346 



LOWER VERTEBRATES. 



tympanic membrane, with its cavity and eustachian tube, is present in all the forms ex- 

 cept the serpents and some footless lizards. The organs of smell are usually well devel- 

 oped, and in some aquatic forms the external nares are provided with valves. The sense 

 of taste is often more or less defective, and its seat is probably not in the tongue alone, 

 but also in sjiecial areas of the mucous membrane of the buccal cavity as well. 



The jaws of all existing reptiles, excepting the turtles, are provided with small, 

 sharp, often recurved, prehensile teeth, which never serve thd office of mastication, but 

 are used to grasp and retain the prey. Teeth may also be on the palatine and ptery- 

 goid bones. (The specialized maxillary teeth of the poisonous serpents will be men- 

 tioned in connection with that group.) Salivary glands are present in both serpents 

 and lizards, and sub-lingual glands are characteristic of the turtles. The oesophagus 

 is long, capable of great dilatation, and its walls are usually longitudinally folded, 

 though in turtles they are provided with papillre ; it leads into a stomach, transversely 

 placed in Chelonia, though longitudinal in other forms. The intestine, in all save 



the herbivorous turtles, is short and 

 but little coiled, and ends in the 

 clo.aca, which opens externally by a 

 round opening, or in serpents and 

 lizards by a transverse slit. 



Respiration is always performed 

 by means of lungs, and these, except 

 in the serpents and the apodal lizards, 

 are of equal size and of the ordinary 

 structure. Air enters through the slit- 

 like glottis and reaches the lungs by 

 passing down an elongated trachea 

 and bronchial tubes. The ribs play 

 a most important part in breath- 

 ing; though tlie rigidity of the cara- 

 pax and plastron of the turtles 

 compels the membei-s of this group 

 to force the air into the lungs by 

 swallowing it. 



The organs of circulation are of 

 particular interest, as they present 

 the several stages of development 

 from that of the batrachians to birds. 

 The right auricle is the larger and 

 receives the systemic, while the left receives the pulmonary veins. The ventricles 

 are only partially separated from each other in the lower forms, and mix, to 

 a greater or less extent, the venous with the arterial blood. Right and left aortic 

 arches are present. The crocodiles, however, have the ventricles separated by a parti- 

 tion, and a structure resembling that of birds is obtained. The right aortic arch and 

 carotids arise from the left ventricle, while from the right ventricle arise a left aortic 

 arch and the pulmonary arteries. 



The renal organs are in the hinder region of the trunk, and the turtles and lizards 

 have the urinary bladder a])pended to the anterior wall of the cloaca, into which also 

 open the genital ducts. The reproductive organs are more or less bird-like, the intromit- 



FlG. 206. — Heart of lizard from beneath ; the aerated blood 

 indicated by dots, the uiiaerated by crosses ; a, abdominal 

 or descending aorta ; f , carotid artery ; I, left auricle ; L, 

 lungs ; jfa, pulmonary artery ; pv, pulmonary vein ; ?•, right 

 auricle ; s, subclavian artery ; /, trachea ; v, ventricle ; /, 

 //, ///, / V, remnants of branchial arches. 



