2i6 Education through Nature 



the food for absorption through the wall of the diges- 

 tive tube. 



Absorption and Circulation. The food, when ab- 

 sorbed, passes either into the lymph of the body-cavity, 

 diffusing thence throughout the body, or else enters 

 the blood circulation and is distributed by it. The 

 simplest form of such circulation exists in the jelly- 

 fish, where the primitive stomach gives off radiating- 

 tubes. In the sponges the food is carried by currents 

 of water to the individual cells, as in protozoa. In 

 the hydra, sea-anemone, and coral, the food is practi- 

 cally in contact with the body-cells, as the stomach is 

 not distinct from the body-wall. In the starfish and 

 sea-urchin a system of water-tubes, the ambulacral 

 system, seems to serve partly for circulation. But even 

 here a primitive, distinct circulatory system exists. 



The Blood is pre-eminently the circulatory medium. 

 It is essentially a liquid tissue with a fluid intercellu- 

 lar substance. In forms as low as echinoderms and 

 mollusks, the blood differs very little from water, 

 having in solution dissolved proteids, salts, and a few 

 corpuscles. In the lower forms, too, it is colorless, 

 and of the same temperature as the medium in which 

 the animal lives. All invertebrate animals, including 

 fishes, amphibians, and reptiles, are cold-blooded. 



The Circulatory System of animals consists of tubes. 

 The most primitive, perhaps, is that of the echinoderms, 

 where it is a pulsating-tube lying alongside the ali- 

 mentary canal, and forming a ring around the latter 

 at the oral and aboral pole. Below the echinoderms 

 a circulatory system cannot be said to exist, though 

 even in protozoa a contractile vacuole is thought by 

 some to serve as a circulatory organ. In worms the 

 circulatory system consists of a dorsal and a ventral 

 tube, united in the anterior segments by anastomosing, 

 pulsating-tubes, or primitive hearts. In mollusks the 

 primitive tube is enlarged into a heart which, by its 



