CHAPTER XXVI 



BRANCH CHORDATA (Continued}. CLASS REP- 

 TILIA: THE SNAKES, LIZARDS, TURTLES, 

 CROCODILES, ETC. 



THE GARTER SNAKE (Thamnophis sp.) 



TECHNICAL NOTE. Garter snakes may be found almost any- 

 where 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, cut- 

 ting 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 possi- 

 ble. 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 



303 



