THE ANIMAL KINGDOM 



II 



gill-slits. A ciliated groove, which is similar in function and in relations 

 to the endostyle of urochords, extends the entire length of the pharynx. 



NERVE CORD^. 



NOTOCHORO- 



-DORSAL ARTERY.~. 



, PHARYNX r-^ 



METAPLEURAL FOLD. 



, NERVE CORD 



PERIBRANCHIAL CAVITY 



GILL APERTURE 



! VENTRAL ARTERY. 



GILL APERTURE 



VENTRAL ARTERY< 



DIGESTIVE GROOVE. 



A. AMPHIOXUS. B.PTYCHODERA. 



Fig. 12. — Diagrams illustrating divergent methods by which the peribranchial 

 cavity is formed. In Amphioxus (A) the pleural folds are separated from the pharynx 

 by paired folds which extend dorsally from the ventral side. In the hemichordate 

 Ptychodera (B), on the other hand, the paired folds begin to form at the dorsal side of 

 the worm and extend ventrally. The peribranchial cavity in urochordates arises in a 

 similar manner. As frequently happens in animals, a similar end-result is attained by 

 divergent means. (Redrawn after Gaskell.) 



LATERAL TRUNK MUSCLES 

 ■SPINAL CORD 

 NOTOCHORD 



DORSAL AORTA- 

 PRECARDINALV 



EPIBRANCHIALGROOVE 



GILL LAMELLAE 



CARTILAGE BAR 

 GILL-RODS 



ge?~;^::j — TRANSVERSE MUSCLE 

 •VENTRAL AORTA 

 )JS 'PERIBRANCHIAL CAVITY 



HYPOBRANCHIAL MUSCLE 

 METAPLEURAL FOLD 



A. AMPHIOXUS B AMMOCOETES. 



Fig. 13. — Cross sections of A, Amphioxus and B, Ammocoetes (larval Petromyzon) 

 through the pharyngeal region showing their fundamental resemblance. 



Opposite it, in the roof of the pharynx, is a somewhat similar epipharyn- 

 geal groove. The liver is a hollow tubular sac, which opens into the 



