9 o MORPHOLOGICAL TYPES 



and the later ones terrestrial, these animals (newts, toads, and 

 frogs) are called Amphibia. In the early larval condition 

 there are external gills, and subsequently three pairs of gill- 

 slits. These disappear when the animals metamorphose and 

 come out on land. 



The aquatic larvae have lateral-line organs, disposed in a 

 similar manner to those of fish, only they are sunk in a groove 

 instead of being in a canal. When the newt emerges from the 

 water in the summer, these organs degenerate somewhat. 

 They reappear when the newt returns to the water, as it does 

 at the next breeding season, if not before. 



The mouth is wide, and the external nostrils are just above 

 it. The eyes are small. The alimentary, excretory, and 

 genital apertures are situated in a cloaca just in front of the 

 base of the tail. 



Skull.- — The cartilaginous neurocranium is very similar to 

 that of Ceratodus. The suspension of the jaws is autostylic, 

 and besides the basal and otic processes, there is also an 

 ascending process. These processes have precisely the same 

 relations to the neighbouring blood-vessels and nerves as they 

 have in Ceratodus. Only a little of this cartilaginous brain 

 case and olfactory and auditory capsules is replaced by cartilage- 

 bone. On each side, anteriorly, are the orbitosphenoids. 

 Posteriorly are the prootics and exoccipitals, which form the 

 condyles with which the skull articulates with the first vertebra. 



The membrane-bones covering the skull dorsally are paired 

 nasals, prefrontals, frontals, and parietals ; on each side of the 

 latter are the squamosals which overlie the quadrates. On the 

 under side are the paired prevomers, pterygoids, and the 

 parasphenoid. 



Paired maxillae are present, and the two premaxillae have 

 fused together in front. The pterygoids (dermal bones) 

 extend freely forwards from the quadrates. In the lower 

 jaw, part of Meckel's cartilage ossifies as the articular, which 

 is encased anteriorly between two membrane-bones ; the 

 (lateral) dentary and the (medial) splenial. The surfaces by 

 which the quadrate and the articular are in contact are carti- 

 laginous. The ceratohyals and the ventral elements of the 



