42 



PHYLUM CEPHALOCHORDA. 



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

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



£2^ 



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FIG. 29. — Transverse section through the branchial region of AmpMoxus 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 Heider). oo aorta ; c cutis layer of 

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

 vessels ; jc vessel in primary bar ; kd pharynx ; Id ligamentum deuticulatum ; m muscle- 

 plate ; mt transverse muscle ; n renal canal ; of metapleural lymph space ; p atrial cavity ; 

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

 uf lymph canals of the atrial floor. 



They lie therefore as small sacs in the preceding myocoele 

 attached to its hind wall by a pedicle (Fig-^ 30 C, 31). Later, the 

 part of the myotome in which they lie becomes separated from 



