//. ACER AT A: GIGANTOSTRACA. 



443 



oesophagus, and in many forms is enclosed in the ventral artery. 



Excretory organs, in the shape of neph- 

 ridia, 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. 



FIG. 455. FIG. 456. 



FIG. 455. Digestive tract of Ctenida ccementaria. (From Lang, after Dug6s.) a, ab- 

 domen ; an, anus; da, rff, diverticula ('liver') of midgut ; g, brain; v6, rectal 

 bladder (stercoral pocket) ; vm, excretory tubules. 



FIG. 456. Lung book of Zilla cadophyla. (After Bertkau.) a, a lung leaf separated 

 from the other leaves, 6 ; st, spiracle. 



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. 461) to the exterior. The anterior wall of each lung sac is 

 made up of thin plates arranged like the leaves of a book, and em- 

 bryology shows that these lung books are gill books drawn into the 

 ventral surface of the abdomen. The tracheae in development 

 pass through a gill-stage and a lung-stage, the tracheal tubes being 

 outgrowths of the spaces between the lung leaves which penetrate 

 all parts of the body. 



The reproductive openings are on the basal somite of the abdo- 

 men. The spermatozoa are motile. The development is direct, 

 there being no metamorphosis. 



Sub Class I. Gigantostraca. 



Marine forms with gills on the 2-6 abdominal appendages; 

 bases of five pairs of cephalothoracic feet masticatory ; a pair of 

 medinn ocelli and a pair of compound eyes on the cephalothorax. 



