CHAPTER VII. 



THE AIR-BREATHING ARTHROPODA. 



AMONG these Arthropoda, no forms absolutely devoid of 

 limbs are at present known, though the appendages are re- 

 duced to two pairs of minute hooks in the vermiform parasite 

 Linguatula. 



The ARACHNIDA have pediform gnathites, and the least 

 modified forms of this group (the Arthrogastra or Scorpions 

 and Pseudo-scorpions) exhibit, in many respects, extraordi- 

 narily close resemblances to the Merostomata among the 

 Crustacea. 



THE ARTHROGASTRA. The anterior part of the body of 

 a Scorpion (Fig. 84) presents a broad, shield-like tergal plate, 

 resembling that of Eurypterus in form. Two large. eyes are 

 situated one on each side of the middle line of the shield, 

 while smaller eyes, which vary in number according to the 

 species, are ranged along its antero-lateral margins. 



Six wide plates, representing the terga of as many so- 

 mites, follow the anterior shield, and are connected only by 

 the soft integument of the sides of the body with their 

 sterna. The seventh is united with its sternum (xv) poste- 

 riorly, while the five following terga and sterna form contin- 

 uous rings, which constitute the joints of the so-called "tail." 

 The anus is situated behind the last sternum. A movable 

 terminal piece, answering to the telson of a Crustacean, which 

 is swollen at its base, and then rapidly narrows to a curved 

 and pointed free end, overhangs the anus, and constitutes the 

 characteristic weapon of offense of the Scorpion. This sting, 

 in fact, contains two glands which secrete a poisonous fluid, 

 and their ducts convey it to the minute aperture situated at 

 the sharp point of the organ. On the sternal surface of 

 the body there are four wide and long sternal plates (xi-xiv), 



