30 



one segment appeariug in the form of an appendage belonging to a 

 different segment, — for example a maxillipede developed as a chela 

 in Cancer, or the end of an antenna bearing a foot in a sawfly^). 



Homoeotic variation seems to be particularly characteristic of seg- 

 mental organs, and it may therefore possibly be considered to give a 

 little additional support to the visceral arch theory of the origin of 

 the paired limbs rather than to the lateral fold view. 



Professor Kerr has suggested an interesting explanation of the 

 meaning of this variation, which supports his views as to the origin 

 of the paired limbs from external gills. According to this view, the 

 development on the pelvic fins of vascular filaments, probably respiratory 

 in function, is a reminiscence of the former exclusively respiratory 

 function of the whole organ, and this hypothesis is made the more 

 probable when we find a similar tendency appearing as a variation in 

 the pectoral fin. It should be remembered that the filaments are 

 situated on the same (ventral) border of the external gills as of the 

 pectoral fins and on morphologically the same border of the pelvic fin. 



Note on the possible respiratory function of the pelvic fin 

 filaments : 



It has been suggested (Graham Kerr, loc. cit.) that the teleo- 

 logical explanation of the enormous development of vascular filaments 

 on the pelvic fin of the male during the breeding season is that it is 

 to obviate the necessity of the fish frequently leaving the nest (an 

 underground excavation in which the male stays after the eggs are 

 laid) in order to rise to the surface to breathe, which, owing to the 

 reduced state of the gill filaments is necessary for the fish in the or- 

 dinary state. During my expeditions into the swamps to look for 

 nests of Lepidosiren I was able to find out that the male remains in 

 the nest till after the end of the larval life of the young. In the nest 

 in which I found the latest young Lepidosirens these had already 

 metamorphosed and had in fact begun to rise to the surface to breathe 

 air, so it is safe to assume that it was very nearly time for them to 

 leave the nest. The old male was still in the nest with the young 

 and the vascular fringe of the pelvic fin had already begun to atrophy. 

 It is clear therefore that the period of functional activity of this 

 vascular appendage coincides with the period during which it would 

 be of most use as an accessory respiratory organ — that is while the 

 fish is guarding its eggs and young in the underground nest. 



1) Bateson, Materials for the Study of Variation, 1894. 



