NO TES. 



45 



these circumstances it is necessary to suppose with EisiG that the 

 lateral line nerve i^Rainus lateralis vagi) arose as a collector. 



The removal of the lateral line from the immediate neighbour- 

 hood of the paired fins in existing fishes is easily intelligible on 

 the ground that the fins have become discontinuous and elaborated 

 into effective locomotor organs. 



It is not impossible that the lateral line nerve {R. lateralis vagi) 

 is homodynamous with the remarkable Ramus cutaneiis quinti 

 {R. rccurrens trigemini et facialis or Nerviis lateralis trigeinini, 

 Stannius) of Teleosteans, which runs to the base of all the fins, 

 paired as well as unpaired ; just as the paired fins themselves are 

 known to be homodynamous with the median fins. In this case 

 the R. cutaneiis quinti would be of primitive significance, notwith- 

 standing the fact that it is absent in Selachians ; and it would be 

 another of those features of organisation in the possession of which 

 Teleosteans exhibit more primitive relations than do the existing 

 Selachians. (Compare the functional pronephros of Teleosteans 

 and the entirely rudimentary pronephros of Selachians.) 



The above suggestion that the lateral line arose in the first 

 instance as a sensory equilibrating apparatus in conjunction with 

 the mechanical equilibrating apparatus effected by the continuous 

 lateral fin-folds, will of course meet with numberless difficulties 

 when it is attempted to carry it out in detail. As in some other 

 respects, so here, a great difficulty is presented by the Cyclo- 

 stomes. It may, however, be pointed out that if the various con- 

 clusions which have been drawn with regard to the morphology of 

 Araphioxus are correct, it must be assumed that the Cyclostomes 

 have entirely lost the lateral fin-folds and that the sense-organs 

 of the lateral line have secondarily become diffused in their dis- 

 tribution over the body. The latter conclusion is also indicated, 

 firstly, by the fact that there is a fairly well developed internal 

 ear in the Cyclostomes which, as noted above, must have been 

 differentiated from a primitive lateral line ; and secondly, by the 

 fact that although the sense-organs are scattered, there is never- 

 theless (at least in Petromyzon) a definite lateral line nerve. 



