The true tree-frogs or tree-toads (Hylidæ) constitute a family especially well represented in tropical America. They have little disk- or pad-like swellings on the tips of their toes to enable them to hold firmly to the branches of the trees in which they live. Some, like the swamp tree-frog and the cricket-frog, are not arboreal in habit, remaining almost always on the ground. The common tree-frog of the eastern States (Hyla versicolor) is green, gray, or brown above with irregular dark blotches, and yellow below. It croaks or trills, especially at evening and in damp weather. Pickering's tree-frog (Hyla pickeringii) makes the "first note of spring" in the eastern States. This tree-frog is the one most frequently heard in the autumn too, but "its voice is less vivacious than in the spring and its lonely pipe in dry woodlands is always associated with goldenrods and asters and falling leaves." The tree-frogs of North America lay their eggs in the water on some fixed object as an aquatic plant, in smaller packets than those of the true frogs, and not in strings as do the toads.

The toads (Bufonidæ) differ from the true frogs in having no teeth and in not having, as the frogs do, a cartilaginous process uniting the shoulder-bones of the two sides of the body. The absence of this uniting process makes the thoracic region capable of great expansion. There are only a few species of toads in North America, but one of these species, the common American toad (Bufo lentiginosus), is very abundant and widespread. It appears also in two or three varieties, the common toad of the southern States differing in several particulars from that of the northern. The toad is a familiar inhabitant of gardens, and does much good by feeding on noxious insects. It is most active at twilight. Its eggs are laid in a single line in the centre of a long slender gelatinous string or rope, which is nearly always tangled and wound round some water-plant or stick near the shore on the bottom of a pond. The eggs are jet black and when freshly laid are nearly spherical. At the time of egg-laying the toads croak or call, making a sort of whistling sound and at the same time pronouncing deep in the throat "bu-rr-r-r-r." The toad does not open its mouth when croaking, but expands a large sac or resonator in its throat. The toad-tadpoles are blacker than those of frogs or salamanders, and undergo their metamorphosis while of smaller size than those of frogs. When they leave the water they travel for long distances, hopping along so vigorously that in a few days they may be as far as a mile from the pond where they were hatched. They conceal themselves by day, but will appear after a warm shower; this sudden appearance of many small toads sometimes gives rise to the false notion that they have fallen with the rain.

Cœcilians (Gymnophiona).—The third order of batrachians, the cœcilians, includes about twenty species of slender worm- or snake-like limbless forms which are confined to the tropics. Some of them are wholly blind and the others have only rudimentary eyes. In them the skin is folded at regular intervals so that the body appears to be rigid or segmented, and in some species there are small concealed horny scales in the skin.

[CHAPTER XXVI]

BRANCH CHORDATA (Continued). CLASS REPTILIA: THE SNAKES, LIZARDS, TURTLES, CROCODILES, ETC.

THE GARTER SNAKE (Thamnophis sp.)

Technical Note.—Garter snakes may be found almost anywhere during the spring and summer months. If possible each student should have a specimen, but in case it is difficult to get enough snakes two students can use a single specimen. If garter snakes are rare, take any other snake. Snakes will live a long time without feeding and specimens should be kept alive until ready to use. Kill with chloroform as directed for the toad (p. 5). After completing the study of the external characters place each specimen in a dissecting-pan and with a pair of scissors cut through the scales on the ventral side, passing backwards from the eighteenth to the fortieth. Pin back the edges of the cut and thus expose the heart. Through its lower end, the ventricle, insert a large canula; inject with a fairly large syringe the glue mass which is described on p. 452. This injection will fill the entire arterial system. To inject the venous system make another cut through the ventral scales, cutting forward from the anal scale through about forty of them. Note the injected mass in some of the vessels already filled. Take one of the large vessels still containing blood and pass two ligatures beneath it. Get ready a small canula and cut a slit in the vessel, elevating the head so that the blood will run out as much as possible. Now wash the blood off, insert the canula in the slit and tie one ligature about the vessel containing the canula; have the other ready to tie after the vein has been injected. Use a new color for the venous system. Leave specimen in cold water for a time until the injection is hard. Then continue the cut from the anal plate forward to the lower jaw and pin out the edges of the cut on both sides in the dissecting-pan.

Structure (fig. [122]).—Note that the snake is covered with horny scales somewhat as the fish is. How do these scales differ from those of the fish? In snakes the scales are not bony, but are true skin structures. Note the modification of the scales on the head, back, and ventral surface. Those on the dorsal surface often have minute ridges, the keels. How do the ventral scales differ from the dorsal ones and others? By a system of muscles these ventral scales are rhythmically moved and as their posterior edges are pushed back against some resisting object the body glides forward. On the head note the pair of eyes. Are there eyelids? In front of each eye note an opening. What are these openings? Thrust a bristle into the opening and see where it enters the mouth-cavity through the internal nares. Does the snake have external ears? Observe the very long jaws and note that they are loosely hinged. Examine the inside of the mouth. Are there teeth? If so where are they situated, and how arranged? Note that all of the teeth point backwards. Food is not chewed. When some object of prey, a frog, or mouse, for example, is seized, the teeth hold it fast to the roof of the mouth and by a backward and forward movement of the lower jaws it is gradually drawn into the large œsophagus. What is the character and situation of the tongue? Just behind the tongue note the narrow slit, glottis, opening into the windpipe, or trachea. Back of the trachea opens the œsophagus.