PHYLUM ARTHROPOD A 



607 



o/berc 



narrow pointed telson. There are usually five pair of limbs sur- 

 rounding the mouth and, with the exception of the first, toothed 

 at the bases in order to perform the functions of jaws ; the last 

 pair are stouter than the others and are expanded so as, ap- 

 parently, to assume the character of swimming paddles. Certain 

 of the more anterior of the free segments bear paired lamel- 

 liform appendages 

 which probably car- 

 ried the branchiae, as 

 in the Xiphosura. 

 The exoskeleton is in 

 many cases elabor- 

 ately sculptured. 



A cartilaginous in- 

 ternal endosternite 

 of the same nature 

 as that which has 

 been described as oc- 

 curring in the Scor- 

 pions is found in 

 Limulus and in cer- 

 tain Spiders, but 

 not in the other 

 groups. 



Coxal glands, 

 similar to those that 

 have been described 

 in the Scorpion, oc- 

 cur also in most 

 Spiders, in the Sol- 

 pugida and Phalan- 

 gida, in some Acarida, 

 and in the Xiphosura. 

 In the Solpugida and 

 Phalangida they oc- 

 cur on the bases of 

 the last pair of legs : 

 in the Araneida arid 

 Xiphosura, as in the 

 Scorpion, they are 

 appendages. 



Alimentary system. — The oesophagus (Fig. 551, ces.) of the 

 Spiders is expanded behind into a special sucking stomach (suck. st.). 

 The mesenteron (mesenl.) gives off in the cephalothorax a pair 

 of large diverticula from each of which arise five narrow 

 diverticula (ccec.) which enter the bases of the pedipalps and legs ; 

 in the abdomen it is surrounded by a mass of cells commonly 



Fio. 540.— Ventral view of Limulus. 1—6, appendages of 

 cephalothorax ; abd. abdomen ; ceph. cephalothorax ; operr. 

 operculum, behind which are seen the series of abdominal 

 appendages : tds. caudal spine or telson. (After Leuckart.) 



found on the bases of the fifth pair of 



