350 LOWER VERTEBRATES. 



infraorbitals ; behind the postorbitals are the subdivided temporals. The nostril opens 

 through a plate, the nasal, which may be and often is divided ; betveeen it and the 

 preorbitals is the loreal, a shield quite characteristic of harmless snakes. Posterior to 

 the mental, and lying between the infralabials, are the submentals and gulars. 



Ordinarily the serpents have regular periods of sloughing the skin, which differ 

 with different forms. Some little time before the change takes place, the waste skin 

 so cleaves from the eyeballs as to render the serpent partially blind, making it 

 unusually irritable. The skin then splits away from the margin of the mouth, and 

 either by gliding through some narrow opening, or by passing through a ring of its 

 own body, the serpent emerges, leaving its old coat turned inside out, but in perfect 

 condition. If, as often happens in confinement, the animal has become ill, the slough- 

 ing is only partial, and, losing all appetite, it eventually dies. 



As a rule, with the exception of a hiss produced by forcibly expelling the breath, 

 the serpents are dumb ; though Krefft maintains that some make a drumming noise, 

 and the Indian Ptyas niucosus is said to give rise to a note like that of a tuning-fork. 

 Garman has observed that some of the boas whine. 



The progression of ophidians is reducible to three modes. The animal may glide, 

 perhaps in a perfectly straight line, by use of its ventral scutes, each, on finding some 

 resistance, forcibly pushing the animal forward. It may walk, by allowing each 

 scute to act as a pair of feet, the lateral portions being alternately carried forward and 

 pushed back ; an undulatory movement like that of myriapods would result from this 

 mode. The third manner is by pushing, as the underground snakes do almost exclu- 

 sively. Ordinarily ophidians combine the three methods. The sea-snakes progres.s by 

 an undulatory movement, and by the sculling action of the paddle-like tail. No ser- 

 pent can move forward on a perfectly smooth surface. 



It is impossible for any opliidian to jump, and it is with extreme difficulty that 

 more than the anterior half of the body can be raised, unassisted, from the ground, 

 though with some support, as the side of a box, the animal can stand almost verti- 

 cally. In habits they are chiefly diurnal, though there are several tropical forms which 

 hunt during the night. Most, if not all, have some j)eriod of the year during which 

 they become inactive. In the torrid zone this may be called estivation, while in the 

 colder climates it is true hibernation, the animal being apparently in deep sleep; 

 though if kept warm and constantly irritated it will pass the winter as it does the 

 summer, and without any ill effects. 



The color.ation of ophidians is varied, and offers some of the most striking illus- 

 trations of adaptation and protective resemblance. Some of the rattle-snakes, which 

 live in more wooded sections, are, on exposure on the hot neighboring plains, changed 

 to a much lighter shade, and the members of all families have the general coloration 

 harmonizing well with their surroundings. The tree-snakes ai-e always of colors 

 resembling either the leaves or twigs among which they live. The common grass- 

 snake, CyclojyJds vernaUs, offers an excellent illustration of adaptive coloi-. In 

 the tropics many jierfectly harmless forms have adopted the coloring of the 

 most venomous, while the Tropidonutus macrophthalmus offers perhaps the most 

 wonderful illustration of protective resemblance known to the order. This inno- 

 cent animal has not only the general form, habits, and markings of the deadly 

 cobra, but even that animal's distensible neck and elongated ribs, — a perfect 

 counterfeit. 



Whether snakes swallow their young has been much discussed. The case stands 



