GENERAL CHARACTERS OF REPTILES. 47 



inhabit their element, and which commonly receive the same designation; for 

 the latter, being air-breathing Animals, are obliged to come frequently to the 

 surface to respire ; and their tail is flattened horizontally, to enable them to do 

 this with facility. The lateral surface of the body of Fish is further extended 

 above, by the projection of the dorsal fin, which is supported on prolongations 

 of the spines of the vertebras ; and below, by the abdominal fin, which also is 

 placed on the median line ; these will, of course, increase the power of the 

 lateral stroke of the body, and can only be moved with the spine. The pec- 

 toral and ventral fins on the other hand, the former of which answer to the 

 superior extremities, and the latter to the inferior extremities of Man, serve, 

 by their independent movements, rather as steering than as propelling organs ; 

 and they also assist in raising and depressing the animal through the water. 

 The scales with which the bodies of all Fishes are covered, are frequently of 

 a bony hardness, and sometimes form a firmly-jointed casing, in which the 

 trunk is completely enclosed ; this is especially the case, when the internal 

 skeleton is imperfectly developed ; so that here we have an approach to the 

 character of the Invertebrata. 



38. The swimming-bladder, as it is commonly termed, of the Fish, is not 

 an organ sui generis ; but is ascertained, by comparison with the pulmonary 

 sacs of the lower Reptiles, to be a rudimentary lung. It does not, however, 

 give any assistance in the aeration of the blood, except in a few instances ; but 

 seems to be in general subservient to the elevation and depression of the body 

 in its element. The heart of the Fish is extremely simple in its construction 

 containing two cavities only; and the course of the circulation is equally simple. 

 The blood, which returns from the body in a venous condition, is received into 

 the single auricle or recipient cavity ; and from this it passes into the ventricle 

 or propellent cavity. The latter forces it into a large trunk, which subdivides 

 into branches that are distributed to the branchial arches on each side ; and in 

 these it undergoes aeration. Being collected from the gills by returning ves- 

 sels, the blood, now become arterial in its character, is transmitted to the large 

 systemic trunk, the aorta, by which it is distributed through the system, 

 returning again to the heart, when it has passed through the organs and tissues 

 of the body. Hence it is evident that the whole of the blood passes through 

 the gills before it goes a second time to the system ; by which the imperfection 

 of the aerating process itself is in some degree compensated. There is a 

 special provision, too, for renewing by muscular power the stratum of water 

 in contact with the gills ; continual currents being sent over them from the 

 pharynx, with which their cavity communicates. It is worth noticing, that 

 whilst, in the Osseous Fishes, there is a single large external gill-opening on 

 either side, with a valve-like operculum or gill-cover, there are, in the Cartila- 

 ginous Fishes, several slits on each side of the neck, one corresponding with 

 each branchial arch. Similar apertures in the neck may be seen in the em- 

 bryo of Man and of other Mammalia, as well as of Birds and Reptiles, at the 

 time that the circulation is in the condition of that of the Fish, the heart 

 possessing only two cavities, and the blood being first propelled through a series 

 of branchial arches. 



General characters of Reptiles. 



39. The class of REPTILES is oviparous and cold-blooded, like that of Fishes ; 

 but the animals belonging to it are formed to breathe air, and to inhabit the 

 surface of the earth, the few which are adapted to make the water their 

 dwelling, being. obliged to come to the surface to breathe. Although they 

 breathe air, however, their respiration is not usually so energetic as that of 

 Fishes; and their general activity is much less. The heart possesses three 



