VERTEBRATES 7 



The union of the digestive tract in vertebrates with the respiratory, 

 just alluded to, is peculiar to them and to chordates. The paired gill 

 clefts in the lateral walls of the pharynx and the paired visceral arches 

 which form their skeletal support are universal throughout the group 

 and are not found in other animals. The character and position of the 

 mouth and the jaws and their relation to the gill system are especially 

 noteworthy. In the great group of Arthropoda the organs of mastica- 

 tion are modified limbs and are lateral in position; in the Mollusca these 

 organs, where they are present, are cuticular or calcareous thickenings 

 on the surface of the mouth or pharynx, and the same is true of other 

 invertebrate groups. In vertebrates, however, the jaws have a dorso- 

 ventral position, and they and the slit-like mouth have had their origin 

 respectively from the foremost pair of visceral arches and the foremost 

 pair of primitive gill-clefts. The fact that the cyclostomate fishes, the 

 most primitive vertebrates, are without jaws seems to indicate that the 

 earliest vertebrates, like them, possessed a circular jawless mouth 

 armed with cuticular teeth and with a series of about seven pairs of 

 gill-clefts supported by cartilaginous arches posterior to it. When the 

 true fishes, the ancestors of all other vertebrates, developed from a 

 cyclostomate stock, the foremost two pairs of cartilaginous arches and 

 the foremost two pairs of gill-clefts lost their primitive functions and 

 were transferred from the respiratory to the digestive system of organs. 

 The foremost pair of gill-clefts became the mouth while the foremost 

 pair of arches reinforced by dermal bones formed the upper and lower 

 jaws ; the second pair of clefts became the spiracles in the elasmobranch 

 fishes and the Eustachian tubes in the terrestrial vertebrates, while 

 the second pair of arches formed the skeletal support of the tongue. 

 The remaining five pairs of arches are gill-bearing in the fishes and 

 support the gill -clefts; in the terrestrial vertebrates, however, they 

 undergo a change of function during the embryonic period and lose 

 their primary significance, being transformed into the cartilages of the 

 larynx and trachea. 



The teeth of all vertebrates are identical in structure, being com- 

 posed of dentine overlaid with enamel, and the identity of this structure 

 with that of the placoid scales of the lowest of the true fishes shows that 

 vertebrate teeth are directly derived from these scales. The actual 

 form of the teeth in each group has been determined by the service 

 they are expected to perform. In most fishes, amphibians and reptiles 

 the teeth are employed only for grasping and holding the prey; they are 

 conical in shape like a placoid scale and are usually more or less loosely 

 attached in the oral cavity. In mammals, which use their teeth not 



