OLFACTORY ORGANS. Igi 
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 
Fic. 190.—Longitudinal section of head of 19 day Petromyzon embrgyo. ch, optic 
chiasma; ep, epiphysial outgrowth; h; hypophysial ingrowth; mes, mesenteron; n, nasal 
eee nc, notochord; oc, oral cavity; op, oral plate; sc, canal of spinal cord; th, 
yreoid. 
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). 
Fic. 191 Fic. 192. 
Fic. 191.—Nario-hypophysial region of Petromyzon, from above. , cartilage of nasal 
capsule; hd, hypophysial duct; of, folds of olfactory membrane; on, olfactory nerve. 
Fic. 192.—Head of Murena, 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 
