OF THE BLOOD AND CIRCULATION. 87 



and some echinoderms, it is variously colored yellow, 

 orange, red, violet, lilac, and even green. 



230. The presence of this fluid in every part of the 

 body is one of the essential conditions of animal life. A 

 perpetual current flows from the digestive organs towards 

 the remotest parts of the surface ; and such portions as are 

 not required for nutriment return, mingled with those which 

 have become useless and need to be renewed or expelled. 

 The blood is kept in an incessant CIRCULATION for this 

 purpose. 



231. In the lowest animals, such as the polypi, the nutri- 

 tive fluid is merely the products of digestion mingled with 

 water in the common cavity of the viscera, with which it 

 comes in immediate contact, as well as with the whole 

 interior of the body. In the jelly-fishes, which occupy a 

 somewhat higher rank, a similar liquid is distributed by pro- 

 longations of the principal cavity to different parts of the 

 body (Fig. 31). Currents are produced in these, partly by 

 the general movements of the animal, and partly by means 

 of the incessant vibrations of microscopic hairs which over- 

 spread the interior, and are hence called vibratile cilia. In 

 most of the mollusks and insects, the blood is also in imme- 

 diate contact with the viscera ; or the vessels, if any, are 

 not continuous, but terminate in various cavities. 



232. In animals of still higher organization, as the verte- 

 brates, the mollusks, and a part of the articulata and 

 echinoderms, we find the vital fluid enclosed in an appro- 

 priate set of vessels, by which it is successively con- 

 veyed throughout the system to supply its wants, and to the 

 respiratory organs, where it absorbs oxygen, or in other 

 words, becomes oxygenated. 



233. The vessels in which the blood circulates are of 

 two kinds : 1. The arteries, of a firm, elastic structure, 

 which may be distended or contracted, according to the 



