14 HUMBLE CREATURES. 



extent of the nature of that organ in the higher ani- 

 mals, inasmuch as its wall, or hmitary membrane, is 

 elastic, and by its contractions propels the blood on- 

 wards. Whilst thus passing along the main artery- 

 it is brought into communication with the atmo- 

 spheric air, to be renewed by the absorption of oxygen, 

 in a manner which we shall presently describe ; and 

 then returns, as already explained, along the second 

 (abdominal) canal, which is united to the former by a 

 number of smaller veins into which the blood also 

 penetrates, so that it may yield its regenerating con- 

 stituents to every portion of the body. 



The aeration of the blood takes place as foUows : — 

 On closely examining. the dorsal surface of the worm 

 with a pocket-lens, a minute circular aperture is 

 distinguishable between each ring and the adjoining 

 one (PI. II. fig. 3) . By these the air is admitted into 

 the body, and being conducted by special canals to 

 the dorsal circulating organ, there gives off its oxygen 

 to the contained fluid. 



So far, then, we find that the nutritive organs of 

 the worm resemble in principle those of the higher 

 animals ; for although it possesses no central organs, 

 such as the stomach and heart, yet, in the alimentary 

 system, the intestiae or canal, with its glands, per- 

 form the functions of the former, whilst the place of 

 the heart is supplied in the circulating system by the 

 contractile dorsal vessel. As, however, these organs 

 are more highly developed in the Fly, the physiology 

 of which we shall consider in a subsequent letter, we 



