246 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



their last stages, be represented by a more or less simple 

 tube having as few as one or two openings on to the 

 surface on each side. In many forms the whole system 

 is simply a caricature of its robuster form in the cartila- 

 ginous and less specialised bony fishes — all the degenerate 

 forms occurring amongst those Teleosts which, for various 

 special reasons, have undergone most adaptation. 



That the lateral sense organs existed at one time as 

 functional organs on the surface of the body necessarily 

 follows from their origin and development. Ayers is 

 inclined to think that those recent forms in which it is 

 still superficial have been produced by a retrogressive 

 development frorn the canal type, but I am not disposed 

 to admit that the evidence favours this view. However 

 that may be, lecent work on the development of the 

 system in Elasmobranchs and Teleosts seems to show that 

 it was at first very confined and entirely limited to the 

 auditory region. From thence it spread (still on the 

 surface) forwards and backwards, producing by division 

 in front the usual tracts, and thus laying down a funda- 

 mental plan, from which the canals were subsequently 

 elaborated. This plan included the tract over the eye, 

 that under the eye, another in relation to the lower jaw, 

 and finally one at the side of the body. The architecture 

 of the system having been determined, canals were then 

 differentiated. Each sense organ sank below the surface, 

 so that it came to lie in a longitudinal furrow. The lips 

 of the furrow then fused, but left anterior and posterior 

 apertures, with the result that each sense organ lay in a 

 short longitudinal closed canal, open, however, at each 

 end. These canals approximated, fused end to end, and 

 thus formed a continuous longitudinal canal. Two con- 

 tiguous primitive apertures fused to form a larger pore, 

 and subsequent sinking on the part of the canal produced 



