636 



PHYLUM CHORDATA : CLASS PISCES — FISHES 



They are usually rather small fishes, with minute rhomboidal shagreen - 

 like scales, and a strong spine in front of each fin, except the caudal. 

 In some genera {Parexus, Climatius) there are two rows of small 

 intermediate spines between the proper pectorals and the pelvics. 



Sub-Class II, Teleostomi 



Fishes with more or less ossified skeletons, especially as 

 regards skull, jaws, operculum, and pectoral girdle. The 

 skull is hyostylic, the jaws being supported by the hyoman- 

 dibular. The pelvic girdles are usually rudimentary or 

 absent. The mouth is usually terminal ; the scales are in 

 the majority soft and cycloid. There is always a gill- 

 cover ; the inter-branchial septa are much reduced ; the 

 gill-filaments project freely from the gill-arches. There is 

 usually a swim-bladder. There are no claspers, no naso- 

 buccal grooves ; there is no cloaca. The fore-brain has 

 a non-nervous roof. The ova are small and numerous, 

 usually meroblastic, sometimes holoblastic. Fertilisation is 

 usually external. The orders 1-3 that follow are still 

 sometimes grouped as " Ganoids." 



Order i. Crossopterygii 



Ancient forms, all of which are extinct except Polypterus and Cala- 

 moichthys from African rivers. Examples, Osteolepis (Lower Devonian), 

 Holoptychius (Devonian), Megalichthys (Carboniferous). 



The skeleton is very bony. The rhombic scales and dermal skull 

 bones are covered with modified dentine called ganoin. The pectoral 

 fins are of the Crossopterygian type, the bulk of the fin being in the 

 form of a thick scale-covered lobe round which the rest of the fin 

 forms a fringe. The dorsal fin is divided up into a series of finlets 

 (1.3 in Polypterus lapradei, 9 in P. senegalus). The tail is diphycercal. 



Inside the mouth the pituitary invagination never becomes shut off, 

 retaining communication throughout life. 



The gills are covered by an operculum, but there is a spiracle opening 

 on the top of the head. The air-bladder is a bilobed, functional lung 

 with a pulmonary blood-supply from the hindmost epibranchial artery. 

 From time to time Polypterus rises to the surface to inspire air — it is 

 said to do this through the spiracles. If it is prevented from reaching 

 the surface, it eventually drowns. The position of the lung is dorsal 

 to the gullet, but its duct curls round to open into the pharynx ventrally. 



There is a spiral valve in the intestine, and a contractile conus 

 arteriosus with longitudinal rows of valves. 



Polypterus is largely carnivorous, living on crustaceans, insects, 

 etc., and at the beginning of the rainy season makes a rough, floating 



