234 ARTHUR WILLEY. 



on the right side. Notice the coeloni just in front of the caecum. The blood- 

 vessel in this region is contractile. The endostyle is farther advanced than in 

 the preceding figure. The right and left walls of the velum are quite opposite 

 to one another. 



Fig. 17. — Stage VIII. Combination of three focussings, showing the last 

 (seventh) pair of slits of a larva in which the ninth primary slit was in a 

 rudimentary condition in the mid-ventral line. 



Fig. 18. — Stage V. Right side (anterior portion only), to show the endo- 

 style, which has grown a long way past the club-shaped gland between the 

 primary and secondary slits. 



Fig. 19. — Stage VI. Similar view of right side, to show that the club- 

 shaped gland has now atrophied, its remains being still visible on the 

 endostyle. 



Fig. 20. — Stage I. Left or oral aspect of a larva. Note pointed anterior 

 extremity of mouth, ciliated groove, external orifice of club-shaped gland, and 

 one element of buccal skeleton. 



Fig. 21. — Stage I. Similar view. Antero-dorsal margin of mouth, com- 

 mencing to hunch up into the ciliated groove. Two elements of buccal 

 skeleton. 



Fig. 22. — Stage II. Similar view. Hunching up and sinking inwards of 

 the anterior portion of the mouth continued. Three elements of buccal 

 skeleton. 



Fig. 23. — Stage II. Similar view. Four elements of buccal skeleton, 

 commencing to grow out as tentacles. When the very young cirri are seen 

 end-on they present a circular appearance. 



Fig. 24. — Stage II. Similar view. Five elements of buccal skeleton. 



Fig. 25. — Stage III. Similar view, to show the anterior half of the mouth 

 bent at a deep focus towards the right side. Note also the two portions — 

 upper and lower — of the oral hood, the latter containing the buccal skeleton. 



Fig. 26. — Stage IV. Similar view. This is an important view of the mouth, 

 showing its change in shape but not diminution in size. The external orifice 

 of the club-shaped gland is covered over by the buccal cirri. 



