398 STUDIES FOR STUDENTS 



of the lateral body folds. The reason for the development of 

 one portion of the lateral fins over another is not well under- 

 stood, but it has been suggested that at points of especial strain 

 in the fold, points where, from the mechanical advantage of their 



m/imotmm m 



Fig. 2. Schematic unpaired fin ; /;, basal , ;', radial ; d, dermal margin of fin 

 (after Smith-Woodward). 



position, certain parts of the fold were able to assist in the pro- 

 pulsion of the body through the water, that the fins were 

 especially well developed, and that there appeared lappetlike 

 prominences in the position of the present paired fins. As these 

 lappets assumed more and more of the function of swimming 

 organs, and less of balancers, they required an even stronger 

 support than before, and this was accomplished by the fusion of 

 the basals together and also of the radials, though to a less 

 extent. As the lappets of the lateral fin grew in importance, 

 the intermediate portion dwindled away until all trace of the 

 original fold is lost between and beyond the paired fins. The 

 final step in the development of the fins was the appearance 

 beyond the distal ends of the radials of fine dermal elements 

 that have much the appearance of the original actinotrichia. 

 These serve the purpose of supporting the web at the extremity 

 of the fins. 



Another line of development of the fishes is closely allied to 

 the development of the fins. The median fin that originally 

 extended from the head of the animal to the tail has in the 

 course of its development gradually retreated toward the poste- 

 rior portion of the body until it is represented by a single caudal 

 fin and one or more separated elements called from their posi- 

 tions the dorsal and the anal fins. The caudal fin has assumed 

 different positions in relation to the terminal portion of the 



