OLFACTORY ORGANS. 



IQI 



olfactory organ and hypophysis. Hence cyclostomes, having but a single nostril, 

 are called monorhinal, in comparison with all other vertebrates which have two 

 nostrils (amphirhinal). The median opening or naris of the cyclostomes connects 

 with a naro-hypophysial duct, on the upper, posterior wall of which is the ol- 

 factory sac, formed of pairs of lateral folds (fig. 191) covered with the olfactory 



FIG. 190. Longitudinal section of head of 19 day Petromyzon embrgyo. ch, optic 

 chiasma; ep, epiphysial outgrowth; h, hypophysial ingrowth; mes, mesenteron; n, nasal 

 epithelium; nc, notochord; oc, oral cavity; op, oral plate; sc, canal of spinal cord; th, 

 thyreoid. 



epithelium and supplied by a pair of olfactory nerves. The lower part of the duct, 

 now purely hypophysial, descends to the hypophysis on the ventral side of the brain, 

 where it either ends blindly (petromyzons) or opens into the dorsal part of the oral 

 cavity (myxinoids). In the latter group the olfactory organ is surrounded by a 

 complicated nasal capsule of enormous size (fig. 153). 



on 



FIG. 191 FIG. 192. 



FIG. 191. Xario-hypophysial region of Petromyzon, from above, c, cartilage of nasal 

 capsule; hd, hypophysial duct; of, folds of olfactory membrane; on, olfactory nerve. 

 FIG. 192. Head of Murcena, after Jordan and Evermann, showing double nostrils. 



All other vertebrates have paired olfactory areas and paired nostrils 

 (nares) are developed in connection with them, and they have at no 

 time any relation to the hypophysis. The mechanism for bringing the 

 water or air to be tested to the olfactory surface differs accordingly as 

 the animals are air or water breathers. In all fishes, with the exception 

 of the dipnoi, the sensory surface is at the bottom of a pit with no 

 connection with the alimentary canal. In the elasmobranchs, in 



