REPTILIA. 267 



the Hooded M. {M. cuaillaim), an American species, rare on this side of the Atlantic, the size of a "Widgeon, 

 with a very hirge fan-like crest, white bordered with black. These have two cceca of moderate length, aud the 

 trachea of the first presents two successive inflations in its course, which are about equal, the same expansions 

 being also visible in the aecond species, wherein the higher is however increased, and the lower one diminished, 

 in addition to the labyrinth at the inferior larynx. To this first group would seem also to belong the M. brazili- 

 eiuh, which is peculiar to South America. 



Finally, the Smew Merganser (M. alhellus) is vciT remarkable for possessing only one minute coecum, resem- 

 bling that of a Heron. It is an extremely beautiful bird, proper to the eastern Continent, and not rare in Britain 

 during the winter, the male of which is bright glistening white, variegated with black markings, and the female 

 like that of the others, except that the adult has a black patch before each eye. It retires far north to breed. 



The great division of web-footed birds might be naturally arranged into five principal 

 groups, continuatory with those indicated at the close of the series of Waders : viz. — 



XI. Natatores (Swimmers) ; incluihng the Flamingo, but corresponding otherwise to 

 the LameUirostres of Cuvier. 



XII. Mergitores (Immergers) ; restricted to the two distinct families of Loons and 

 Grebes. 



XIII. PiscATORES (Fishers); or the ToAj/iaZniaii, which are all exclusively piscivorous. 



XIV. Y AG ATORES (IVanderers); or the Lonr/ipennes; containing the two perfectly distmct 

 groups of the Terns, Gulls, and Skuas, and of the Albatrosses and Petrels. 



XV. Urinatores (Divers) ; more properly so designated ; and composed of the separate 

 families of Auks and Penguins. 



THE THIRD CLASS OF VERTEBRATED ANIMALS. 

 REPTILIA. 



These have the heart so constructed that at each contraction a portion only of the 

 blood received from the various parts of the system is sent into the lungs, the remainder 

 of this fluid returning into the general circulation without having- passed through the 

 lungs, and consequently without having been subjected there to respiration. 



Hence, it results that the action of oxygen upon the blood is less than in the 

 Mammalia, and that, if the amount of respiration of the latter, wherein the whole of 

 the blood is obliged to pass through the lungs before returning into the system, be 

 expressed as unity, the quantum of respii'ation of Reptiles should be expressed as a 

 fraction of unity proportionately small, as the quantity of blood propelled into the 

 lungs, at each contraction of the heart, is diminished. 



As respiration imparts the warmth to the blood, and the susceptibility of the fibre 

 to nervous irritamen. Reptiles have cold blood, and their aggregate muscular energy 

 is less than in the Mammalia, and much less than in Birds. Hence, their movements 

 can scarcely be performed otherwise than by crawling or swimming : and though 

 several of them leap and run with celerity on certain occasions, their habits are gene- 

 rally sluggish, their digestion excessively slow, their sensations obtuse, and, in cold or 

 temperate climates, they pass nearly the whole winter in a state of lethargy. Their 

 proportionally very diminutive brain is less necessary than in the two preceding 

 classes for the exercise of their animal and vital functions ; their sensations seem to be 

 less rcferrlble to a common centre ; they continue to live and to execute voluntary 

 movements, for a very considerable while after having been deprived of the brain, and 

 even when the head is severed. The connexion with the [main trunks of the] 

 nervous system is also much less necessary for the contraction of the muscular fibre ; 



