ii8 



CEPHALOCHORDATA 



or alternate bending of the body from side to side in swimming 

 or burrowing can be performed. There are usually, on each side, 



Pig. 72. — Braiichiostoma lanceolaturn. Diagrammatic transverse section of the pharyn- 

 geal region, passing on the right through a primary, on the left through a secondary 

 branchial lamella. «o, Dorsal aorta ; c, dermis ; ec, endostylar portion of coelom ; 

 /, fascia, or investing layer of myotome ; fh, compartment containing fin-ray ; g, 

 gonad ; gl, glomerulus ; k, branchial artery ; kd, pharynx ; Id, combined atrial and 

 coelomic wall (ligamentum denticnlatum) ; m, myotome ; tnt, transverse muscle ; 

 n, nephridium ; n.ch, notochord ; of, metapleural lymph space ; p, atrium ; sc, 

 coelom ; si, ventral aorta ; sk, sheath of notochord and spinal cord (sp.cd) ; uf. 

 spaces in ventral wall. (From Korschelt and Heider, after Boveri and Hatschek.) 



35 myotomes in front of the atriopore, 14 between the atriopore 

 and the anus, and 11 postanal, making 60 in all: some species 

 have only about 50 myotomes, and some as many as 85. (See 



