LARVAL DEVELOPMENT. 



139 



that it forms a convenient pnnctnm fixnm with relation to 

 which the growth of neighbouring structures, particularly 

 that of the endostyle, can be determined. 



The upper and lower limbs of the endostyle are inclined 

 to one another at an acute angle, and may be said to form 

 two unequal sides of a triangle, the apex of which is 

 directed backwards between the rows of secondary and 

 the primary gill-clefts (Fig. 77). 



Between the two rows of slits on the right side of the 

 body there is a blood-vessel, representing the anterior 

 continuation of the sub-intestinal vessel, which ends blindly 

 in front above the first primary slit. This is the future 

 ventral branchial artery, with which we are already ac- 

 quainted. When its final situation in the mid-ventral line 

 below the endostyle is remembered, its position in the 

 larva high up on the right side, as in Fig. 74, will appear 

 very striking. 



Continued Migration of Primary Gill-slits. 



The secondary slits now go on growing in size, and the 

 primary slits gradually tend to disappear entirely from the 

 right side until, as in Fig. 78, only the original upper por- 



in 



3.0 



Fig. 78. Anterior portion of larva from right side, to show the backward 

 growth of the endostyle between the primary and secondary gill-slits. (After 

 WlLLEY.) 



s.o. Sense-organ of praeoral pit. p.s' ' . First primary slit. in. Internal opening 

 of club-shaped gland, e. Endostyle. p.b. Peripharyngeal ciliated band. 



