OPHIDIA. 429 



than merely holding the prey fast, and the Snakes are provided 

 with no genuine masticatory apparatus. The heart nas three 

 chambers, two auricles and a ventricle, the latter imperfectly 

 divided into two cavities by an incomplete septum. The lungs 

 and other paired organs are mostly not bilaterally symmetrical, 

 one of each pair being either rudimentary or absent. There is 

 no urinary bladder, and the cloacal aperture is transverse. 



Of these characters of the snakes, the most obvious and 

 striking are to be found in the nature of the organs of locomo- 

 tion. The front limbs, with the scapular arch and sternum, 

 are invariably altogether absent ; and the hind-limbs, if not 

 wholly wanting, are never represented by inore than an im- 

 perfectly-developed series of bones concealed within the 

 muscles on each side of the anal aperture, and never exhibiting 

 any outward evidence of their existence beyond the occasional 

 presence of short horny claws or spurs (" calcaria "). In the 

 entire absence, then, or rudimentary condition of the limbs, 

 the Snakes progress by means of the ribs. These bones are ■ 

 always extremely numerous (sometimes amounting to more 

 than three hundred pairs), and in the absence of a sternum, 

 they are, of course, extremely movable. Their free extremities, 

 in fact, are simply terminated by tapering cartilages, which are 

 attached by muscular connections to the abdominal scales or 

 " scuta " of the integument. By means of this arrangement 

 the Serpents are enabled to progress rapidly, walking, so to 

 speak, upon the ends of their ribs; their movements being 

 much facilitated by the extreme mobility of the whole vertebral 

 column, conditioned by the cup-and-ball articulation of the 

 bodies of the vertebra with one another. 



The body in the Snakes is covered with numerous scales, 

 developed apparently in the lower layer of the epidermis, and 

 covered by a thin, translucent, superficial pellicle, which is peri- 

 odically cast bff and renewed. On the head and along the 

 abdomen these scales are larger than over the rest of the body, 

 and they constitute what are known as the " scuta " or shields. 



The only other points in the anatomy of the OpJiidia which 

 demand special attention are the structure of the tongue, teeth, 

 and eye. 



The tongue in the Snakes is probably an organ more of 

 touch than of taste. It consists of two muscular cylinders, 

 united towards their bases, but free towards their extremities. 

 The bifid organ, thus constituted, can be protruded and 

 retracted at will, being in constant vibration when protruded, 

 and being in great part concealed by a sheath when retracted. 



As regards the eye of Sernents /'fig. 166, A), the chief 



