198 



COMPARATIVE ANATOMY 



Myxine this tube is long, and is supported by rings of cartilage. In 

 the larval lamprey the organ is at first ventral and unpaired (Fig. 

 125), but subsequently becomes sunk in a common pit with the 

 pituitary invagination and takes on a dorsal position : it is almost 

 completely divided into two lateral halves internally by the forma- 

 tion of a fold of the mucous membrane. The pituitary sac thus 

 extends backwards from the ventral side of the organ above the 

 mucous membrane of the mouth : in Petromyzon it ends bhndly, 

 but in Myxine it opens into the oral cavity, perforating the skull 

 floor from above instead .of from below as in other Vertebrates. 



Fishes.— The position of the olfactory organ in Elasmobranchs 

 (Fig. 157, A) differs from that seen in Cyclostomes in lying on the 



Fig. l.'JT. — A, Ventral View of the Head of a Dogfish {Sci/l/ium i-anicula). 

 y, external nostril ; JI, mouth ; HSO, integumentary sense-organs. 



B, Lateral View of the Head of a Pike (Ef^ox lucims). a and h, the anterior 

 and posterior openings of the external nostrils ; f, fold of skin separating a 

 and 6 ; Ag, eye. 



under instead of the upper surface of the snout, and thus i-etains 

 the more primitive position. From these Fishes onwards the 

 organ is always paired, each sac being more or less completely 

 enclosed by a cartilaginous or bony investment forming an outwork 

 of the skull. 



From the Ganoids onwards it always has a similar position with 

 regard to the skull, being situated between the eye and the end of • 

 the snout, either laterally or more or less dorsally : originally, 

 however, it is ventral. In the course of development each external 

 nostril of Ganoids and Teleosts becomes completely divided into 



