574 



COMPARATIVE ANATOMY 



mouth is not nervous, as was once assumed, but glandular. A chemical 

 sense, however, amphioxus indisputably has, since it responds to chemical 

 stimulation. Neurosensory cells occur in clusters on the oral cirri and 

 scattered over the surface of the body, and these may be assumed to be 

 the chemoreceptors. 



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

 pits innervated by olfactory nerves. The erroneous notion that cyclo- 

 stomes are monorhinal is based upon the fact that the olfactory pits open 

 into the median unpaired hypophysis, the external opening of which lies 



on the dorsal side of the snout. This 

 evidence is, however, misleading, since 

 the monorhinal condition in the 

 embryo results from the fusion of 

 primarily paired anlagen, and the 

 olfactory pits develop as paired organs 

 connected with paired olfactory lobes 

 of the brain. The proximity of the 

 olfactory pits and their connexion 

 with the hypophysis may be explained 

 as the result of the great enlargement 

 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 cyclo- 

 stomes is a many-layered ciliated 

 membrane, beset with neurosensory 

 cells like those of worms. By the 

 folding of this epithelium, as in fishes, the number of hair-cells is multiplied, 

 and the sensitiveness of the organ correspondingly increased. In petro- 

 myzon 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 plicated olfactory epithelium, 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 



Fig. 473. — Head of human embryo 

 with pharyngeal floor removed. Cut 

 surfaces lined. Compare with Fig. 

 471; b, lung; cs, cervical sinus; e, eye; 

 h, hyoid arch; hd, hypophysial duct 

 (Rathke's pocket) ; /, lung, Ig, lacrimal 

 groove; md, mandible; n, naris; on, 

 oronasal groove; tr, trachea. (From 

 Kingsley's " Comparative Anatomy of 

 Vertebrates," after Hertwig.) 



