o:^i. 



ARTHROPODA 



forms is enclosed in the ventral arter}'. Excretory organs, in the shape of 

 nephridia, are frequently present and open to the exterior at the base of the 

 second or the lifih pair of appendages. Malpigliian tidjcs may occur, but 

 these, unlike those of other tracheates, are entotlcrmal in origin and hence 

 not homologous with them 



The respiratory organs are either gills, lungs or trache;r. The gills 

 are borne on some of the alidominal appendages. The lungs are sacs on 

 the anterior abdominal somites opening by narrow slits (fig. 4J1) to the 

 exterior. The anterior wall of each lung sac is made up of tltin plates ar- 

 ranged like the leaves of a book (fig. 410), and embryology shows that these 

 lung books are gill books drawn into the ventral surface of the abdomen. 

 The trachea" replace one pair of the lungs and are apparently homologous 

 with them. They penetrate all parts of the body. The reproductive 

 openings are on the basal somite of the abdomen. The spermatozoa 

 are motile. The development is direct, there being no metamorphosis. 



Sub Class I. Giganloslraca. 

 ^Marine forms with gills en the 2-6 abdominal appendages; bases of 

 live pairs of cephalothoraciac feet masticatory; a pair of median ocelli arid 

 a pair of compound eves on the cephalothorax. 



Fic. 417. Fig. 41S. 



Fig. 417. — Liiiiiiliis pulyplicmiis* horseshoe crab (orig.). 



Fig. 41S. — \'entral surface of Limiilus moliociiiius (from Ludwig-Leunis'l. i, 

 chelicerx; 2-5, walking feet; 0, iiushing toot; 0", llaboUuin; 7, genital operculum; 

 S. gills (there should be live); 9, base of telson. 



Order I. Xiphosura, 



_ Cephalothorax large; abdomen with joints fused, terminated bv a long 

 spiniform telson. Limiitus fyol yph cuius * king crab or horseshoe crab. Other 

 species on eastern sliorc of eastern continent. 



