STRUCTURE OF THE ADULT ANIMAL. 



39 



of the body are not arranged in pairs, but alternate with 

 one another along the whole length of the animal ; and this 

 lateral asymmetry, one of the most marked features of the adult 

 Amphioxus, affects 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. 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 ccelomio canal. 

 P, metapleural canal. B-, left dorsal aorta. S, cardiac aorta. X, myotome. Y, 

 suspensory fold of pharynx, separating the dorsal coclomic 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. 



The alimentary canal is a nearly straight tube, the anterior 

 part 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 



