870 COMPARATIVE ANATOMY 



As the dorsal portion of the head then continues growing, the nares are 

 carried toward the tip of the snout or, in the elasmobranchs, toward the 

 ventral side of the head. We have already discussed the forms of the 

 nasal capsules with the skeletal system. In the higher groups, glands 

 form which keep the epithelium moist. 



Beginning with the amphibia and appearing in the higher groups 

 of animals, there is an accessory olfactory organ known as the organ of 

 Jacobson (Fig. 339), which is probably used to test the character of food 

 while it is in the mouth. The first and fifth cranial nerves supply the 

 organ of Jacobson, which lies near the' internal nostrils. 



Cyclostomes have but a single nostril. In all other vertebrates there 

 are paired nostrils called nares. In the cyclostomes, the nostril is directly 

 connected with the hypophysis, a condition not found in any other ver- 

 tebrates. In animals living within the water there are folds formed on 

 one side of each naris which practically divides it into two, in fact, in 

 many of the teleosts, each primitive nostril is actually divided into two ; 

 this permits water to circulate through the olfactory sac, thus carrying 

 various sensations of odors to the extensive sensory surface of the sac. 



In air-breathing vertebrates, including lung fishes, contrary to the 

 water-breathing groups, the nasal cavity has a connection with the ali- 

 mentary tract. In some elasmobranchs the first traces are seen of an ar- 

 rangement for drawing air over the sensory surface. This arrangement 

 is an oro-nasal groove which leads from each nostril to the angle of the 

 mouth (Fig. 296, nas. gr) ; in fact, this groove may, in some species, 

 form a definite tube. From the lung fishes upward a similar groove is 

 formed on each side before the skeletal parts form. As this closes, the 

 edges of each groove unite to form a duct leading from the nostril into 

 the oral cavity where an internal naris or choana is thus formed. The 

 position of these choanae varies in the different groups of animals. 

 Maxillary and pre-maxillary bones arise ventral to the nasal passage, 

 causing the ducts to appear as though running through the skull. 



If the oro-nasal groove does not close, harelip results, just as the 

 failure of the palatines to come together causes a cleft palate. 



In some of the urodeles a projection occurs from the lateral wall. 

 This is one of the first indications in the rising groups of vertebrates 

 of the conchae, well known in all higher groups. Often the ventral por- 

 tion of the nasal passage is lined only with ordinary epithelium. This 

 portion is then called the respiratory duct. The more dorsal portion 

 lining the tract with sensory epithelium is known as the olfactory duct. 

 The organ of Jacobson (Fig. 339) is on the medial side of the nasal 

 cavity in the lower urodeles. In the higher urodeles it is ventral, while 

 in the highest it has rotated to the lateral side. 



The external nares are closed by the smooth muscles in some of 

 the amphibia. There is little change in the choanae between amphibia 

 and reptiles. The reptiles, however, show a tendency to have a differ- 



