574 



ARTHROPODA. 



carry, as a rule, four pairs of jointed ambulatory legs. Abdominal 

 limbs are present in the embryo of various Arachnida, but ambula- 

 tory appendages are never developed on the abdominal somites of 

 the adult. In the Scorpions, the first abdominal segment carries a 

 minute " operculum," formed by the coalescence of a pair of ap- 

 pendages, which covers the opening of the reproductive organs ; 

 and the second abdominal segment carries the peculiar organs which 



Fig. 431. — A, Scorpio afer, viewed from above, and somewhat reduced in size ; b, Upper sur- 

 face of the cephalothorax of the same, enlarged ; c, Buthus Kochii, with the terminal segments 

 and the ends of the appendages on one side omitted. m t Maxillary palpi (behind these are 

 the four pairs of ambulatory legs) ; c, Chelicerae ; t, Telson ; o, Lateral ocelli ; o', Central, larger 

 ocelli ; g, Opercular plate, covering the opening of the reproductive organs ; r, One of the 

 " combs " ; s, One of the stigmatic openings, (c is after Prof. Ray Lankester.) 



are known as the " combs." The extremity of the abdomen carries 

 in the Spiders the so-called " spinnerets " (two, four, or six in num- 

 ber), which have sometimes been regarded as modified abdominal 

 appendages. A " telson " is present in the Scorpions, in which it 

 forms a poisonous " sting " ; and a jointed caudal filament is present 

 in Thelyphonus. 



The higher Arachnida are air-breathers, and possess distinct res- 

 piratory organs in the form of " trachea? " or " pulmonary sacs," the 



