12 AMERICAN SPIDERS 



in size of the carapace and appendages, and often of the abdomen 

 as well, must take place immediately following molting when the 

 integument is still soft. 



One characteristic of all the arthropods is the fact that their 

 bodies are divided transversely into numerous well-marked rings or 

 segments (in some cases most indications of segmentation are lost). 

 The segments in front, which go to form the guiding center of the 

 animal, are usually dissimilar and so greatly modified and fused that 

 their exact limits are obscured. Thus, the head in one group is not 

 necessarily the same as the head in another; it may be composed of 

 more segments or carry more appendages, and the appendages of 

 the same segment may be vastly different. From primitive append- 

 ages have been derived mouth parts, swimmerets, legs, spinnerets, 

 antennae, and many other organs. They are used for feeding, swim- 

 ming, running, silk-spinning, mating, and for sensory perception. 

 The hind portion of the animal, which is called the abdomen, is like- 

 wise not the same in all the arthropods. In the centipedes and milli- 

 pedes it is a multisegmented trunk, provided with numerous jointed 

 legs, in some instances nearly two hundred pairs. In the insects the 

 abdomen completely lacks appendages except at the caudal end. In 

 spiders the only abdominal appendages are the spinnerets. 



With such marked difference in the external form of the Arthro- 

 poda as compared with vertebrates, it is not surprising that the 

 internal anatomy should also be quite distinct. The various systems 

 for carrying on living, such as those for digestion, respiration, excre- 

 tion, and reproduction, show marked differences. 



In the horseshoe crabs and most of the crustaceans, the respira- 

 tory organs are external gills, which aerate the blood by absorbing 

 through their delicate walls the oxygen and other gases dissolved 

 in the water. Whereas most of the other arthropods long ago aban- 

 doned an aquatic life, some individualists among the insects have 

 secondarily returned to its security during parts of their life, but 

 not before they devised new means of living. Respiration in the 

 land arthropods is effected by means of internal aerating chambers 

 called book lungs and tracheae, or, less frequently, by breathing 

 directly through a soft outer covering. The book lungs of the 

 arachnids are closely packed sheets of body surface bound together 

 like the leaves of a book, to give the maximum surface for aeration. 

 The tracheae in the arachnids are small tubes that lead into the 

 body and sometimes ramify to form complex systems. In the myri- 

 apods and insects, the air is conducted directly to the tissues by 



