452 EMBEYOLOGY OF THE LOWER VEETEBEATES oh.. 



about amongst the vegetation of shallow water and- we may sup- 

 pose that in this early stage the limb was of a crude styliform 

 shape such as we see exemplified in the metamorphosed external 

 gill of the Urodele balancers, or in the actual limb of the larva of 

 Lepidosiren. 



On this hypothesis the ancestral styliform limb has pursued two 

 divergent lines of evolution. The one of these is found in those 

 Vertebrates which have developed along the lines of becoming 

 specialized for efficient swimming. Here it has become a fin, 

 an early stage of this evolution being represented by the crude 

 paddle of Ceratodus. That this biserial archipterygial type does 

 actually represent an extremely early type in the evolution of fins 

 seems to be demonstrated by two facts taken in conjunction with one 

 another — 



1. That this thick and clumsy organ represents functionally 

 a relatively inefficient type of swimming organ as compared with 

 the thin flat fin of most existing Fishes, and 



2. That palaeontology shows it to have been a widely distributed 

 type of fin in the early days of the evolution of the main groups of 

 Fishes. It was in fact the predominant type of limb amongst ancient 

 Elasmobranchs, Ganoids and Lung-fishes. 



Evidence is not entirely wanting to show how the Crossopterygian 

 type of fin on the one hand (as seen in the existing Polypterus) and 

 the Actinopterygian type on the other (as seen in Amia and other 

 Ganoids and Teleosts) may have been evolved out of the biserial 

 archipterygial type. This evidence cannot be gone into here but so 

 far as Crossopterygians are concerned the student should note 

 the close resemblance of the pectoral fin of the young Polypterus 

 (Fig. 197, E) and of its supporting skeleton (Fig. 169, F) to the 

 modified archipterygial fin of the ancient Shark Pleuracanthus (Fig. 

 169, B). 



Along the other line of evolution the styliform limb has given 

 rise to the pentadactyle leg with its expanded foot and its 

 characteristic jointing. It is of great interest in this connexion to 

 watch the clumsy movements of a Lepidosiren larva and to note that 

 the hind limb by which the creature pushes itself along becomes bent 

 twice upon itself precisely in the way which would give rise to the 

 ankle and knee-joints of one of the lower Tetrapoda. Occasionally 

 the appearance is rendered still more suggestive by the tip flattening 

 out slightly into a foot-like expansion. The observer watching 

 a Lepidosiren larva performing such movements finds it difficult 

 to avoid the suspicion that he is witnessing something very like 

 what took place in the early stages of the evolution of the penta- 

 dactyle limb. Should this be the true history of the origin of that 

 type of limb it would explain the unsatisfactory and wholly uncon- 

 vincing results of the efforts of comparative anatomists to derive the 

 skeletal elements of the pentadactyle limb from those of one or 

 other type of fin. 



