PISCES 129 



There are in this order four extinct families belonging to the sub- 

 order Osteolepida, and one family, represented by two genera of 

 present-day African "lobe-fins" of the sub-order Cladista. Both of 

 these types are essentially antique, true " living fossils," although no 

 real fossils of these genera have as yet been discovered. 



A somewhat detailed description of Polypterus bichir will serve to 

 introduce the reader to the characters of the Crossopterygii that are 

 of most importance. 



According to Harrington, Polypterus bichir inhabits the deeper 

 waters of the Nile but does not bury itself in the mud like a true mud- 

 fish. The large lobate pectoral fins are used as balancers and to some 

 extent as paddles in swimming, but the most significant function of 

 these fins is their use as supports; for when resting on the bottom, the 

 head is held up from the mud by the tips of the fins, much as a mud- 

 puppy (Necturus) is supported by its fore legs. The excellent figure 

 (Fig. 102, A), after Osborn, shows these fins used as supporting limbs. 

 Such a habit suggests the mode of origin of the terrestrial limb from 

 the aquatic limb. The air-bladder in this primitive fish is not merely 

 or principally a hydrostatic organ, but is an accessory respiratory 

 organ. It is connected by a primitive trachea with the pharynx and 

 is used as a lung. The supporting character of the pectoral fin and 

 the primitive lung are considered of especial importance as furnishing 

 the beginnings of adaptations for terrestrial life. 



The larva of Polypterus is interesting on account of its similarity 

 to amphibian larvae. Budget has described a larva of P. senegalus 

 (Fig. 70, C) as quite a striking object, beautiful in color and markings. 

 Its most remarkable feature is the single pair of external gills, which 

 are pinnate in structure and are evidently homologous with the exter- 

 nal gills of amphibian larvae and those of the perennibranchiate 

 urodeles. The larva, like the adult, uses the pectoral fins as support- 

 ing appendages. The median fin system is of the primitive diphy- 

 cereal type like that of a tadpole, but differs from the latter in having 

 cartilaginous supports or rays. The species of Polypterus figured 

 (Fig. 70, A) is P. senegalus. 



The other living genus of Crossopterygii, Calamichthys (Fig. 70, B) 

 is much like Polypterus, but is a greatly elongated eel-like edition 

 of the latter. It is confined to middle western Africa, living in the 

 small muddy rivers of that region, swimming about with an undulat- 

 ing, snaky motion and feeding mainly on crustaceans. 



