CHAPTER XIV 



PHYLUM ARTHROPODA (EXCLUSIVE OF CRUSTACEA 

 AND INSECTA) 



Class 2, Acerata 



The horseshoe crabs, spiders, scorpions, mites, a number of 

 degenerate forms, and some important extinct animals which 

 are known only from fossil remains are grouped as a class to which 

 the name Acerata is applied. This name signifies the lack of 

 antennae common to all of the members of the class. The body- 

 usually consists of a cephalothorax and abdomen, though in the 

 mites these two divisions are not separated. Six pairs of appen- 

 dages upon the cephalothorax are arranged about the mouth. 

 The bases of one or more pairs of these appendages are modified 

 to serve as mandibles. The abdomen is composed of a variable 

 number of somites which, in the embryo, bear appendages, but 

 these are lost or highly modified in the adult, except in the 

 Xiphosura. 



Many modifications of the respiratory organs are found in 

 this group. Gills occur on the abdomen of some but correlated 

 with the air-dwelling habit the gills become drawn into the body 

 where they are found as lung books which open to the exterior 

 through narrow slits. In some instances, the lung books are 

 replaced by tracheae which penetrate to all parts of the body as 

 in the insects. 



The basal segment of the abdomen bears the genital opening. 

 In development, there is no metamorphosis. 



Subclass GiGANTOSTRACA 



The extinct eurypterids and the horseshoe crabs appear 

 strikingly hke Crustacea. In details of organization and in 

 development, however, they show an intimate relationship with 

 scorpions and other Acerata and are consequently recognized as 

 constituting a subclass to which the name Gigantostraca has 

 been applied. 



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