NEAR RELATIVES OF INSECTS. 239 



is also unsegmented, and is joined to the cephalo-thorax 

 by a slender stalk. At the base of the abdomen are 

 located the openings of the respiratory system ; and at the 

 apex of the abdomen are located the external organs of 

 the spinning mechanism peculiar to spiders. 



The appendages of the head are the labium, the 

 maxillae, the mandibles, and the palpi. The mandibles 

 are two- jointed; and near the point of the second claw- 

 like segment there is a small opening, the outlet of the 

 poison gland. This poison kills or disables the insect 

 victim; but its effect upon the human family differs in 

 different cases, ranging from no effect whatever to serious 

 inflammation in some cases. Possibly it may be with 

 spider bites as we say of liability to a disease. We 

 desire to believe, and it may be a tenable theory, that the 

 perfectly healthy person is immune from all disease, the 

 natural warders of the body, the white blood corpuscles, 

 being in such a person present in such numbers as to be 

 capable of disposing of all disease germs that may enter 

 the system, with safety to the individual. 



The walls of the cephalo-thorax are heavily chitinized. 

 From this portion arise the four pairs of legs; and here 

 must also be attached the many muscles used in the swift 

 movements of the spider. The abdomen is soft and non- 

 chitinized. The legs consist typically of seven segments 

 coxa, trochanter, femur, patella, tibia, tarsus, and 

 metatarsus. The tarsus is usually composed of several 

 segments, and is one-, two-, or three-clawed. 



Spiders breathe as insects do, by means of tracheae; 

 but to the tracheae are added two sack-like cavities, in 

 which are numerous plates called pulmonary lamellae. 

 These sacks are really rudimentary lungs and have their 

 openings on the front ventral surface of the abdomen; 



