THE ENTBEOPNEUSl^A. 57 



Fig. 2.— Similar view of an older animal, with two gill-slits. (Obj. A, oc. 2.) 



Pig. 3. — Similar view of a three-gill-slit larva. (Obj. A, oc. 2.) 



Fig. 3a. — Posterior end of the same in a retiacted state, seen in profile 

 (from a preserved specimen). (Obj. A, oc. 2.) 



Fig. 4.— Side view of larva with five pairs of gill-slits. The fold of the 

 operculum covering part of the first gill-slit is semi-transparent. (Obj. A, oc. 2.) 



Fig. 5. — Side view of preserved specimen with nine pairs of gill-slits 

 Owing to the contraction of the body and the protrusion of some of the 

 valves, few only of these are visible. (Obj. AA, oc. 2.) 



Fig. 6.— The adult animal {^). (x 2 diameters.) 



Fig. 7. — Longitudinal vertical section (not quite median) of a larva in 

 Stage F. (Obj. C, oc. 2.) 



Fig. 8. — Cells of mesoblast of anterior pouch, from a transverse section of 

 a larva shortly after this pouch is closed off from the hypoblast. (Obj. F, 

 oc. 2.) 



Fig. 9. — The same tissue from the posterior third of the proboscis of a 

 rather older larva, showing the proliferation and commencing differentiation of 

 the mesoblastic elements. (Obj. D, oc. 2.) 



Fig. 10. — Section similar to Fig. 7, from a rather older larva to show 

 anterior, dorsal structures (Stage G — H). (Obj. D, oc. 2.) 



Figs. 11 — 20 represent transverse sections of a larva in Stage G — H, with 

 the exception of Figs. 12 and 16, which were drawn under Obj. F, oc. 2. All 

 these figures were drawn under Obj. CO, oc. 2. In most of these figures the 

 ectoderm is only indicated on a short arc of the circle of the whole section. 

 They are numbered from before backwards. 



Fig. 11. — Transverse section of proboscis cavity. Mesoblastic elements 

 distributed nearly uniformly all round the interior. 



Fig. 12. — Portion of these mesoblastic elements more highly magnified, to 

 exhibit the differentiations. 



• Fig. 13. — Section taken behind Fig. 11. The mesoblastic layer is thinner 

 dorsally than elsewhere. 



Fig. 14. — In this section the septum separating the two horns of the cavity 

 is reached. 



Fig. 15. — Still further back the first rudiment of the proboscis gland is 

 reached (cp. Fig. 29). 



Fig. 16. — The rudiment of the gland lying in the septum more highly 

 magnified. 



Fig. 17. — Section across the proboscis stalk. The anterior end of the 

 notochord is reached. The back of the gland is nearly passed. The meso- 

 blastic horn of the left side is apparently divided into two parts, this appear- 

 ance is due to shrinking. 



Fig. 18. — The lumen of the notochord is reached. The extreme ends of the 

 two anterior mesoblastic horns and the posterior apex of the left horn from 



