//. ACERATA: GIGAA'TOSTJIACA. 



443 



CBSOX)hagiis, 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 

 tuljes may occnr, but these, unlike those 

 of other tracheates, are entodermal in 

 origin and hence not homologous with 

 them. 



an. 



Fig. io-o. Fio. 4:")('.. 



Fig. 4o.5.— Digestive tract of Ctenlihi ccemcntnria. i From Lmijk, aftfr Diigi^s.) n, ali- 



(lomen : mi, anus; da, dt, diverticula Cliver'i of midt^ut ; j/, lu-aiu ; vl\ rect.il 



Ijladder (stercoral pocket) ; vm, excretory tubules. 

 Fir;. .1.50.— LuTlK book of ZiHn cadopliiila. (^Vfter Bertkau.) a, a luii^ leaf separated 



from the other leaves, b ; st, spiracle. 



The respiratory organs are either gills, lungs, or trachea'. The 

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

 arc 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 leayes 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 sj^aces 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 arc motile. The dcvelopmeut is direct, 

 there being no metamorphosis. 



Sub Class I. Gigantoslraca. 



Marine forms with gills on the 2-(J abdominal appendages; 

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

 median ocelli and a i)air of compound eyes on the cephalothorax. 



