GILL-SLITS. 23 



geal wall intervening between two primary slits the 'primary 



^i„r i^'"^'^°l* transverse section through Amphioxus in the hinder part of the 



pharyngeal region showing the brown canals (after Lankester, from Perrier). The division 

 ^Ln*"?'"";- ^-eptral portion of the myotomes into two groups of fibres separated by a 

 connective tascia is shown ; a atrium ; arf root of dorsal aorta ; 6 primary bar • 6' secondlrv 

 bar; c hver (caecum); ca lymph space; cd notochord ; c« tube of' atriocoe°omic 

 funnel ; en connective tissue framework of the body ; c p somatic wall coveZa 

 caecum; et skeletal plate of endostyle ; e ectoderm; / branchial slit VhS 

 pharyngeal groove ; gv endostyle ; H ventral canal of notochord ; h dok^l ca^^Tot 

 the notochord ; m transverse muscle of atrial floor ; M mvotomes ; T^rmetapleure 

 iV spinal cord ; «a ventra. np dorsal root of a spinal nerve ; o gonadTp poS of a 

 fold in dorsal wall of atrium ; ph pharynx ; r dorsal fin ray ; s so-called lymph-spaces 

 In the case of the hepatic caecum, s points to the blood-vessels ^ ^ 



