GILL-SLITS. 23 



^eal wall intervening between two primary slits the primary 



Fio. 10.— Diagram of a transverse section through Amphioxus in the liinder part of tlie 

 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 ; b' secondary 

 bar ; o, liver (caecum) ; ca lymph space ; cd notochord ; ce tube of atriocoelomic 

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

 caecum ; ct skeletal plate of endostyle ; e ectoderm ; / branchial slit ; gd hyper- 

 pharyngeal groove ; gv endostyle ; li ventral canal of notochord ; Is dorsal canal of 

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

 N spinal cord ; na ventral, np dorsal root of a spinal nerve ; p gonad ; p jxirtiou of a 

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

 In theVase of the hepatic caecum, s points to the blood-vessels. 



