112 OF THE BLOOD 



merous in birds than in mammals, and more abundant in the 

 latter than in fishes. In man and other mammals they are 

 very small and nearly circular, (Fig. 78 ;) they are some- 

 what larger, and of an oval form, in birds and fishes, (Figs. 

 79, 81 ;) and still larger in reptiles, (Fig. 80.) 



229. The color of the blood in the vertebrates is bright 

 red ; but in some invertebrates, as the crabs and mollusks, 

 the nutritive fluid is nearly or quite colorless ; while in the 

 worms 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 per- 

 petual current flows from the digestive organs towards the 

 remotest parts of the surface ; and such portions as are not 

 required for nutriment and secretions return to the centre of 

 circulation, mingled with fluids which need to be assimilated 

 to the blood, and with particles of the body which are to be 

 expelled, or, before returning to the heart, are distributed in 

 the liver. 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 simply the product of digestion (chyme) 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 fringes, called 

 vibratile cilia, which overspread the interior. In most of 

 the mollusks and articulates, the blood (chyle) is also in 

 immediate contact with the viscera, water being mixed with 

 it in mollusks ; the vessels, if there are any, not forming a 



