42 



PHYLUM CEPHALOCHORDA. 



myocoele. They soon come to project into the cavity of the 

 somite in front, pushing the myosepta before them (Fig. 30). 



FIG. 29. Transverse section through the branchial region of Amphioxus showing on^the left 

 the condition of a secondary, and on the right that of a primary gill-bar, diagrammatic 

 (after Boveri and Hatschek, from Korschelt and Heiderj. ao aorta ; c cutis layer of 

 myocoele ; ec endostylar coelom ; / fascia layer ; fh dorsal fin canal ; p genital sac ; gl renal 

 vessels ; k vessel in primary bar ; kd pharynx ; Id ligamentum denticulatum ; mfmuscle- 

 plate ; mt transverse muscle ; n renal canal"; of metapleural lymph space ; p atrial ca\ :ty ; 

 sc dorsopharyngeal coelom ; si sub-pharyngeal vessel ; sk skeletogenous layer of sclerotome ; 

 / lymph canals of the atrial floor. 



They lie therefore as small sacs in the preceding myocoele 

 attached to its hind wall by a pedicle (Figs. 30 (7, 31). Later, the 

 part of the myotome in which they lie becomes separated from 



