546 



CLASSIFICATION OF VERTEBRATES 



the gill slits are covered by an operculum ; an air bladder is 

 present, but it does not function as a lung ; there are no claspers ; 

 and they lay large numbers of small eggs. There are three primitive 

 orders, mainly extinct but including the modern sturgeons 

 (Acipenser) and some others, which share, in varying degrees, the 

 elasmobranch characters of muscular conus, spiracle, heterocercal 

 tail, and spiral valve. The fourth order, the Teleostei, which 

 includes the vast majority of fishes, including nearly all those 

 familiar in British waters, such as the trout [Salmo trutta) and 

 cod (Gadtis callarias =G. morrhua), differs from the Chondrichthyes 

 in these points also. The tail of teleosts is homocercal (Fig. 427), 

 that is, it is externally symmetrical but goes through an asym- 

 metrical stage in the embryo and may retain internal vestiges of 

 this. Alone amongst the lower vertebrates, the teleosts have no 

 cloaca. 



CLASS III — CHOANICHTHYES ( =CROSSOPTER YGIl) 



Neoceratodus, Lepidosiren and Protopterus are the lung-fishes of 

 Queensland, South America and Africa respectively ; their air- 

 bladders (one in Neoceratodus, a pair in each of the others) function 



4f' 



Fig. 427. — A diagram of the haddock. — From Thomson. 



a., Anus ; a/'., a/2., anal fins ; b., barbule ; br.m., branchiostegal membrane (a continuation of the gill cover) ; 

 cf., caudal fin ; dfA-df'i., dorsal fins ; g., genital opening ; na., nasal openings (double on each side) ; 

 op., operculum or gill cover ; pf., pectoral fin ; pvf., pelvic fin ; u., urinary opening. 



as lungs, and to fit this use there is a special blood supply and 

 there are internal nares. All three are tropical and live in 

 water which must often be poor in oxygen. Lepidosiren and 

 Protopterus normally survive the season when the rivers in 

 which they live dry up, by resting in the mud. The lung-fishes 

 are known as Dipnoi, and are associated with other extinct 

 forms as the Choanichthyes. They are very important in any 

 discussion of the origin of land vertebrates, but as the elementary 

 student is never likely to meet them we shall not consider them 



