388 



ARTHROPODA 



forms is enclosed in the ventral artery. Excretory organs, in the shape of 

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

 second or the fifth pair of appendages. Malpighian tubes may occur, but 

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

 not homologous with them 



The respiratory organs are either gills, lungs or tracheae. The gills 

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

 the anterior abdominal somites opening by narrow slits (fig. 421) to the. 

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

 ranged like the leaves of a book (fig. 416), and embryology shows that these 

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

 The tracheae 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. Gigantosiraca. 



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

 five pairs of cephalothoraciac feet masticatory ; a pair of median ocelli and 

 a pair of compound eyes on the cephalothorax. 



.. 



FIG. 417. FIG. 418. 



FIG. 417. Limulus polyphemus* horseshoe crab (orig.). 



FIG. 418. Ventral surface of Limulus moluccanus (from Ludwig-Leunis). i, 

 chelicen-e; 2-5, walking feet; 6, pushing foot; 6, flabellum; 7, genital operculum; 

 8, gills (there should be five) ; 9, base of telson. 



Order I. Xiphosura. 



Cephalothorax large; abdomen with joints fused, terminated by a long 

 spiniform telson. Limulus polyphemus* king crab or horseshoe crab. Other 

 species on eastern shore of eastern continent. 



