304 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1951 



thought to be traces of a pelvis and hmd limbs. Attached to each fe- 

 mur there is on the surface of the body a conical, clawlike spur. These 

 spurs are larger in the male python and no longer function as limbs 

 but serve as excitor organs when scraped against the female during the 

 act of mating. We see a somewhat similar parallel in the vestigial hind 

 limbs of certain whales, undoubted mammals which live entirely in 

 water and whose quadruped ancestors are as much a mystery as those 

 of snakes. 



Serpents superficially resemble w^orms, for which they may be mis- 

 taken, but a true worm has no bony jaw^s, tongue, or well-developed 

 eyes and is classed with the animals that have no backbone. The 

 wormlike snakes vary greatly in length, from a 4-inch species like 

 Glauconia dissimilis to a 30-foot giant like the Malay python {Python 

 reticulatus) . Their body may be long and slender, or short and fat, 

 according to the species. 



The backbone of a snake is composed of numerous vertebrae which 

 are of two kinds — those of the body, which carry each a pair of ribs, 

 and those of the short tail, which have instead long transverse proc- 

 esses. The vertebrae are connected by "ball-and-socket" joints which 

 allow for great flexibility. At the same time, because of certain pro- 

 jections on each vertebra that lock with the adjacent ones in a kind 

 of dovetailed joint, a snake's body possesses a rigidity that is re- 

 markable for such a delicate mechanism. The slightest blow will 

 fracture or dislocate a snake's backbone, yet the animal can twist and 

 rear into positions impossible in other vertebrates. Many a pet snake 

 owes its escape from captivity to the strength and suppleness of its 

 backbone. 



The dovetailed jointing to some extent limits the body movements 

 in the vertical plane but does not interfere with the extensive lateral 

 play typical of a snake in movement. Illustrations by early natural- 

 ists and modern cartoonists often depict the progress of snakes and 

 "sea serpents" in vertical undulations. Such movement is entirely 

 foreign to snakes and to reptiles in general ; it is, in fact, quite impos- 

 sible, because of the way the backbone is' constructed. 



The numerous curved ribs which are joined in pairs to the trunk 

 vertebrae are capable of certain movements. Lateral movement is 

 seen under certain circumstances, as when the snake is flattening the 

 body in the sunning attitude or when allowing passage of a meal, which 

 can be detected as a bulge along the body. No breastbone exists in 

 snakes to hamper their movements. The movement of the ribs back- 

 ward and forward was at one time thought to play an important part 

 in locomotion, but this has recently been questioned. Locomotion is 

 now thought to operate under muscle action which is visible to the 

 eye in the movement of the body surface, especially on the lower side. 



