The olfactory organs are at first found in the same form in all the Gnathostomes, as a pair of depressions in the fore part of the skin of the head, above the mouth; hence, they are also called the Amphirhina ("double-nosed"). The Cyclostoma are "one-nosed" (Monorhina); their nose is a single passage in the middle of the frontal surface. But as the olfactory nerve is double in both cases, it is possible that the peculiar form of the nose in the actual Cyclostomes is a secondary acquisition (by adaptation to suctorial habits). A third essential character of the Gnathostomes, that distinguishes them very conspicuously from the lower vertebrates we have dealt with, is the formation of a blind sac by invagination from the fore part of the gut, which becomes in the fishes the air-filled floating-bladder. This organ acts as a hydrostatic apparatus, increasing or reducing the specific gravity of the fish by compressing or altering the quantity of air in it. The fish can rise or sink in the water by means of it. This is the organ from which the lungs of the higher vertebrates are developed. (FIGURE 2.250. Fully developed man-eating shark (Carcharias melanopterus), left view. r1 first, r2 second dorsal fin, s tail-fin, a anus-fin, v breast-fins, h belly-fins.) Finally, the fourth character of the Gnathostomes in their simple embryonic form is the two pairs of extremities or limbs--a pair of fore legs (breast-fins in the fish, Figure 2.250 v) and a pair of hind legs (ventral fins in the fish, Figure 2.250 h). The comparative anatomy of these fins is very interesting, because they contain the rudiments of all the skeletal parts that form the framework of the fore and hind legs in all the higher vertebrates right up to man. There is no trace of these pairs of limbs in the Acrania and Cyclostomes.