CHOANICHTHYES 



547 



in detail here. In general, apart from the possession of lungs, 

 their characters are primitive and elasmobranch-like, and in 

 two respects they are even more primitive than the cartilaginous 

 fishes ; the gut of the modern Dipnoi is ciliated, and there are no 

 vertebrae, the notochord being persistent. The Choanichthyes 

 were well estabhshed in the Devonian period. A living form, 

 Latimeria (Fig. 549), which closely resembles the extinct species,' 

 has recentty been discovered in African waters. 



Tetrapoda 



The rest of the vertebrates, called the Tetrapoda, or four- 

 footed creatures, agree in being air-breathing and in having 

 pentadactyl limbs. As we shall see later the adjective ' penta- 

 dactyl ' is not to be taken too hterally ; it describes a typical 

 condition, in which there are five fingers (or toes) but is used also 

 for any limb which is built on the same general plan, however 

 much it may be reduced. A few tetrapods have so Httle trace of 

 limb-skeleton that one cannot credit them with possessing even 

 vestiges of pentadactyl limbs, but their other features associate 

 them with animals which are clearty not fish. Tetrapods also 

 differ from all the earlier classes in that if they have unpaired 

 fins these never contain skeletal rays ; in the possession of a 

 tympanic cavity in the ear ; and in the possession of a cloacal 

 bladder for storing waste products. 



CLASS IV — AMPHIBIA 



The modem amphibians are adequately characterised by the 

 possession of a tadpole larva with functional gills ; this meta- 

 morphoses into an air-breathing 

 adult which retains some 

 primitive and fish-like features, 

 such as a long conus arteriosus, 

 symmetrical arterial arches and 

 a mesonephric kidney (p. 617). 

 Many of the extinct forms are 

 difficult to separate on the one 

 hand from Choanichthyes and 

 on the other from reptiles, but fig. 428.— stegocephaiia.— From 

 show a primitive type of skeleton Swinnerton. 



i_ '^u^ ^ ^^^4. :^ (a) Mastodonsaurus (Upper Trias); {b) Cacops 



such as one might expect m ^ (Permian). 



