DIGESTIVE SYSTEM 729 



There are two general types of mouth forms. The first is found in 

 the great group of Cyclostomata (cycle mouths). There are no true jaws 

 (Fig. 422). The mouth is round and cannot be closed. Examples of this 

 form are the lampreys and hagfishes. This type of mouth is called suc- 

 torial. The cyclostomata are the only vertebrate parasites known. They 

 attach themselves to a living fish and suck their way directly through 

 the muscles of the host. 



The second type of mouth belongs to that group called the Gnathos- 

 tomata (jaw-mouths). This type of animal has movable bones or carti- 

 laginous jaws, usually possessing teeth formed of dentine and underlaid 

 with enamel. The jaws are developed from one pair of visceral arches. 

 The teeth are quite similar to the placoid scales of certain fishes which 

 have been modified in various ways. There are two theories held in re- 

 gard to the Gnathostomata or jaw-mouth fishes. First, that the mouth 

 is like that of the cylostomes, to which the gill arches with their asso- 

 ciated teeth have been added; and the second, that this jaw-mouth is a 

 new opening which originally consisted of a pair of gill-slits which later 

 became fused in the mid ventral line, the first mouth then being lost. 

 Probably the latter view has more supporters, because in the selachians, 

 where there is supposed to be a more primitive condition, the jaw-mouth 

 is not at the extreme cephalic end of the animal, but on the ventral side 

 with a long rostrum extending cephalad to it. Later, in some of the 

 ganoids this jaw-mouth has a secondary position at the very tip or termi- 

 nal end. 



The pharyngeal pockets develop from a row of outpushings meeting 

 a similar set of inpushings from the outside (Fig. 295). If the point of 

 contact is broken through, as in fishes, such openings are called gill-slits. 

 These are four to eight in number which permanently form a communica- 

 tion between the pharynx and the exterior, thus allowing the escape of 

 water taken in by the mouth for use in breathing. 



One or more of these slits appear in the early stages of amphibians 

 and in a few forms persist throughout life. In reptiles, birds, and mam- 

 mals, there are similar inpushings and outpushings during the embryonic 

 period, but only two or three contact-points ever form openings, and then 

 only for a short time. However, the most anterior of these which ap- 

 pears in the selachians as the spiraculum or blow hole, persists in all 

 higher vertebrates as the Eustachian tube and the greater part of the 

 middle ear. The other pouches disappear, although cartilages, muscles, 

 arteries, and glands arise in the embryo in connection with these pouches. 

 Sometimes there is an arrested development so that an open com- 

 munication persists between the pharynx and the exterior of the jaw 

 either upon one or both sides, as a cervical fistula. This is supposed 

 to be a permanent gill-slit that for some mechanical or chemical reason 

 has not continued growing as it normally does in the higher forms. 



The nasal cavities in fishes lie above the stomato-pharyngeal cavity 



