THE TOUNG NATUBALIST. 



23 



and the other of a simple structure. In the Crocodilia however, we find a 

 distinctly higher organization ; the heart is four-chambered, but as the walls 

 of the two aortic arches are in contact where they cross one another, there is 

 a mixture of venous and arterial blood, though this takes place outside 

 the heart, not within it, as in the lower reptiles. In birds the heart is four- 

 chambered as in mammals, there is no mixture of the pure and impure blood 

 and therefore they are warm-blooded, more so in fact than any other animals. 

 The respiration of birds is more active than that of any other vertebrates, that 

 is, they consume more oxygen and form more carbonic acid in proportion to 

 their size. Their lungs are not so minutely sub-divided as those of mammals, 

 but the surface over which the air can act upon the blood is immensely ex- 

 tended, by a provision which is peculiar to this class. The air introduced by 

 the windpipe, passes not only into the lungs, properly so-called, but into a 

 series of large air-cells, which are placed in various parts of the body, and 

 which even send prolongations into the bones, especially in birds of rapid 

 flight, whose whole skeleton is thus traversed by air. iSoine lacertilians (as 

 Chameleons and Geckos) also have have air-tubes passing from the lungs into 

 the bones, and some of the fossil pterosaurians possessed similar structures, 

 surely a suggestive factor in evolution. 



The curious lacertilian reptile Hatteria punctata, sole living representative 

 of the very ancient and once numerous order Rhynckocephalia, possesses in 

 its head a curious sense organ, which seems to point not merely to the 

 existence of a common ancestor of all the vertebrates, but in the still more 

 remote past to a common ancestor of vertebrates and molluscs. This sense- 

 organ arises in the middle dorsal line, as a hollow outgrowth of the third 

 ventricle of the biain; and in both batrachians and reptiles becomes divided 

 into two parts, one of which retains connection with the brain, and the other 

 in most cases becomes separated, and is developed into a highly organized 

 invertebrate eye, lying under the parietal foramen ; a depression of the skin 

 of the head occurs immediately over this spot, but does not lead down into 

 it. This organ is found in different stages of uselessness in different lizards, 

 in some, as the slow-worm quite isolated from the brain, in others, as Hat- 

 teria, Iguana, C/iameleo vulgaris, and Lacerta ocellata, a distinct nerve con- 

 nection with the brain exists. The discovery of a molluscan eye in the head 

 of an archaic lizard is most suggestive of a common origin of the invertebrates 

 and vertebrates. 



Before treating of the higher Sauropsidans, it will be very interesting to 

 briefly examine the order Ophidia. We may be justified in regarding snakes 

 as lizards, which have, through ages of disuse, gradually lost their limbs. 

 ^Excepting in the Boas and Hock-snakes, they are absolutely wanting. No 



