478 HISTORY OF THE HUMAN BODY 



then its associated mouth (palaostoma) must have been the 

 later nasal cavity, and its unpaired condition in cyclostomes, in 

 spite of the contradictory testimony of the olfactory nerves, 

 would then be primitive and not secondary. 



In all other vertebrates the nose is strictly amphirrhine, that 

 is, it consists of two lateral cavities, symmetrically placed. 

 These cavities begin as localized thickenings of the ectoderm, 

 which invaginate and form nasal sacs, a condition that per- 

 sists in fishes. In the Dipnoi, the first air-breathers, these sacs 

 become prolonged posteriorly and break through into the 

 mouth cavity, forming the choana [posterior nares'], forma- 

 tions which are present also in amphibians and all higher ver- 

 tebrates. When these are present, the lower part of the cavity 

 is used more or less exclusively for respiration, and the olfac- 

 tory sense becomes limited to the more dorsal portion, thus 

 dividing the cavity into a pars respiratoria and a pars olfactoria. 



Within possible limits the greatest diversity exists in the 

 location of the nasal cavities, and especially their openings, 

 the anterior and posterior nares. The former may be placed 

 ventrally, as in dog-fish, and may occupy all intermediate 

 positions to an extreme dorsal one. Marked adaptations in 

 this respect are seen in those air-breathers which have become 

 secondarily aquatic, enabling them to breath at the top of the 

 water without being seen. Thus in the whales and porpoises 

 the nostrils seem to be moved to the top of the head, the 

 deception being due to very short frontal and nasal bones and 

 to an excessive anterior prolongation of the maxillaries and 

 premaxillaries. In some birds, like the albatrosses and pe- 

 trels, the nostrils become prolonged into tubes formed by the 

 beak, so that they open near the tip of that organ instead of 

 at its base. An extreme case is seen in the Dipnoi, in adap- 

 tation to their annual hibernation within a cocoon of dry 

 clay; for in these animals the openings of the anterior nares 

 lie within the mouth cavity, and the mouth is connected with 

 the exterior during hibernation by means of a long tube com- 

 posed of slime secreted by the animal. 



As regards the choanae, their original position is shown by 



