Spiders and Their Near Relatives 



ages bears a pair of gills. These are termed book-gills, each 

 being composed of many thin plates, like the leaves of a book. 



In its more general characteristics, Limulus is closely allied 

 to the Arachnida, except that its respiration is aquatic. So 

 close is this resemblance in structure to the Arachnida that many 

 zoologists, and among them some of those who have studied 

 Limulus most carefully, regard the Xiphosura as an order of 

 the Arachnida. A more conservative view is to regard the 

 Palaeostracha and the Arachnida as two closely allied but dis- 

 tinct classes. 



The most familiar of the fossil representatives of the Palae- 

 ostracha are the Trilobites. 



VI. Class Arachnida (A-rach'ni-da). — The members of 

 this class are air-breathing arthropods, in which the head and 

 thorax are usually grown together, forming a cephalothorax, 

 which have four pairs of legs, and which apparently have no 

 antennas. The characteristics of the Arachnida are more fully 

 discussed in the next section of this chapter. 



TABLE OF CLASSES OF ARTHROPODS 



The more striking of the distinguishing characters of the 

 six classes of arthropods mentioned above can be stated in a 

 tabular form as follows: 

 A. With two pairs of antennae and at least five pairs of legs; 

 respiration aquatic. Crustacea 



AA. With one pair of antennae or apparently with none; respira- 

 tion, except in Limulus, aerial. The number of legs 

 varies from six to many. 

 With one pair of feeler-like antennae. 

 C. With more than three pairs of legs, and without wings. 

 D. With two pairs of legs on some of the body-segments. 



Diplopoda 

 DD. With only one pair of legs on each segment. 



Chilopoda 

 CC. With only three pairs of legs, and usually with wings 

 in the adult state. Hexapoda 



BB. Apparently without antennae. 



C. Respiration aquatic. Pal^ostracha 



CC. Respiration aerial. Arachnida 



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