646 PHYLUM CHORDATA : CLASS PISCES — FISHES 



(Fig. 370). The scales,are small and deeply buried in the 

 skin, the limbs reduced to short styles. There are four 

 gill -clefts bearing much -reduced gills. The external 

 gills of the larva .are not retained in adult life, but during 

 the breeding season, when the male Lepidosiren is on guard 

 over the developing eggs in a deep burrow at the bottom of 

 the swamp, the pelvic limbs, and occasionally the pectoral 

 limbs also, develop plumose highly vascular filaments, 

 and so become accessory breathing organs, thus freeing 

 the male from the necessity of leaving the nest and coming 

 to the surface to breathe. Lepidosiren may reach a length 

 of 4 ft. Its diet is partly vegetarian and partly carni- 

 vorous, Ampullaria, a large fresh-water snail, being a 

 favourite article of food. Lepidosiren passes the dry season 

 at the bottom of a deep tubular burrow. Afterwards, 

 during the rainy season, the eggs are laid in L-shaped 

 burrows excavated in the bottom of the swamp. The 

 eggs are 6-7 mm. in diameter. The larvae, like those of 

 Protopterus, have external gills and glandular cement- 

 organs. 



General characteristics. — The notochord persists 

 throughout life. Its sheath forms a tube of cartilage 

 which does not segment into vertebras. The (diphycercal) 

 tail supports show a primitive arrangement. The limbs — 

 tapered paddles in Ceratodiis, slender and dwindling in 

 Protopterus and Lepidosiren — are of the biserial archiptery- 

 gium type, a jointed rod of cartilage bearing more or less 

 well-developed radials on either side (see Fig. 372). The 

 skull is autostylic and is largely a persistent chondro- 

 cranium with the addition of some membrane bones. 

 Parts of the girdles and backbone and the cycloid scales 

 are bony. The teeth are large compound grinding teeth 

 of a characteristic type. The intestine has a spiral valve. 



There is no spiracle, and the gill-clefts are covered by an 

 operculum. In Lepidosiren the gill lamellae are greatly 

 reduced. 



The air-bladder is highly developed to form a breathing 

 lung, in addition to its normal hydrostatic function. It 

 is single in Ceratodiis^ double in Lepidosiren and Protopterus. 

 It develops as a ventral outgrowth of the pharynx, and in 

 course of growth twists round the right side of the gullet, 



