4i6 ZOOLOGY FOR MEDICAL STUDENTS chap. 



has disappeared entirely. The latter is the case in various other Urodeles 

 and also in the Apoda. 



The arterial system is laid down on the same general plan as in the 

 Fishes, there being a series of six aortic arches which undergo modifica- 

 tions of the same general type as will be described under the heading 

 Reptilia. 



The venous system is also laid down on the same general plan as in 

 the lung-fish. There is a well-developed posterior vena cava which drains 

 both the kidneys and which, over the greater part of its extent has lost 

 its original sheath of liver-substance. 



The duct of Cuvier in the adult amphibian as in the adults of the 

 higher vertebrates is termed the anterior vena cava (in Man the superior 

 vena cava). 



The primitively irregular lymphatic spaces have in the amphibians 

 become more sharply defined. In the terrestrial Frogs and Toads large 

 lymph cisterns are provided immediately beneath the skin. Others of 

 the lymph spaces have become definite lymphatic vessels which serve to 

 return the lymph to the blood-vessels. Where they open into the 

 veins the lymphatic vessels may be provided with a rhythmically con- 

 tractile muscular wall forming lymph-hearts which pump the lymph 

 into the blood-stream. 



The skeleton of the Amphibia is in great part bony. The notochord 

 becomes replaced by a chain of vertebrae — ranging in number from over 

 250 in some of the Apoda down to seven — ^the smallest number known 

 to occur in any vertebrate — in the African toad Hymenochirus. The 

 vertebrae are at first cartilaginous — ^the cartilage not, however, invading 

 the secondary sheath : the centra may be amphicoelous but more usually 

 they fit together by rounded joint surfaces, the concave surface being 

 behind (opisthocoelous — Salamanders) or in front (procoelous — Anura). 

 The ribs in the Amphibia are very short. 



There is a well-developed cartilaginous cranium strengthened by 

 investing and replacing bones to which names are given corresponding 

 with those used in the higher vertebrates. The skeleton of the visceral 

 arches becomes in Tadpoles converted into an elaborate basket-work 

 which acts, like the gill-rakers in Fish, to prevent food-particles from 

 passing into the gill-cleft. In the adult, however, it becomes degenerate 

 and simpHfied, forming a broad, flat, plate-like "hyoid apparatus" 

 which supports the floor of the mouth. 



Correlated with the mode of progression the base of attachment of 

 the limb-skeleton to the trunk has been strengthened. In the case of 

 the pectoral limb this has been brought about by the expansion of the 



