^O AYERS. [Vol. VI. 



canals are not so large as those of the pigeon, have nearly the 

 same diameter, but have decidedly different shape and indi- 

 vidual peculiarities of spatial relation, resembling very closely 

 in these things the song thrush {Tin^dus miisiais). A close 

 agreement between these two forms was to be expected. The 

 anterior ampulla of Mimus is much smaller than either the 

 posterior or the horizontal, and of these two the former is 

 the larger. 



ENDOLYMPHATIC DUCT. 



In the Torpedo the endolymphatic duct is the long tube 

 which runs from the funnel-shaped cap of the utriculo-sacculus 

 through the muscles, skull, and skin to open out on the dorsal 

 surface of the head just in front of the so-called aural line of 

 canal organs.^ It does so by means of a funnel-shaped depres- 

 sion in the skin. Just beneath the skin it swells out into a 

 sac-like enlargement which is usually filled with otolithic crys- 

 tals. The communication thus kept up between the sea-water 

 and the internal ear renders the liquid contained in the ear very 

 like the sea-water, and analysis gives only a trace of organic 

 matters, such as mucin and other albuminoids. The sharks 

 preserve this duct in its simplest form ; i.e. in its most primi- 

 tive form. In the remaining Ichthyopsida and in the Saurop- 

 sida the organ is much reduced and its course of development 

 changed. In the forms just named it does not open upon the 

 surface of the head, but it may communicate with the lymph 

 spaces about the central nervous system. In other forms it 

 ends blindly in a saccular enlargement, usually after having 

 penetrated into the cranial cavity, and finally it may be present 

 as an inconspicuous protuberance on the dorso-median portion 

 of the ear sac. 



When it communicates with the brain spaces, its inclusion 

 within the skull is due to its being inclosed by the membrane 

 bones during development, — the bones which normally pro- 

 tected its parent organs. In this connection I wish to call 

 attention to some modifications of this duct and of the utriculo- 



1 In some Elasmobranch forms the endolymphatic duct opens on the surface behind 

 the aural canal. In all species the tube pierces the skin obliquely, and is here much 

 contracted ; the opening, whether slit-like or a funnel-shaped depression, is usually 

 situate on the top of a dermal hillock. 



