INTRODUCTION. XXVII 



account of previous association. The name proposed for the species grouped 

 under Crotalophorus of Gray, Holbrook, and others, but not of Linne or 

 Gronow, is Sistrurus (asioTpov, a rattle). 



Batrachia. 



The Batrachians are vertebrates, such as the Csecilians, Salamanders, 

 Toads, and Frogs, the most of which are affected by a metamorphosis, 

 during which a branchial and fish-like respiration is changed to one in 

 which the main dependence is placed on lungs. Whether breathing by 

 means of gills in the earlier stages, or by lungs in the later, these animals 

 are possessed of a supplementary cutaneous respiration. It is by the aid 

 of the latter that they ai*e enabled to pass periods of several months to a 

 couple of years buried in the earth or mud during hibernation. Moisture 

 is at all times absolutely necessary; deprived of it they soon die. The 

 skin is naked, either smooth or rough. In cases the skin over the whole 

 body is glandular; in others the cutaneous glands are aggregated in par- 

 ticular portions of the body, as the parotoids of Toads and some Sala- 

 manders. A few of the snake-like batrachians have rudimentary scales 

 hidden in the skin. The slough is stripped off more or less entire, and 

 generally eaten. The gills are retained by certain forms throughout their 

 whole existence; in these, however, the lungs are partially developed, and 

 the respiration is at the same time branchial and pulmonary. The heart 

 has but a single ventricle, and the atria are incompletely separated. 

 There are two occipital condyles. For the most part, the very young 

 feed on vegetation; in such the intestine is elongate. Those beginning 

 their independent existence after the tadpole stage has been passed, and 

 the later stages of all members of the class, are carnivorous. The prey 

 is swallowed entire. There are a few instances in which the embryo is 

 developed and the young hatched in the oviduct. Commonly the eggs are 

 fertilized externally, as or after they are laid. Eggs of batrachia have been 

 favorites of embryologists in their researches; it was in them Prevost and 

 Dumas first noticed the cleavage masses, and in them Newport saw the 

 spermatozoon creep through the outer envelopes to the yolk. We know no 

 species of which the bite is poisonous. The nearest approach to venom is 

 in the acrid, milky secretion of the parotoids, which produces considerable 

 irritation when brought in contact with the membranes of eyes or mouth. 



