414 MANUAL OF ZOOLOGY. 



spines ; and in some cases their shape is even undistinguishable 

 from the horny scales of the typical Teleostean fishes. In all 

 cases, however, whatever their form may be, they have the dis- 

 tinctive ganoid structure, being composed of an inferior layer 

 of true bone, and a superior layer of enamel. It is to be 

 remembered, however, that these ganoid plates and scales are 

 not confined to the fishes of the order Ganoidei, but that they 

 occur in two sub-orders of the Bony fishes namely, the Plec- 

 tognathi and Lophobranchii and in some others of the Teleostei 

 as well. 



III. As to the fins, both pectorals and ventrals are usually 

 present, and the ventrals are always placed far back in the 

 neighbourhood of the anus, and are never situated in the im- 

 mediate vicinity of the pectorals. In some living and many 

 extinct forms the fin-rays of the paired fins are arranged so as 

 to form a fringe round a central lobe (fig. 175). This structure 

 characterises a division of Ganoids called by Huxley, for this 

 reason, Crossopterygidce, or "fringe-finned." The form of the 

 caudal fin varies, the Ganoids being in this respect intermediate 

 between the Bony fishes, in which the tail is " homocercal," 

 and the Sharks and Rays, in which there is a " heterocercal " 

 caudal fin. In the majority of Ganoids, then, the tail is un- 

 symmetrical or "heterocercal/' but it is sometimes equi-lobed or 

 " homocercal." 



IV. As to the structure of the respiratory organs, the Ganoid 

 fishes agree essentially with the Bony fishes. They all possess 

 free pectinated gills attached to branchial arches, and enclosed 

 in a branchial chamber, which is protected by an operculum, 

 and is closed by a branchiostegal membrane, usually supported 

 by branchiostegal rays. Besides the ordinary branchiae there 

 is frequently an additional gill, called the " opercular branchia," 

 attached to the interior of each operculum, and below this 

 a false gill or "pseudo-branchia," which receives arterialised 

 blood only. 



V. There is always a swim-bladder, which is often divided 

 by partitions into several cells, and is always connected with 

 the gullet by an air-duct, as in the Malacopterous division of 

 the Teleostean fishes. 



VI. As to the structure of the heart, the Ganoids differ from 

 the Bony fishes, and agree with the Sharks and Rays in having 

 a rhythmically contractile bulbus arteriosus, which is furnished 

 with a special coat of striated muscular fibres, and is separated 

 from the ventricle by several rows of valves (fig. 171, B, C). 

 This is a decided advance in structure, as in this way the 

 arterial bulb is enabled to act as a continuation of the ventricle. 



