2o6 



COMPARATIVE MORPHOLOGY OF VERTEBRATES 



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, 



Fig. 212. — Longitudinal section of head of 19 day Petromyzon embryo, ch, optic 

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

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

 thyreoid. 



with the exception of the dipnoi, the sensory surface is at the bottom 

 of a pit with no connexion with the alimentary canal. In the elas- 

 mobranchs, in order that water may flow more readily through this 

 pit, a fold is developed on one side of each naris, which practically 

 divides it into two. In many teleosts there is an actual division of 



on 



Fig. 213. Fig. 214. 



Fig. 213. — Nario-hypophysial region of Petromyzno, from above, c, cartilage of 

 nasal capsule; hd, naro-hypophysial duct; of, folds of olfactory membrane; on, olfactory 

 nerve. 



Fig. 214. — Head of Murana, after Jordan and Evermann, showing double nostrils. 



each primitive nostril into two, which may be at some distance from 

 each other, often at the ends of prominent tubes (fig. 214). Inside 

 the nasal capsule the olfactory epithelium is variously folded in order 

 to increase the sensory surface, often forming a labyrinth of consider- 

 able complexity (fig. 215). 



In air-breathing vertebrates, beginning with the dipnoi, means are 



