Evolution of Animals 481 



If we next select for comparison the skin-glands, these form 

 an equally important feature in common for Batrachia and 

 Mammalia, while their absence in reptiles is in striking con- 

 trast. The abundant and diversified glands in salamanders 

 and other urodeles, which often secrete several diverse sub- 

 stances, only find an evolved parallel in the sweat, the sebace- 

 ous, the meibomian, the mammary, and other glands of mam- 

 mals. 



Much stress has been laid on the similarity of the teeth 

 and tooth groupings in some fossil reptiles to the mono- or 

 diphyodont and diversely grouped teeth of mammals. But 

 many facts go to show that this is probably a case of parallel 

 development or homoplasy. We may therefore review here 

 the entire question of tooth formation amongst craniate animals. 



We have already learned that horny teeth are a frequent 

 characteristic of nemerteans. These persist as the median 

 horny tooth of Myxine or as the numerous scattered teeth of 

 Petromyzon. Such seem unquestionably to have been the 

 primitive tooth type, and are reproduced as the only func- 

 tional teeth in many fish genera, and over considerable areas 

 of the buccal cavity. But these again correspond to the der- 

 mal plates or denticles, that have constantly tended to appear 

 in several lines of animal development. In this connection 

 Wiedersheim (i^5; 30) says: "The first and most primitive 

 hard structures in Vertebrates are ... in the form of 

 small pointed denticles (placoid organs) in the skin; these con- 

 sist of enamel and dentiney resting on a basal plate of bone, 

 thus resembling in structure ordinary oral teeth'' [author's ital- 

 ics]. Again Bridge {139: 248) says, "True calcified teeth first 

 make their appearance in fishes, where they assume the form 

 of modifications of exoskeletal structures. The teeth of elasmo- 

 branchs are identical in essential structure, as well as in the 

 manner of their development, with the ordinary dermal spines 

 of the skin, and in the embryo the dermal spines form a con- 

 tinuous series with those which invest the jaws and eventually 

 become teeth. It is only later, when lips become apparent, 

 that the continuity of the teeth and dermal spines is inter- 

 rupted, and the two structures assume their distinctive char- 

 acters." 



Now alike amongst fishes and batrachians such teeth con- 

 stantly appear and disapj)ear over definite areas, and funda- 



16 



