STRUCTURE OF SERPENTS. 479 



The Chamteleons exhibit numerous anatomical peculiarities. As in 

 the Amphisbsenas, there is no epipterygpid nor interorbital sejjturn. 

 The pterygoid does not directly articulate with the quadrate which is 

 ankylosed to the adjacent bones of the skull. 



Order Ophidia. Serpents or Snakes. 



The elongated limbless form of snakes seems at first sight 

 almost enough to define this order from other Reptiles, but 

 it must be carefully noticed that there are limbless lizards, 

 limbless amphibians, and limbless fishes, which resemble 

 serpents in shape though very different in internal structure. 

 For the external shape seems in great part an adaptation to 

 the mode of life, to the habit of creeping through crevices 

 or among obstacles. Even in the thin-bodied weasels is 

 there not some suggestion of the serpent ? Yet the limbless- 

 ness of serpents is not a merely superficial abortion, for there 

 is no pectoral girdle nor sternum, and never more than a 

 hint of a pelvis. 



The skin is covered with scales, which being simply folds 

 of the epidermis have much coherence, and are periodically 

 shed in a continuous slough. The scales on the head form 

 large plates, and those on the ventral surface are transverse 

 shields. There are no separate eyelids, but the thin trans- 

 parent epidermis extends over the staring eyes. The nostrils 

 lie near the tip of the head ; there are no external ear-open- 

 ings. In many cases there are odoriferous glands near the 

 cloacal aperture. 



The muscular system is very highly developed, and the 

 limbless serpent, Owen says, " can outclimb the monkey, 

 outswim the fish, outleap the zebra, outwrestle the athlete, 

 and crush the tiger." 



There are many remarkable peculiarities in the skeleton. 



The vertebrae are very numerous, some pythons having 

 four hundred ; they are proccelous, and are distinguishable 

 only into a pre-caudal and caudal series. 



All the pre-caudal vertebrae except the first — the atlas — 

 have associated ribs, which are movably articulated and 

 used as limbs in locomotion. In the caudal region, the trans- 

 verse processes, which are elsewhere very small, take the place 

 of ribs. 



