THE METABOLIC MAClllNEin OF ANIMALS :i()l 



coelenterates, tliere is no need for a specialized transportation system 

 for circulating digested foodstuffs other than that furnished by the 

 ramifications of the gastrovascular system. Since the organism is 

 composed of only two layers of cells, each is capable of securing the 

 necessary materials forits metabolism either from outside of the body 

 or from a neighboring cell lining the cavity. 



However, in the flatworm Planaria, a more highly developed gastro- 

 vascidar system appears. In animals of this type the gut ramifies 

 between nonspecialized cells composing the parenchymatous tissue 

 in which the various organ systems of the body are embedded. As 

 the food is digested it is circulated directly throughout the gastro- 

 vascular cavity by means of contractions of the body, the food readily 

 passing from the branched gut to surrounding tissues of the body 

 by diffusion. The waste products reach the gastrovascular cavity 

 and by similar muscular contractions are passed to the outside, or 

 they may be excreted through the flame cell excretory system (see 

 page 320). 



Still further advances in the development of specialized circulatory 

 devices occur in types having a body cavity, or coelom. In a number 

 of invertebrates the coelom is filled with a lymphlike fluid which may 

 contain corpuscles resembling white corpuscles, or leucocytes. This 

 may be looked upon as an advance over the gastrovascular type of 

 distributing system. And, as we ascend the animal scale and the cir- 

 culatory devices tend to become more complex, we note the tendency 

 to develop definite tubes in which the circulatory fluids may l)e con- 

 fined. These types are usually muscular and contractions of the 

 body facilitate the movement of the fluid. In segmented forms like 

 the earthworm the coelomic fluid supplements the work of the regular 

 circulatory system. 



Open Circulatory Systems 



This type of transportation reaches its peak of development in the 

 Crustacea. The lobster or crayfish, both aquatic forms, furnish 

 familiar examples, in which the blood serves the three purposes 

 of respiration, transportation of foodstuffs, and the elimination of 

 wastes. As in all well-developed circulatory systems, there is a 

 muscular pumping mechanism, or heart, which, by its contractions 

 forces the blood along a group of so-called arteries. The.se in turn 

 usually break down into smaller vessels terminating in the tissues. 

 The blood bathes the tissues and then finds its way back, usually along 



