188 



AMPHIOXUS 



5. The pharynx is of an inverted heart-shape in transverse 

 section. 

 a. The branchial bars, or gill-arches. Owing to their 

 obliquity and extreme closeness to one another 

 twenty or more bars may be cut on each side in a 

 single section. Each bar is clothed on its outer 

 surface by the atrial epithelium, which is a single 



Fig. 41. — AmpMoxus 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. O, notocliord. D, connective- 

 tissue sheatli surrounding notocliord. E, cavity of pharynx. F, hyper- 

 branchial groove of pharynx. G-, hypobranchial band of pharynx, which in 

 this anterior part is flattened out or even convex, H, atrial cavity, J, trans- 

 verse muscles in floor of atrial cavity. Iff, dorsal coslomic canal, P, meta- 

 pleural canal, R, left dorsal aorta. S, cardiac aorta, X, myotome. Y, 

 suspensory fold of pharynx, separating the dorsal ccelomic canal from the 

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

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



