

vii.] THE EXTERNAL SKELETON, 255 



branchus], and even (as in Pleikodori) teeth upon the under 

 surface of the back part of the skull. 



Fishes teach us that teeth may be developed in yet other 

 situations, for in some of those animals (e.g. the Salmon and 

 Qdontoglossum), there are teeth upon the tongue ; and in 

 most Fishes teeth are developed at the extreme back of the 

 mouth, on the bones of those arches (branchial arches) 

 which represent permanently what have been spoken of in 

 the First Lesson as the transitory posterior visceral arches of 

 the early stages of human development. The branchial 

 arches of Fishes which support the gills, and are placed on 

 each side beneath the back of the head, sometimes (as in 

 the Perch), develop tooth-like bodies on their inner surfaces, 

 and the hinder of these lateral arches terminate (Fig. 114) 

 in bones called pharyngeal, 1 which most frequently support 

 teeth (sometimes of very complex structure, as in Scarus], 

 termed from their situation pharyngeal teeth. Moreover, 

 the exactly reverse conditions to those which obtain in 

 man may be met with, as in the Carp and Tench, where the 



FIG. 222. PHARYNX OF A TENCH OPENED FROM BELOW, AND THE TWO ROWS 



OF PHARYNGEAL TEETH DIVARICATED. (After OlVtn.) 



b, basi-occipital tooth ; ph, pharyngeal teeth ; o, oesophagus. 



margins of the jaws are as free from teeth as is man's throat, 

 while the posterior aperture of the mouth is bounded by 

 teeth, partly pharyngeal, and in part attached to a prolonga- 

 tion downwards of the hindmost bone of the base of the 

 skull ! 



We shall see that tooth-like tissue may be developed in still 

 more anomalous situations, but as some of the structures 

 referred to cannot be called teeth, and others can only doubt- 

 fully be so called, notice of them may be postponed. 



22. As to IMPLANTATION, the changes which take place in 



1 From being placed in the "pharynx." For this see Lesson XI. 



