THE STURGEON 15 



gill laminae project outwards as filaments, and the 

 branchial skeleton, supporting the pharynx between 

 the clefts, is movable ; water is taken in through the 

 mouth, and by the contraction of the pharynx is sent 

 through the gill-clefts and over the gills, and so out- 

 wards via the gill-openings. The nasal sacs are 

 paired and lateral ; each is usually provided with two 

 external nostrils. An air-bladder is typically 

 present. 



Of four subclasses of the Pisces, two, the Crosso- 

 pterygians and Dipnoans, are represented at the 

 present day by only a few species in the fresh waters 

 of tropical countries. The remaining two, the 

 Chondrosteans and Teleosteans, include British fresh- 

 water species. 



The Sturgeons (Acipenseridce) and the Paddle- 

 fishes (Polyodontida] are the specialized and in many 

 ways the degenerate living members of a very ancient 

 and generalized group of fishes. The essential 

 characters of the subclass Chondrostei to which they 

 belong are as follows : 



The bones overlying the primary pectoral arch 

 and connecting it with the skull are four in number ; 

 the rays of the dorsal and anal fins are much 

 more numerous than their endoskeletal supports, 

 which they simply overlap ; the pelvic fins, like the 

 dorsal and anal, have each a muscular basal lobe 

 containing a well-developed series of radial segments 

 of their skeletal rods ; the caudal fin is heterocercal, 

 i.e. the axis of the tail is turned upwards, the upper 

 (supra-caudal) lobe is reduced to a series of spines, and 

 the lower (infra-caudal) lobe is strongly developed. 



