3 8o 



MANUAL OF ELEMENTARY ZOOLOGY 



li. Teleostoml. 



the enlargement of the skeleton of the pectoral fin, are 

 shown in Fig. 268. Both ovaries are present. 



The Teleostomi have bone as well as cartilage in the 

 skeleton, an air-bladder in the abdominal 

 cavity, a peculiar, thin, non-nervous roof to the 

 cerebrum, and a bone-supported fold, the operculum, over 



the external openings of the 

 gill clefts, which lead straight 

 outwards through the sides of 

 the throat, and are not en- 

 larged into gill pouches like 

 those of the Elasmobranchii 

 (p. 350). Thus the arches are 

 narrow from within outwards, 

 and cannot accommodate the 

 full width of the gills, which 

 project into the chamber under 

 the gill cover. The scales of 

 the Teleostomi are flat, bony 

 plates, only in a few cases 

 provided with small vestiges 

 of the enamel-capped spines 

 of the placoid type, em- 

 bedded in the skin, and 

 usually overlapping like tiles 

 on a roof. 



Certain small sections of 

 the Teleostomi, 

 loosely known as 

 "Ganoids," of which the 

 Sturgeon (Acipenser) is an 

 example, keep a varying number of the following features 

 of Elasmobranchii : the spiracle, spiral valve, common vent 

 or cloaca, muscular conus arteriosus (instead of a non- 

 muscular "bulbus arteriosus"), and obviously unsym- 

 metrical or " heterocercal " tail (the bony framework of 

 the apparently symmetrical "paracercal" tail of an ordinary 

 fish is really unsymmetrical, except in certain cases, such 

 as the cod, etc., where, by the absence of the upturned 

 tip of the backbone, it reaches a complete secondary 

 symmetry). Most ganoids have for scales stout, bony 



Fig. 272. — Tails of fishes. 



Protocercal (Cyclostomata and it Qanolds " 



Dipnoi); B, Heterocercal (Elasmo- 

 branchii and Ganoids) ; C, C", 

 Paracercal (Teleostei). 



