OPHIDIA. 607 



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 

 snakes in shape though they are very different in internal 

 structure. For the external shape is in great part an adapta- 

 tion 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 limblessness 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. 



GENERAL CHARACTERS. The skin is covered with scales, 

 which, being simply folds of the epidermis, have much co- 

 herence, and periodically shed a continuous slough. 



There are never any hints of anterior appendages, girdles, 

 sternum, or episternutn ; but in pythons, boas, and a few 

 others, there are rudiments of a pelvis, and even small clawed 

 structures which represent hind-legs. 



The mouth is expansible ; maxillcc, palatines, and ptery- 

 goids are movable ; and the rami of the mandible are con- 

 nected only by elastic ligament. The teeth are fused to the 

 jaws ; there are no separate eyelids, the thin transparent 

 epidermis extending over the eyes. There are no external ear 

 openings, and the nostrils lie near the tip of the head. 



The bifid, mobile, retractile tongue is a specialised organ of 

 touch. In the mouth there is often a poison gland, which is a 

 specialised salivary gland. 



There are many peculiarities in the skeleton. The numerous 

 vertebra are all proccelous. 



The brain only gives off ten nerves. The sense of hearing 

 is often slightly developed, and there is no tympanic cavity. 



The heart is three-chambered, the ventricular septum being 

 incomplete, as in all Reptiles except Crocodilians. 



There is a transverse cloacal aperture. In the males a 

 double saccular and spiny copulatory organ is eversible from 

 the cloaca. 



Snakes are widely distributed, but are most abundant in 

 the tropics. 



