ARTHROPODA 203 



less mazes of threads, frequently spun in corners of rooms. The 

 orb weavers^ Epeiridae (Figure 95), construct webs that are mar- 

 vels of efficiency, and sometimes span streams twelve feet wide. 



Poisonous Spiders. — The b/ack widow spider has caused a num- 

 ber of deaths. Kellogg states (191 5) that a diadem spider of 1.4 gr. 

 contains sufficient poison to destroy completely all the blood cor- 

 puscles in 2.5 litres of rabbit blood. Comstock says that the 

 tarantula^ frequently found in bunches of bananas, is incapable of 

 seriously injuring man. 



Bibliography on Venomous Spiders 



Baerg, W, J. 1923. The effects of the bite of Lactrodectes mactans. 



Jour. Parasit., vol. 9, pp. 161-169, March. 

 Bogen, E. 1926. Arachnidism, spider poisoning. Arch, of Int. Med., 



vol. 38, no. 5, pp. 622-632, Nov. 15. 

 Comstock. The Spider Book. 



Kellogg, V. L. 191 5. Spider poison. Jour. Parasit., vol. i. 

 Lloyd, J. S. 1921. Spiders used in medicine. Sc. Amer. Mon., vol. 



60, July. 

 Reese, A. M. 1921. Venomous spiders. Sc, vol. 54, no. 1399, Oct. 



21. 



Reese, A. M. 1929. Economic Zoology. 3d Edition. 



Other Classes. Xiphosura. — The king crab or horseshoe crab 

 (Figure 96), is the only living representative of an ancient group. 

 Related fossil forms from the Carboniferous indicate that it origi- 

 nated in fresh water. The horseshoe crab, Limulus, is a large 

 marine form (2 feet long), commonly found on our Atlantic coast. 

 It lives in shallow water and feeds on worms. It has a large 

 cephalo-thorax and a relatively large abdomen with a long caudal 

 spine. It is used for fertilizer and to feed hogs. It is particularly 

 interesting to us because of the theory of the origin of vertebrates, 

 developed by William Patten. (See p. 218.) 



Pycnogonida. — Sometimes classed as relatives of the Arachnids, 

 these marine forms have four somewhat primitive eyes resembling 

 those of spiders. They have a slender cephalo-thorax and abdomen 

 and four pairs of jointed legs with seven or eight segments each. j 



The alimentary canal extends as ceca^ into the legs. Organs of / /^X6 

 respiration are absent. The ma/es have four to seven cement glands " 



on certain of their appendages, usually the third. The secretion 



