The Lateral Sensory System in the Muraenidae. 145 



this ring of bone, at a certain age, alone separating- the lateral 

 sensory and auditory canals. One is necessarily struck by the simi- 

 larity of appearance, in these sections, of these two canals. Anterior 

 to the region of these sections the cartilage of the cranial wall disappears 

 and the squamosal here forms a part of the side wall of the labyrinth 

 cavity, appearing, in sections, as a ring of bone with plate-like 

 portions extending dorsal and ventral to it, as shown in figure 20. 

 The ring of bone lodges the lateral sensory canal, and the anterior 

 vertical, and horizontal semicircular canals pass close to its inner sur- 

 face. So closely are these canals related that it would even seem as 

 if vibrations in the lateral canal might here be transmitted through 

 the bone to the fluids in the labyrinth cavity, and hence to the ear. 

 Certainly this manner of transmission of sound waves is as direct and 

 feasible as the one assumed by Sagemehl (No. 23), through the mouth 

 or gill openings to the outer wall of the bulla acoustica. 



Houghton (Nos. 19 and 20) found and described the squamosal 

 canal of the common eel, but he could not make out its relations to 

 the lateral system. The relations of this canal to the ear may ac- 

 count for his assertion that the eel "is commonly supposed to hear 

 well*'. The canal is deeply buried beneath the overlying dorsal ex- 

 tension of the adductor mandibulae muscle, this relation to the muscle 

 indicating that the dorsal extension of the muscle is acquired after 

 the canal has sunk beneath the surface, the muscle pushing upward 

 between it and the overlying skin. That the canal cuts through the 

 muscle, from its outer to its inner surface, is wholly improbable. 



In the squamosal canal there are two sense organs. Nos. 9 and 

 10 infraorbital. No. 9 lying approximately opposite the hind edge of 

 the postorbital process of the skull, that is at about the middle of 

 the total length of the squamosal, and No. 11 lying about half-way 

 between organ No. 9 and the hind end of the bone. Between these 

 two organs there is no indication whatever, even in my youngest 

 specimen, of a primary tube, though sucli a tube and its pore. No. 10 

 infraorbital, should normally have here been formed. The two termina 

 tubes and pores of the squamosal canal. Nos. 9 and 11 infraorbital, 

 are also absent as independent tubes and pores, both of them, however, 



Internationale Monatsschrift für Anat. u. Phys. XX. 10 



