140 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



mass, as in the Salpida?, where it forms the so-called 

 " nucleus." 



In those Vertebrates that breathe by gills the 

 water of respiration enters by the mouth ; in the air- 

 breathing forms the air enters the mouth by the nostrils, 

 so that in their case also the most anterior portion of 

 the digestive tract serves as an ante-chamber to the 

 respiratory organs. Leaving these functions aside 

 for the moment (see page 231) and confining ourselves 

 to the mouth as a part of the digestive apparatus, we 

 observe that it is rounded in shape in the lowest, 

 the Cyclostomata (lampreys and hags), and merely 

 supported by cartilages ; in all the rest it is more or 

 less slit-like, and a pair of branchial arches give rise 

 to jaws (&iiatliostomata). These jaws are either 

 covered by connective tissue, or horny plates (tor- 

 toises, birds, monotremata), or, as in all Mammals, 

 except the lowest and the whales, they are guarded and 

 aided by movable muscular structures, which are 

 known as lips. These aid in the taking in of food, 

 or in retaining it when it has entered the mouth 

 cavity ; in the production of sounds, and especially of 

 human speech ; and they are an important factor in 

 the production of the expression of the emotions. 



In the vertebrate series we apply the term teeth 

 to those hard bodies which are set in the mouth, and 

 are developed from cells of epiblastic origin ; in their 

 simplest condition these organs are more or less 

 simple spiny bodies, exactly comparable to the spines 

 which are found on the skin of many sharks. Nor 

 is the community a community of structure 

 merely; from within the limits of the history of 

 an individual it is possible to draw sufficient evidence 

 to prove that there is a community of origin 

 between what have been well called dermal 

 denticles and what we call teeth. The accom- 

 panying figure, which represents a section of the lower 



