Ii8 Invertebrate Zoology. 



AN ARACHNID 

 (Epeira rip aria). 



External Anatomy. The body is here again divided 

 into a cephalothorax and an abdomen, the two being united 

 by a small peduncle. The cephalothorax is flattened above 

 and bears six pairs of appendages. The most anterior pair, 

 the chelicerce, are provided with strong cutting jaws and are 

 pierced, near their tips, by a small opening which leads from 

 a. poison-gland. They lie anterior to the mouth. 



'Thepedipalps, which lie immediately behind the chelicerae, 

 and on each side of the mouth, are more leg-like. They 

 consist of a strong-bodied, basal portion and of an elongated, 

 many-jointed feeler ex palpus. In the males the palpi are 

 specialized into organs for bearing the spermatophores. 

 Compare this pair of appendages with the same of Limulus. 



The four pairs of legs take their origin from behind the 

 mouth, along the lateral portion of the cephalothorax. Do 

 they all agree in having the same number of joints? Note 

 the hook- like organ terminating each. 



Examining the abdomen from above, do you find any- 

 thing (grooves or transverse bands) that might suggest that 

 it was once segmented ? 



The lower surface of the abdomen presents several organs 

 of interest. The external openings of the lung-sacs appear 

 as two transverse slits, one on either side of the anterior por- 

 tion of the abdomen. In the median line, between them, is 



