198 



OF THE BLOOD AND CIBCTJLATION. 



suring 1 -200th of a line in diameter ; they are, therefore, only 

 somewhat larger than in man. In the invertebral series of 

 animals they are generally irregular, granular, rounded cor- 

 puscles.*] 



Fig. 214. Blood and lymph glo- Fig. 215. Blood-globules of 



bules of the loach (Cobitis fossilis); the Ammocetes branchialis ; a, 



a, a, b, perfect blood-globules ; d, a, b, perfect blood-globules ; c, 



a blood-globule altered by the ac- lymph-globule. The blood-glo- 

 tiou of water, and shewing its nu- bules are exactly similar in the 

 cleus ; c, lymph granules. lamprey (Petromyzon], and un- 



like those of all other fishes, whe- 

 ther cartilaginous or bony. 



353. The colour of the blood in the vertebrata is bright 

 red ; but in some invertebrata, as the crabs and mollusca, 

 the nutritive fluid is nearly or quite colourless, while in the 

 worms, and some echinoderms, it is variously coloured, yellow, 

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



354. 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 and the secretions, return to the centre of circu- 

 lation, 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 

 through the liver. The blood is kept in an incessant circula- 

 tion for this purpose. 



355. 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, Medusa, which occupy a some- 

 what higher rank, a similar liquid is distributed by prolongations 

 of the principal cavity to the different parts of the body (fig. 

 173). Currents are produced in these, partly by the general 

 * Professor Wagner's Physiology, p. 233, et seq. 



