XXI] MAIN DIVISIONS 543 



It retains throughout life two gill-slits and three external gills on. 

 each side. Necturus is abundant in the shallows of the St 

 Lawrence, wriggling in and out around the roots of aquatic plants. 

 A somewhat similar animal, Proteus, with more rudimentary limbs, 

 is found inhabiting the limestone caverns of Carniola in Austria. 

 Lastly, there is the aberrant Siren, which has a horny beak en- 

 sheathing the premaxilla and dentary ; it has no hind-limbs, but, 

 is similar to Necturus in its gills : it is found inhabiting the 

 swamps of the Southern United States. 



Since the Ichthyoidea possess both gills and lungs it is at first 

 sight tempting to regard them as the little modified descendants of 

 an animal just making the transition from water -breathing to 

 air-breathing life. There are however insuperable difficulties in 

 the way of such an explanation. If we turn to other groups of the 

 animal kingdom we find that the first step in fitting an animal for a 

 land life is the covering up of the respiratory organ so as to protect 

 it against drying up. But in hardly any fish are the respiratory 

 organs so exposed as in Necturus, Proteus and Siren. 



Further, it was pointed out that the great gap between fishes 

 and Amphibia is to be found in the structure of the limb. But the 

 Ichthyoidea do not in any way assist in bridging the gap. On the 

 contrary their limbs are obviously degenerating, a fact which seems 

 to show that the aquatic life has been re-acquired. Now when the 

 similarity between say Necturus and the late larva of Molge is 

 borne in mind, and the further fact that these larvae may abnormally, 

 become sexually ripe, the conclusion is irresistibly suggested that the* 

 Ichthyoidea are larvae in which the adult stage has been suppressed.; 

 In the case of one large American newt, Amblystoma tigrinum, the 

 larva (the " Axolotl ") often breeds under certain circumstances and 

 was at one time regarded as a distinct genus (Siredon). 



The second division of Urodela, the SALAMANDROIDEA, are in : 

 general very similar to Molge, both in appearance, anatomy and size* 



As in Ichthyoidea, so likewise in Salamandroidea is North 

 America very rich. These have been divided into families on grounds; 

 of differences in the skeleton which have little effect on the external 

 appearance. The most abundant are the AMBLYSTOMATINAE repre- 

 sented by the genus Amblystoma of which there are many species,^ 

 nine being found in the Eastern States and Canada.; The members 

 of this family are distinguished by having the palatine bones, 

 directed transversely, so that the vomeropalatine rows of teeth 

 run across the roof of the mouth instead of along it, and by having 



