12 INTKODUCTION. 



that from the skin of one of the lower animals, say a worm, a 

 ramified and villous prolongation arose by local excrescence 

 which, as contrasted with the general respiration hitherto carried 

 on by the skin, was a specially qualified organ of respiration — a 

 true gill. This gill must be in such connection with the vessels 

 of the body, or with the cavities which contain the circulating 

 blood, that the absorption of oxygen by the blood may be more 

 easily effected here than in other parts of the skin ; it cannot 

 otherwise be designated as a true gill (or branchia). But, in 

 order to exercise the same respiratoiy activity as the skin, 

 these gills must possess a certain rigidity, so that their whole 

 surface may be in contact with the water that surrounds them, 

 for this would be impossible to soft and pendulous gills ; more- 

 over, certain auxiliary organs must be connected with them, to 

 sacure the requisite change of water by producing a constant 

 current. This renewal of the supply of water for respiration is 

 frequently effected by the active movement of the branchiae 

 themselves, or by the constant motion of the animal ; but in 

 every case where such organs have ceased to be superficial on 

 the skin by its induplication, or have become internal, special 

 auxiliary organs are found, as in Crustacea for instance. Fishes, 

 Mollusca, &c., whose sole duty is to keep up a constant stream 

 bathing the gUls. Thus the physiological efficiency of the 

 principal organ depends not alone ' on the capability of the 

 epidermal cells to absorb oxygen from the surrounding water 

 (by osmosis), but also on those auxiliary organs which con- 

 stantly supply the branchiae with fresh water for respiration, 

 and, by keeping up their rigidity, prevent any diminution of 

 the respiring surface by collapse. 



If furthermore we suppose that the branchiae, which 

 originated, perhaps, by adaptation to an increased demand on 

 the respiratory organs, were permanent during the transmu- 

 tation of the first species into several new ones, while at the 

 same time they preserved their character of independent appen- 

 dages of the outer skin, they might very likely come to act as 

 organs not merely of respiration, but also of locomotion. For 

 by their position, rigidity, and power of independent movement 

 — all indispensable to their efficiency as branchiae — they are, 



