GILL-SLITS. 23 



geal wall intervening between two primary slits the primary 

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ci 



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FIG. 10. Diagram of a transverse section through Ampkioxus in the hinder part of the 

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

 of the more ventral portion of the myotomes into two groups of fibres separated by a 

 connective fascia is shown ; a atrium ; ad root of dorsal aorta ; b primary bar ; ft 1 secondary 

 bar ; c liver (caecum) ; ca lymph space ; cd notochord ; e^'tube of atriocoelomic 

 funnel ; en connective tissue framework of the body ; c p SQmatic wall covering 

 caecum ; ct t skeletal plate of endostyte ; e ectoderm ; / branchial slit ; g$ hyper- 

 pharyngeal groove ; gv endostyle , li ventral canal of notocljorol ; Is dorsal canal of 

 the notochord ; m transverse muscle of atrial floor ; M myotomes ; mt metapleure ; 

 A T spinal cord ; na ventral, np dorsal root of a spinal nerve ; o go*n.ad ; p portion of a 

 t Id in dorsal wall of atrium ; ph pharynx ; r dorsal fin raj' ; s' so-called lymph-'spaces. 

 i the case of the hepatic caecum, s points to the blood-vessels. 



