Ichthyopsida 437 



attempts to discover phylogenetic relationships, we are often embar- 

 rassed by the fact that evolution progresses sometimes by addition and 

 sometimes by subtraction. 



SUBCLASS V. DIPNOI 



Lungfishes. Lungs — single or two, well-developed, and vitally 

 important in addition to gills — are the characteristic to which this 

 group owes its name ("Di-pnoi," very broadly translated, referring to 

 the double breathing mechanism). Aside from lungs, various other 

 characteristics of these fishes require making a special subclass for 

 them. Their more important peculiarities are as follows. 



Single lung-sac (in Epiceratodus) , or pair of sacs (in Protoplerus 

 and Lepidosiren), opening into esophagus through an aperture (glottis) 

 in its ventral wall. Nasal cavities have openings (internal nares) into 

 mouth-cavity. 



Gill-clefts — five pairs (Epiceratodus; Protoplerus) or four pairs 

 (Lepidosiren), opening externally, on each side, beneath an operculum 

 supported by thin plates of bone. Spiracles closed in adults. 



Skeleton incompletely ossified. Cranial part of skull mainly 

 cartilaginous. Notochord persistent, vertebrae represented only by 

 neural and hemal arches, partly bony; no centra. Tail-fin diphycercal. 



Paired fins long, narrow, pointed, each having a thick muscular 

 basal "lobe," and supported by a jointed cartilaginous axis bearing 

 (in Epiceratodus) on each side of it a series of projecting rays of carti- 

 lage (Fig. 338) — a "biserial" fin. Pelvic fins at extreme rear of trunk. 



Scales, thin, overlapping plates of dermal bone. 



Intestine contains a well-developed spiral valve. Anus and urino- 

 genital ducts open into a common cloaca. 



Heart three-chambered, although the partition between right 

 and left auricles is not complete. 



There are only three living genera of Dipnoi: Epiceratodus 

 (Ceratodus or Neoceratodus) of Australia (Fig. 329F) ; Protop- 

 lerus of Africa; and Lepidosiren of South America. These fishes all 

 inhabit rivers in warm climates subject to prolonged dry seasons during 

 which the streams may become reduced to pools of more or less stag- 

 nant water, or may completely dry up. The fishes survive the dry 

 season by breathing air through the nasal passages into the lungs. The 

 African and South American dipnoans burrow into the mud, leaving 

 only the tip of the head and the nostrils exposed to the air. 



The Dipnoi exhibit a motley collection of characteristics. Having 

 lungs, and nasal cavities opening into the mouth, they possess the 

 respiratory equipment typical of land vertebrates. The intestinal spiral 

 valve and the cloaca are elasmobranch features. In degree of ossifica- 



