

SECTION THROUGH PHARYNX. 



209 



. Transverse Section through the Anterior Part of the 

 Pharynx. 



1. General characters. The section is triangular in shape, 

 the apex being formed by the dorsal fin, the base by 

 the ventral surface, and the basal angles by the 

 metapleural folds. The pharynx, perforated by the 

 gill-slits, and surrounded by the atrial cavity, lies 

 below the notochord. 



Pig. 43. Amphioxus lanceolatus. Transverse section through the 

 anterior part of the pharynx of an adult specimen. The boundary 

 of the atrial cavity is indicated by a thick black line. (a. m. m.) 



A, skeleton of dorsal fin. B, spinal cord. C, notochord. D, connective- 

 tissue sheath surrounding notochord. E, cavity of pharynx. F, epibranchial 

 groove of pharynx. G, endostyle, which in this anterior part is flattened out or 

 even convex. H, atrial cavity. J, transverse muscles in floor of atrial cavity. 

 M, dorsal coelomic canal. P, metapleural canal. R, left dorsal aorta. S, 

 endostylar coelomic canal in which the cardiac aorta lies. X, myotome. 

 Y, suspensory fold of pharynx, separating the dorsal coelomic canal from the 

 atrial cavity. Z, gill-arch or branchial bar : the triangular white spot repre- 

 sents the cut surface of the skeletal rod of the arch. 



