Metazoa A mpJiioxus. 



257 



a large cavity the coelom which was further subdivided by 

 the various mesenteries or transverse plates. In Amphioxus 

 the coelom is also present, although much more irregular in 

 in its nature. In transverse section it can be seen in 

 various places surrounding the alimentary canal, and ante- 

 riorly as a much reduced space above the pharynx. The 



FIG. 126. TRANSVERSE SECTION THROUGH ANTERIOR PHARYNGEAL REGION 

 OF Amphioxus lanceolatus. (Milnes Marshall.) 



A, dorsal fin ; B, spinal cord ; c, notochord ; D, connective-tissue sheath ; 

 E, pharynx ; F, hyperbranchial groove ; G, hypobranch : al groove ; 

 H, atrial cavity ; j, transverse muscle-bands ; M, p, coelom ; R, left 

 dorsal aorta ; s, cardiac aorta ; x, myotome ; Y, fold of pharynx ; z, 

 branchial bar. 



compressed character of the body and the addition of the 

 atrial cavity, which it is, however, to be noted is really a 

 part of the outer world, render the limits of the coelom more 

 difficult to define. If, however, the primary stages in the 

 development of the atrial cavity be borne in mind, no diffi- 

 culty will be experienced in understanding the relationships 

 of the two cavities. We have now to glance at the 



