438 CHORD AT A. 



They are world-wide in distribution, and are freshwater, 

 pelagic, littoral, katantic and abysmal in habitat. 



They are divided into two unequal divisions — the 

 Crossopterygti, mainly extinct but including Polypterus of 

 the Nile, and the Actinopterygii which embrace the 

 remainder. These are in their turn divided into three 

 sub-orders : — 



1. The Chondrostei are mainly extinct types, together 

 with the sturgeon of sub-arctic regions and one or two 

 species found in North America. Their skeleton is 

 cartilaginous. 



2. The Holostei include the bony pike ( Lepidosteus) 

 of North America and several extinct forms. The skeleton 

 is osseous and there is a spiral valve in the intestine. 



3. The Teleostei ^ constitute an immense number of well- 

 known fishes. Their skeleton is osseous, they usually have 

 horny (cycloid or ctenoid) scales, they have no conus 

 arteriosus to the heart and no spiral valve in the intestine. 

 Their principal groups are as follows : — 



1. Physostomi (air-bladder communicating with the oesophagus), 



most freshwater fishes and common marine forms, such as 

 herring, sprat, eels. 



2. Anacanthiiii (air-bladder closed, the fins are soft), comprising 



cod, haddock and the flat-fish. (In the flat-fish the air- 

 bladder is absent. ) 



3. Acanthopteri (fin-rays are spiny, air-bladder closed), including 



perch, mackerel, gurnard. 



4. Pledognathi and 5. Lophobranchii^ two small groups with very 



specialised members. The Pledognathi usually have a 

 hard bony exoskeleton and few powerful teeth with certain 

 bones of the jaw fused. The Lophobi-anchii have tufted 

 gills and may assume peculiar shape and habits ; they 

 include the pipe-fishes and the sea-horses. 



Order II. — Elasmobranchii. 



This order includes the Sharks and Skates. Their tail 

 is heterocercal, the scales are placoid. The gill-slits are 



* The archaic freshwater types like the sturgeon, bony-pike, and Polypterus 

 all the extant Teleostomi, except Teleostei) used to be combined in an order called 

 Ganoidei, but their genetic relationships to the Teleostei are now recognised. 



