STRUCTURE OF THE ADULT ANIMAL. 39 



of Jhe body are not arranged i n pa.irs. hut, altprnatA ^fr\f ^. 

 o ne .another along the whole length of the anima l ; and this 

 lat eral asymmetry , one of the most marked features of the adult 

 Amphioxus, a ffects the nerves, blood-vessels, and other structures 

 as well. The ventral surface of the body in the anterior two- 



FiG. 12. — 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. The section is taken at about 

 the level of the reference line R in Fig. 11. (From Marshall and Hurst.) 



A, skeleton of dorsal fin. B, spinal cord. C, notochord. J), 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 ccelomic canal. 

 P, metapleural canal. R, left dorsal aorta. S, cardiac aorta. X, myotome. "Y", 

 suspensorv fold of pharynx, separating the dorsal ccelomic canal from the atrial 

 cavity. Z, gill-arch or branchial bar ; the white triangular spot represents the cut 

 surface of the skeletal rod of the arch. 



thirds of the animal is covered by a thin sheet of muscle (Fig. 

 12, j), the fibres of which run transversely from side to side. 



Th e alimentary canal is a nearly straight tube, the anterio r 

 p art of which is modified for respiration, as in fish. 



The buccal orifice (Fig. 11) is a large oval opening, on the 

 ventral surface of the anterior end of the body ; it is fringed on 

 each side by a series of ciliated tentacles, but there are no 



