202 



HIGH SCHOOL ZOOLOGY. 



^•^^^ 



Fig. IZ&.Scm'pio europmus, 

 with its combs and ocelli. 



developmnnt of the latter seem to point to its being more closely 



allied to the Arachnida. This resemblance is strongest to the 



Scorpions (Fig. 136), a group of Arach- 



nida confined to the warmer zones of 



the Old and New Worlds, In these 



the appendages of the cephalothorax 



are similar to those of Limulus, the 



first two pairs (clielicerae and pedipalpi) 



acting as jaws and prehensile arms, 



while the others are walking legs. 



Respiration is performed by four pairs 



of " lungs," wliicb are cavities on the 



third to the sixth abdominal seijments, 



containing leaflets that recall the gills 



of Limulus, and opening by slits on the ventral surface. 



Development shows that these lungs arise as infoldings at the 



bases of appendages, and that they are homologous with the 



gills of the horse-shoe ci-ab. The abdomen differs from that 



of the crayfish, in being differentiated, into 

 two regions, a preabdomen of seven, and 

 a postabdomen of six segments, the last of 

 which terminates in a curved claw, per- 

 forated by the duct of a poison-gland. 



The little book-scorpions {CheUfer) have no 

 poison-gland in the tail, nor is the abdomen sub- 

 divided into two regions ; they belong to an in- 

 dependent sub-order, the Pseudoscorjnonlna, as 

 do the daddy-long-legs (Phalangina), with their 

 short abdomen and long walking legs. Both of 

 these groups feed on minute insects and mites ; 

 with the Scorpions they form the order Artliro. 

 gastra. 



17. Of the various orders of Arachnida, 



7ij'. \Z~—Aqdle)ia lui'via, , ., ,. .. ^ ■ , , k • \ 



V ith the ocelli the gpider.s (Araneina) and mites (Acarina) 



(After Einerton) 



