390 MANUAL OF ZOOLOGY. 



spines; and in some cases their j/^a/-? 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 stiperior layer of ehamel. It is to be re- 

 membered, 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 Pkc- 

 tognathizxidi Lophobranchii — and in some others of the TekosU. 

 as well. 



III. As to ih&fins, both pectorals and ventrals are usually pre- 

 sent, and the ventrals are always placed far back in the neighbour- 

 hood of the anus, and are never situated in the immediate 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. 146). This structure characterises a division 

 of Ganoids called by'Huxley, for this reason, Crossopterygida, 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 unsymmetrical 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 

 /ri9f 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 opercuhim, 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. 142, B, C). 

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

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



VII. The intestine is often furnished with a spiral redupli- 



