304 METAZOAN PHYLA 



sinuses, and veins. In the tissues the blood flows through irregular 

 spaces which lack continuous walls; it is, therefore, not a closed system. 



The respiratory organs are two air sacs from the anterior walls of 

 which arise from 15 to 20 leaflike folds supplied with blood capillaries. 

 These folds are sometimes called collectively a lung book, and the whole 

 apparatus is generally termed a hook lung. There are tracheae in the 

 abdomen but the tracheal system is not extensive and does not play a 

 large part in respiration. 



In addition to the malpighian tubules, which the spiders share with 

 insects and myriapods, they also possess in many cases two coxal glands 

 in the cephalothorax, which are homologous with the green glands of the 

 crayfish. 



The nervous system of the spider consists of a bilobed ganglion above 

 the esophagus and a subesophageal ganglionic mass, with nerves running 

 to various parts of the body (Fig. 207). This represents a condensation 

 in the nervous system greater than that met with in any other type which 

 has been studied and goes along with a high degree of coordination of 

 activities. 



325. Metabolism. — The food of spiders consists of juices from their 

 prey, which are drawn into the sucking stomach by the contraction of 

 muscles attached to the wall of the cephalothorax. When a spider feeds 

 upon an insect, the latter is well crushed between the bases of the che- 

 licerae and is then held against the mouth while the sucking process is 

 going on. From the sucking stomach the food passes onward through 

 the alimentary canal; it is acted upon by digestive fluids secreted by the 

 digestive stomach and also by a secretion from the so-called liver, which 

 is a large digestive gland surrounding the alimentary canal in the abdo- 

 men, the secretion of which resembles the pancreatic secretion of higher 

 forms. Elimination takes place through the malpighian tubules, the 

 coxal glands being generally small and frequently quite degenerate. 



326. Reproduction. — The sexes of spiders are always separate, and 

 fertilization is internal. The male transfers the sperm cells from his 

 genital opening to the seminal receptacle of the female by means of his 

 pedipalpi, which in this sex are very large and modified in many and 

 curious ways characteristic of different species. The eggs are laid in a 

 silken cocoon which may be attached to some object or carried about by 

 the female (Fig. 208). The young leave the cocoon soon after hatching. 

 They undergo no metamorphosis. 



327. Spinning Activities. — The characteristic feature in the life of 

 all spiders is the production of silk, which may be used in the construc- 

 tion of a hiding place or home for the spider, and which also serves in 

 the construction of a web for the capture of prey. Small silken threads 

 are used in spinning the silk of attachment discs which serve to fasten 

 larger threads in place, for making a swathe of silk to be thrown about the 



