LUNG-FISH 



389 



other fishes. The olfactory organ calls for special mention as its mode 

 of development shows features that are of importance in relation to 

 conditions met with in the higher vertebrates. As in other cases the 

 olfactory organ is morphologically an ingrowth of the ectoderm of the 

 lower surface of the head in front of the mouth. In the larva of 

 Protopierus the opening of this becomes elongated and then assumes the 

 outline of a dumb-bell, the central portion becoming narrow and slit- 

 like (Fig. 169, B, olf), and finally becoming obliterated so that the cavity 

 of the organ now has two separate openings (Fig. 169, C, olf^ and olp). 

 Here we have established for the first time what in the higher vertebrates 



ABC 



Fig. 169. 



Roof of mouth in larvae of Lung-fish [Protopterus) to show the division of the olfactory opening 

 nto anterior and posterior nares. olf, Undivided olfactory opening ; olf^, anterior naris ; 

 olp, posterior naris. 



are termed, the anterior (or external) and the posterior (or internal) 

 nares. In these higher vertebrates the ridge forming the anterior 

 boundary of the buccal cavity passes between the two openings on each 

 side so that the external naris is left on the outer surface of the head 

 while the internal naris is enclosed within the buccal cavity. Terrestrial 

 vertebrates are enabled by this arrangement to breathe through the 

 nose : the Lung-fish, however, has not got this length. It breathes 

 through the mouth and uses the new arrangement of its olfactory organ 

 merely as an adjunct to its sense of smell, for so-to-say sniffing the 

 water — more especially in its search for food. 



The Lung-fishes deposit their eggs amongst water-plants {Ceratodus) 

 or in burrows at the bottom of the swamp (Lepidosiren, Protopterus). 



