i 4 AMERICAN SPIDERS 



contract, they increase the size of the stomach and there results a 

 strong sucking action that pulls the predigested food into the mid- 

 gut. All the absorption occurs in the midgut, which is notable for 

 a series of large blind sacs in the cephalothorax extending as four 

 thick arms on each side, and voluminous glandular extensions from 

 the main digestive tube in the abdomen. The hindgut provides a 

 channel of egress for the fecal material, a thick, whitish liquid that 

 is accumulated in a large bladderlike sac called the stercoral pocket, 

 and voided through the anus. 



In addition to the tubular Malpighian vessels opening into the 

 hindgut, which serve as excretory organs, spiders have a pair of 

 coxal glands located opposite the coxae of the first and third legs, 

 and these discharge their products through tiny openings behind 

 the coxae. It is believed that the coxal glands are modified nephridia, 

 the primitive excretory organs of earthworms and other animals, 

 and that from similar glands in other parts of the animal have de- 

 veloped the various silk glands and perhaps the poison glands as well. 



The activities of the arthropods are governed by a nervous 

 system quite different from that found in the vertebrates. In the 

 simpler forms it consists of a double nerve cord lying below the 

 alimentary tract, which is enlarged in each segment to form a center 

 or ganglion, from which lesser nerves arise. The most anterior pair 

 of ganglia lie above the pharynx, and, joined to the pair immediately 

 behind and below the pharynx by nerve connections, is called the 

 brain. A very considerable modification of this generalized condi- 

 tion is to be seen in most of the spiders, which have contained 

 within the cephalothorax all the central nervous system. The 

 ganglia in the cephalothorax have been consolidated into a single 

 mass around the esophagus and below the digestive system. From 

 the dorsal brain arise the nerves for the eyes and the chelicerae, and 

 from the lower mass large nerves go to the appendages and back 

 into the abdomen through the narrow pedicel. 



Sensation from the external environment is communicated to the 

 central nervous system by means of structures called receptors. 

 The most obvious ones are the eyes, which are remarkable organs 

 in some insects but by comparison very feebly developed in spiders. 

 Also very poorly represented in the arachnids are receptors for 

 chemical stimuli, such as smell and taste, and perhaps the former 

 sensation, as it is understood in vertebrates, is not even present in 

 spiders. The receptors for touch are numerous and varied in the 



