Basic Structure of Vertebrate 



Poison canal 



PuIp ^nI§ 



Dentine 



Fig. 55. Cross sections of poison-fangs. (Left) The poison-canal is an open 

 groove. (Right) The canal is closed. The arrows suggest the process of folding 

 whereby a groove may be arched over to form an enclosed duct. 



it is not. It develops as a superficial groove whose opposite edges later 

 overarch and enclose it. Accordingly it is lined by enamel. In a cross 

 section of the fang, the pulp-cavity is seen to be concentric with the 

 poison-canal (Fig. 55). Well-developed replacing fangs are embedded 

 in the jaw close behind the functional fangs. Of mammals, most ant- 

 eaters and some whales are toothless, while sperm whales and dolphins 

 have numerous teeth (over 200 in some dolphins) which are homodont. 

 Horny Teeth. Certain vertebrates which lack calcareous teeth 

 produce functional substitutes for them by development of epidermal 

 horny structures. The round-mouthed eels (cyclostomes) have small, 

 conical, horny teeth on the surface of the suctorial disk which sur- 

 rounds the mouth. These purely epidermal teeth are, from time to 

 time, shed and replaced by others which are already well formed be- 

 neath the old teeth. The jaws of the tadpoles of many of the frogs 

 and toads bear numerous minute, spiny, horny teeth, but these larval 

 teeth are not retained by the adult. Larvae of salamanders do not 

 develop horny teeth. In the toothless turtles and tortoises and in birds, 

 the jaws are ensheathed by a thick layer of horn derived from the 

 stratum corneum of the epidermis. In some turtles the horny sheath 

 develops spines which simulate teeth. In others, and in birds the horn 

 may be so shaped as to provide the jaws with cutting edges, or formed 

 into the hooked beak of the hawksbill turtle, parrot, and other birds. 



II. Respiratory Organs 



GILLS AND LUNGS 



Gills are structures which serve to bring blood into close relation 

 to the external water in which the animal lives. They consist of very 

 thin membranous plates (branchial lamellas: Fig. 56), or some- 



