392 MANUAL OF ELEMENTARY ZOOLOGY 



is less clear than that of the vas deferens. It is never a urinary duct, 

 and often (as in the frog, p.. 503) develops independently, but since 

 in some cases (as in the dogfish) it arises in the embryo by splitting off 

 from the original kidney duct, it is usually regarded as a separated part 

 of the latter. 



Lizards (Lacertilia) and Snakes (Ophidic?) are closely 

 related : indeed, it is difficult to distinguish 

 Reptiles!' between them, for there are lizards, like the 

 Blindworm, which have no outward trace of 

 limbs, and snakes, like the Python, which have vestiges of 

 hind legs. The true snakes, however, may be distinguished 

 by the absence of any trace of a shoulder girdle or urinary 

 bladder. The shoulder girdle of lizards (Fig. 417) shows a 

 bony scapula, coracoid, and precoracoid, with a cartilaginous 

 epicoracoid, meeting a broad cartilaginous sternum, to 

 which some of the ribs are usually prolonged, as in birds 

 and mammals, but not in amphibians. There are also 

 clavicles and a long, median inter clavicle, which lies upon 

 the sternum. Both snakes and lizards have two penes — 

 hollow sacks opening into the hinder wall of the cloaca, 

 through which fhey can be protruded by being turned 

 inside out. 



A snake crawls or climbs by means of its ribs. These 

 are attached to broad scales on the belly (seen in Fig. 281 B), 

 which they raise in turn, and thus cause to grip the surface 

 over which the animal is ' moving. The backbone is 

 enabled to stand the strain of this process by the presence 

 on each vertebra of two knobs, the zygosphenes, which fit 

 into pits, the zygantra, on the hinder side of the preceding 

 vertebra. 



Turtles and tortoises (Chelonia) are characterised by the 

 bony shields {carapace and plastron) which enclose their 

 broad bodies above and below, by the peculiarities, 

 already mentioned, of their strong, toothless skulls, by the 

 absence of a sternum, and by the presence of a single 

 penis. In the aquatic turtles the hands and feet are 

 converted into paddles, in tortoises they are adapted for 

 walking. In some turtles the skeleton of the limbs shows 

 all the features of the typical pentadactyle limb (Fig. 19). 

 The only other case in which this is found is that of the hind 

 limb of certain newts. The sharp, bony jaws are covered 



