THE PLACE OF SPIDERS IN NATURE 13 



means of tracheae which, however, are dissimilar to those of the 

 arachnids and develop in a different way. Although simple diffu- 

 sion through the skin or into the body by means of the tracheae 

 often is sufficient in small, inactive arthropods, in large and more 

 active forms some sort of breathing takes place, usually through 

 rhythmical movements of parts of the body by special muscles of 

 the abdomen. 



The type of circulatory system in the arthropods is a specialized 

 one, seemingly highly efficient within the size limits of these crea- 

 tures. Instead of closed tubes that carry the blood to every part 

 of the body and ramify in great profusion to reach all the tissues, 

 the system is at least in part an "open" one. The place of the veins 

 is taken by expansive channels, or sinuses, filled with blood, in which 

 the organs and tissues are bathed. In a large pericardial sinus lies 

 the heart, which expands to allow the blood to enter the pumping 

 vessel through paired lateral openings, and contracts to send the 

 blood coursing through the arteries to all parts of the body. The 

 blood is ordinarily a clear liquid in which are suspended numerous 

 pale blood cells. A disadvantage of having the internal organs 

 bathed in blood is the seriousness of accidental rupture of the outer 

 covering. Any breaking of the body wall might prove fatal to the 

 creature, since the blood would quickly drain from the body, but 

 the tough exoskeleton guards against this. Injury to an appendage 

 could also be fatal, but in many arthropods the injured member is 

 removed by breaking it off (a process called autotomy) at a point 

 where healing is rapidly accomplished. 



The digestive system is a tube that extends from one end of the 

 body to the other, and is often subjected to various types of elabora- 

 tion by coiling and compounding to increase the amount of absorp- 

 tive surface. In the spiders and their relatives, this is accomplished 

 by extending arms in many directions from the main tube. A con- 

 siderable diversity exists among the arthropods as regards the de- 

 tails of the digestive system, but all are alike in having a foregut 

 and hindgut derived from the infolding ectoderm, and an expansive 

 midgut, in which absorption is accomplished by means of the en- 

 zyme-producing epithelium. 



The foregut in spiders is modified to pull in the liquid food. It 

 consists of a pharynx into which the small mouth opens, an esopha- 

 gus, and a so-called sucking stomach. The former are rather simple 

 tubes, but the sucking stomach is an enlargement behind the esopha- 

 gus supplied with powerful muscles on its four sides. When these 



