LATER LARVAL DEVELOPMENT OF AMPHIOXUS. 205 



elements of buccal skeleton commencing to grow out as 

 tentacles or cirri. 



Stage IV. Right Side. — Thirteen primary slits, all bend- 

 ing under the pharynx, first very small and thirteenth closing; 

 seven secondary slits, the new one appearing in front ; the 

 larger secondary slits commencing to double in, i.e. to form 

 tongue-bars ; endostyle extending a short way beyond the 

 club-shaped gland; length of endostyle, 24; atrium closed 

 anteriorly. 



Left Side. — Primary slits are now partly on this side; 

 average apparent length of mouth, 17 ; oral hood containing in 

 its lower half incipient cirri. 



Stage V. Right Side. — Twelve primary slits just visible 

 at base of pharynx, twelfth closing ; the first primary slit 

 undergoes atrophy ; eight secondary slits with complete tongue- 

 bars in the larger ones; endostyle extends a long way past the 

 club-shaped gland ; length of endostyle, 28 ; club-shaped gland 

 undergoes atrophy. 



Left Side. — Primary slits not fully arrived on this side 

 yet; tongue-bars commencing in the primary slits ; average 

 apparent length of mouth, II ; junction of upper and lower por- 

 tions of oral hood. 



Stage VI. Right Side — Eight secondary slits, length 

 equal to the breadth, measuring about 11 units, both in a direc- 

 tion parallel to long axis of body and in a direction at right 

 angles to it ; length of endostyle, about 34. 



Left Side. — Ten primary slits (i.e. Nos. 2 to 11 inclusive), 

 the eleventh closing; tongue-bars complete in the larger slits, 

 which are equal in size to the corresponding secondary slits ; 

 mouth same as in Stage V; buccal skeleton commencing to 

 penetrate the upper portion of the oral hood. 



Stage VII. — Slits on both sides commencing to elongate 

 in a vertical direction; average length of the central slits, 14. 



Right Side. — Eight slits; length of endostyle, 39. 



Left Side. — Nine slits, ninth (i.e. tenth primary) usually 

 closing ; apparent length of mouth or velum, 4 ; four velar 

 tentacles. 



