CHAPTEE VII. 



THE OEGANS OF SENSE. 



Chaeacteristic of the Organ of Smell in Fishes is that it 

 Jias no relation whatever to the respiratory function, with the 

 exception of the Dipnoi, in which possibly part of the water 

 received for respiration passes through the nasal sac. 



The olfactory organ is single in Branchiostoma and the 

 Cyclostomcs. In the former a small depression on the front 

 end of the body, clothed with a ciliated epithelium, is re- 

 garded as a rudimentary organ of smell. In the adult 

 Petromyzon a membranous tube leads from the single opening 

 on the top of the head into the cartilaginous olfactory capsule, 

 the inside of which is clothed by membranes prolonged into 

 a posterior blind tube (Fig. 30, s), which penetrates the carti- 

 laginous roof of the palate, but not the mucous membrane 

 of the buccal cavity. In the Myxinoids the outer tube is 

 strengthened by cartilaginous rings like a trachea ; the capsule 

 is Kned by a longitudinally folded pituitary membrane, and 

 the posterior tube opens backwards on the roof of the mouth ; 

 the opening is provided with a valve. 



In all other Fishes the organ of smell is double, one being 

 on each side; it consists of a sac lined with a pituitary 

 membrane, and without, or with one or two, ojDenings. The 

 position of these openings is very difierent in the various 

 orders or suborders of Fishes. 



In the Dipnoi the nasal sac opens downwards by two wide 

 openings which are within the boundaries of the cavity of the 



