. 



In elasmobranchs the olfactory organs are paired pre-oral pits, lined 

 with pUcated olfactory epithehum and, as in other fishes generally, uncon- 

 nected with the mouth. In some genera such as Pristiurus, however, 

 nasobuccal grooves extend from the nasal pits to the corners of the mouth. 

 Morphologists see in these grooves the beginnings of the narial passages 

 which, in higher vertebrates beginning with the Dipnoi, connect the nasal 

 pits with the pharynx. As a special adaptation to aquatic Hfe the nasal 

 pits of elasmobranchs become incompletely divided by a transverse fold 

 of skin so that, as the fish swims, water flows into the anterior and out of 

 the posterior opening. 



The Dipnoi take an important step forward in the evolution of the 

 organ by acquiring a connexion between the olfactory pits and the 

 mouth. True choanae opening into the pharynx first appear in this group, 

 and thus the functions of smelling and breathing become associated. In 

 the embryo, the narial passages are formed by the closure of the edges 

 of paired nasobuccal grooves, such as are seen in adult elasmobranchs. 



The connexion between olfactory organs and respiratory passages 

 which was invented by Dipnoi, is found also in Amphibia; but the lamel- 

 lated olfactory epithehum disappears as unsuited to life in the air. Nasal 

 passages enlarge in Amphibia, and become divided into a more dorsal 

 olfactory region and a more ventral respiratory passage. Olfactory hair- 

 cells are limited to the upper region. A supplementary olfactory organ, 

 the organ of Jacobson, arises in connexion with each nasal passage and 

 thus with the mouth. It may serve to test the food taken into the mouth. 



