126 GENERAL ZOOLOGY 



esophagus there is a muscular sucking stomach which pumps 

 the nutritive fluids from the prey. The food is digested 

 to some extent in the large true stomach, and then passes 

 on into the intestine to be acted upon further and absorbed. 

 There are five large sacs, or caeca, connected with the stom- 

 ach. These serve to increase its digestive and absorp- 

 tive surface. Digestion is also assisted in the intestine by 

 a number of pouches which branch out to such an extent 

 that they fill most of the space within the abdomen. 



After being absorbed the food is taken into the circulatory 

 system ; passes through sinuses and veins to the pericardial 

 chamber. A spider's heart is wholly within the abdomen 

 and is fastened to the walls of the chamber in which it 

 lies by little elastic threads. These pull it out into an ex- 

 panded condition after each contraction. While expanding 

 the heart fills with blood through three pairs of lateral 

 slits, or ostia. During contraction the blood is forced out 

 through the arteries to all parts of the body. Backflow 

 through the ostia is prevented by valves. Most of the 

 blood coming to the heart has been purified by passing 

 through the "leaves" of the book lungs, which are the 

 respiratory organs of spiders. There is a pair of openings 

 on the ventral side of the spider's abdomen, which lead 

 into little chambers within the body in which the lung- 

 books lie. Air is drawn in through the openings and 

 passes between the leaves of the book, thus giving the blood 

 within a chance to give up waste gases and acquire oxygen 

 through the thin epidermal covering. The excretory 

 organs of spiders consist of slender Malpighian tubules. 

 Their free ends are closed and lie among the internal 

 organs. Their proximal ends open into the intestine, and 

 there discharge absorbed waste products. 



Self -protection. Spiders, like most predaceous animals, 

 are solitary in their habits. They usually sit quietly in 

 their webs waiting for prey. If danger threatens, a spider 

 quickly seeks some retreat or drops to the ground. Before 

 it goes, however, a thread is attached and a silken strand, 



