202 THE HISTORY OF CREATION. 
of all solid parts, could, unfortunately, as little as the 
Skull-less animals leave fossil remains. From its whole 
organization and ontogeny it is quite evident that it 
represents a very important intermediate stage between 
the Skull-less animals and Fishes, and that its few still 
existing members are only the last surviving remains of 
a probably very highly developed animal group which 
existed towards the end of the primordial period. On 
account of the curious mouth possessed by the Hags 
and Lampreys, which they use for sucking, the whole class 
is usually called Rownd-mouthed animals (Cyclostoma). 
The name of Single-nostriled animals (Monorrhina) is still 
more characteristic. For all Cyclostoma possess a simple, 
single nasal tube, whereas, in all other Vertebrate animals 
(with the exception of the Amphioxus) the nose consists 
of two lateral halves, a right and a left nostril We are 
therefore enabled to comprise these latter (Anamnionata 
and Amnionata) under the heading, double-nostriled animals 
(Amphirrhina). All the Amphirrhina possess a fully 
developed jaw-skeleton (upper and under jaw), whereas it 
is completely wanting in the Monorrhina. 
Apart also from the peculiar nasal formation, and the 
absence of jaws, the Single-nostriled animals are dis- 
tinguished from those with double nostrils by many 
peculiarities. Thus they want the important sympathetic 
nervous system, and the spleen which the Amphirrhina 
possess. Of the swimming bladder, and the two pairs of legs 
—which all double-nostriled animals have, at least in their 
embryonic conditions—not a trace exists in the Single- 
nostriled animals, which is the case also in the Skull-less 
animals. Hence, we are surely justified in completely 
