xr 



PHYLUM ARTHROPODA 



651 



surrounding 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, apparently, 

 to assume the character of swimming paddles. Certain of the 

 more anterior of the free segments bear paired lamelliform appen- 

 dages which probably carried the branchiae, as in the Xiphosura. 

 The exoskeleton is in many cases elaborately sculptured. 



A cartilaginous in- 

 ternal endosternite 

 of the same nature as 

 that which has been 

 described as occurring 

 in the Scorpions is 

 found in Limulus and 

 in certain Spiders, but 

 not in the other groups. 



Coxal glands, 

 similar to those that 

 have been described 

 in the Scorpion, occur , 

 also in most Spiders, 

 in the Solpugida and 

 Phalangida, in some 

 Acarida, and in the 

 Xiphosura. In the 

 Solpugida and Phalan- 

 gida they occur in the 

 bases of the last pair 

 of legs ; in the Ara- 

 neida and Xiphosura, 

 as in the Scorpion, 

 they are found on the 

 bases of the fifth pair 

 of appendages. 



Alimentary sys- 

 tem. The mouth of 



the Spiders leads into FIQ. 559. Ventral view of lamulus. 1 (^appendages of 



a WharvTiY fTTicr ^ifil cephalothorax; abd. abdomen; ceph. cephalothorax; 



iiyiiA ^ ig. i, operc. operculum, behind which are seen the series of 



vh) followed bv a abdominal appendages : tels. caudal spine or telson. 



" ' (After Leuckart.) 



narrow oesophagus 



(oes.) expanded behind into a special sucking stomach (suck. st.). The 

 mesenteron (mesent.) gives off in the cephalothorax a pair of large 

 diverticula from each of which arise five narrow branches (ccec.), 

 the last four of which enter the bases of the legs ; in the abdomen 

 it is surrounded by a mass of cells commonly termed " liver " (hep.), 

 the ducts of which open into it. The rectum or proctodceum gives 

 off dorsally a large cloacal sac (rect. ccec.) into which open a pair of 



