90 ARTHROPODS 



The Abdomen. — (i) Examine the dorsal side of the 

 abdomen to see if there is any indication of segmentation. 



(2) Look on the ventral side of the abdomen, near its 

 union with the cephalothorax. Do you find small openings ? 

 These are the lung sacs which the air enters and purifies 

 the blood. How do they differ from the spiracles of in- 

 sects? Look in the opening, to find if possible, how the 

 blood is purified. On the median line a little further back 

 may be found the opening of the reproductive organs. 



(3) At or near the posterior end find the spinning ap- 

 paratus, which consists of blunt protuberances called spin- 

 nerets. How many are there? Look closely to see if you 

 can distinguish any markings on the surface. Each spm- 

 neret consists of numerous openings through which exudes 

 the material that is hardened by the air into a solid flexible 

 thread. All the threads of the spinnerets unite to produce 

 the zueb of the spider. A small spiracle may be found iu 

 front of the spinnerets. 



(4) Are these spinnerets evidences of segmentation? 

 Do you think the abdomen has ever been composed of sep- 

 arate segments the divisions of which showed around the 

 body? If so by what process has the segmentation been 

 reduced to its present condition ? 



Draw ventral side of the abdomen (X4)- 



Development. — (i) The eggs of the spider are usu- 

 ally placed in a sac, or cocoon surrounded by a slik web. 

 In some species the cocoons are carried around by the 

 mother spider who will sometimes die rather than abandon 

 it. In other species the cocoons are concealed by bark or 

 covered with dirt or pebbles, while some abandon the eggs 

 as soon as they are laid. 



