Vertebrata. 341 



there is a succession of teeth; before falling out a tooth 

 loses its connection with the bone, the socket is reabsorbed. (For 

 the special conditions of replacement in Mammalia, see that 

 group). 



Among other hard parts connected with the oral cavity, and occasionally 

 occuiTing among the Vertebrata, may be mentioned certain horny 

 structures, locally thickened and hardened portions of the cuticle, which 

 are developed here just as on the outer skin: the horny teeth of Oyclo- 

 stomes and Monotremes, the horny jaws of Aves, Chelonia, and others. 



The tongue projects from the floor of the mouth, and is inti- 

 mately connected with the visceral skeleton, especially with its 

 unpaired portions. It is a feeble structure in Pish, but in Mammals it 

 is well-developed, muscular, and very movable, and thus of great 

 importance in introducing food into the mouth. It exhibits a great 

 diversity of form, as will be noticed more particularly in the various 

 groups : it is rarely altogether absent. Various glands are also 

 connected with the mouth, pouring into it secretions which serve to 

 moisten the food, etc. ; they are absent from Fish, but are developed 

 in all other groups. As a rule they are embedded in the wall of the 

 buccal cavity, but in the Mammalia some lie apart from it, and only 

 communicate with it by their ducts ; here they have, moreover, attained 

 to a considerable size as "salivary glands"; this is only 

 exceptionally the case in other groups. 



Developmentally, the thyroid gland is also connected with the mouth. 

 It arises* as one or more evaginations of the floor of the oral cavity, from 

 which, however, it soon separates, and later forms a. large, independent, duct- 

 less gland, which consists chiefly of epithelial sacs, filled with fluid, and united 

 by connective tissue. The function of the thyroid was until recently a complete 

 riddle ; medical researches of the last few yeai-s, however, have demonstrated 

 with certainty that it fonns a product of vital importance, which is carried by 

 the blood to the rest of the body. The enigmatical thymus al>o arises as 

 a series of evaginations, which separate later from the buccal cavity. It is 

 well developed in embryos and yoimg animals (in many young Mammalia it is 

 an extensive organ, stretching far back into the thorax), but it degenerates in 

 after life. In the adult the thymus consists chiefly of cellular connective 

 tissue. 



The oe sop ha I)- us is short and wide in Fish and Amphibia, 

 but in Reptiles and Birds may be longer, in consequence of the 

 formation of a neck ; in the Mammalia it is not only fairly long, but 

 also rather narrow, although capable of great distension, whilst in 

 all the others it is very wide. The stomach is a wider region 

 varying in form, with innumerable small tubular glands in its walls. 

 The small intestine is a straight tube in the Cyclostomes and 

 some other Fish, but is usually coiled. In Fish, Amphibia, and many 

 Reptiles it is still comparatively short : in Birds and Mammals on the 

 contrary, it attains a considerable length, several times the length of 



* According to general opinion, the thyroid corresponds to the eudostyle in the 

 branchial sac of Ascidians (q.v.). 



