VERTEBRATA : FISHES. 



501 



various extinct forms have "ganoid" scales. The pectoral and 

 ventral limbs are both present, but have (in Lepidosireii) the form 

 of awl-shaped, filiform, many-jointed organs, of which the former 



f 



Fig. 272. Dipnoi. Lepidosiren annectens, 



only have a membranous fringe inferiorly. The ventral limbs are 

 attached close to the anus, and the pectoral arch has a clavicle ; 

 but the scapular arch is attached to the occiput. In Ceratodus 

 (fig. 273) the pectoral and ventral limbs have the same form as 

 in the Crossopterygious Ganoids. The caudal fin is symmetri- 

 cal, or, in some extinct forms, may be heterocercal. The heart has 

 two auricles and one ventricle. The respiratory organs are two- 

 fold, consisting, on the one hand, of free, filamentous gills con- 

 tained in a branchial chamber, which opens externally by a single 

 vertical gill-slit ; and, on the other hand, of true lungs in the form 

 of a double cellular air-bladder, communicating with the oesopha- 

 gus by means of an air-duct or trachea. The branchice are sup- 

 ported upon branchial arches, but these are not connected with the 

 hyoid bone ; and in some cases, at any rate, rudimentary external 

 branchiae exist as well. The nasal sacs open posteriorly into the 

 throat. 



If these characters are examined a little more minutely, it 

 is easy to point to those in which the Dipnoi approach the 

 Fishes, and to those in which they resemble the Amphibians. 

 They resemble the "fishes in the shape of the body, and in the 

 possession of a covering of horny overlapping scales of the true 

 cycloid character ; whilst the limbs are more like those of fishes 

 than of reptiles. The fin, also, which clothes the posterior 

 extremity of the body, is of a decided fish-like character. The 

 most marked piscine feature, however, is the presence of free 

 branchiae, attached to branchial arches, and placed in a bran- 

 chial cavity, which opens internally into the pharynx by a 

 number of fissures, and communicates externally with the outer 

 world by means of a single vertical gill-slit. 



On the other hand, the Dipnoi approximate to the Amphi- 

 bians in the following important points : The heart consists 

 of three cavities, two auricles, and a single ventricle. True 

 lungs are present, with a trachea and glottis, returning their 



