2 o8 CHARACTERS OF VERTEBRATE ANIMALS 



getting down into the lungs, and there are soft projections at 

 the back of the mouth-cavity which can be brought together so 

 as to keep it out of the gullet. In other words, the hinder part 

 of the mouth-cavity is partitioned into a pharynx (see p. 34), 

 which passes back into the wide g^lllet. This in its turn com- 

 municates with a stomach which is singularly bird-like, since part 

 of it is modified into a muscular gizzard (see p. 146). The 

 resemblance is strengthened by the crocodile's habit of swallow- 

 ing stones, which presumably help, as in a bird, to grind up the 

 food. This is the more necessary since^ the teeth are not suited 

 for chewing, and the prey is either swallowed whole or in 

 large pieces, according to its size. As in Birds and Lizards 

 (see pp. 146 and 200) the large intestine terminates in a cloaca, 

 which opens to the exterior by a longitudinally oval aperture 

 situated on the under side of the root of the tail. 



The blood, heart, and blood-vessels (fig. 132) are much like 

 the corresponding parts in the lizard (see p. 200), the most 

 important difference being that there are two ventricles, a feature 

 possessed by no other known reptile. This limits the impure 

 blood to the right side of the heart and the pure blood to the 

 left, as in mammals and birds, but the advantage gained is less 

 than at first sight appears, for the two kinds of blood mix outside 

 the heart. This is so because we still have, as in the Lizard 

 (see p. 201), two aortic arches, a right which takes origin in 

 the left ventricle and carries off pure blood from it, and a left 

 which conducts away impure blood from the right ventricle. It 

 is clear, therefore, that the dorsal aorta, which is formed by the 

 union of the two arches and which supplies the body behind the 

 fore-limbs, must contain mixed blood, and the two arches also 

 communicate by a small hole just where they leave the heart. 

 A reduction of the aortic arches has taken place as compared 

 with the Sand Lizard (see p. 191), for each of them is here single 

 instead of being formed by the partial union of two. If the left 

 arch were entirely done away with we should have a condition 

 closely resembling that found in the bird (see fig. 102), and all 

 parts of the body would be supplied by pure blood. There can 

 be little doubt that some of the remote reptile-like ancestors of 

 birds possessed circulatory organs much like those of the crocodile, 

 and that in them part of the body was consequently supplied with 

 mixed blood. 



