62 



CHARACTERS OF VERTEBRATE ANIMALS 



MID BRAIN 



'twixt brain 

 fore brain 



cerebral 

 hemisphere 



VISCERAL ARCHES 



of the most distinctive characters of Vertebrates, which for this 

 reason are often termed Chordata. 



In the vast majority of Vertebrates we find, at the beginning 

 of the digestive tube, jaws having an up-and-down movement. 

 Certain of the higher Invertebrates, e.g. lobster, scorpion, cock- 

 roach, also possess parts which are called jaws, but they are really 

 limbs which move from side to side, are placed entirely outside the 

 mouth, and do not form, as in a Vertebrate, a part of the head. 



In all groups of Vertebrates we find that at some period or 

 other of life there are certain slit-like openings on the side of the 

 throat, by means of which the cavity of the pharynx communicates 



with the exterior (fig. 

 35). These openings 

 in aquatic forms, such 

 as fishes, become the 

 gill-slits of the adult. 

 Probably everyone has 

 lifted up the flap or gill- 

 cover, which is found 

 on each side of the head 

 in -such a fish as a sal- 

 i^on or herring, and 

 seen the red comb-like 

 gills attached to the bars 

 between these slits. And 

 those who have not done 

 this are likely to have 

 noticed gold-fish or the 

 like in an aquarium, and if so, will have observed that the 

 animals continually open and shut their mouths, taking in water, 

 which passes out again through these gill-slits in the process of 

 breathing. In such terrestrial forms as Mammals, Reptiles, and 

 Birds such slits are present in the embryo only (fig. 35), but they 

 never perform any function, and ultimately close up altogether, 

 only serving as a token that in the remote past the ancestors of 

 these groups were aquatic forms, possessing gill-slits and breathing 

 by means of gills. Such a statement as the last might appear 

 very far-fetched, were it not that in the life-history of the frog, 

 and other Amphibia, we can actually see a gill-possessing fish-like 

 creature gradually turn before our eyes into a terrestrial animal, 



F^S- 35- — Front part of Chick Embryo 



