HABITS, ETC., OF OEKEBRATULUS LACTEUS. 195 



Fia. 3.— Male (left) and female (right) Cerebratulus, the latter showing 

 normal anal papilla. Photograph from life, one eighth life size. 



Fig. 4. — Cerebratulus with proboscis sheath cut open to show how the 

 proboscis is coiled when withdrawn. Photograph from preserved specimen, 

 one eighth life size. 



All the remaining figures, with the exception of Nos. 11, 53, 54, and 

 59, have been drawn with a camera lucida on a Leitz microscope. The 

 magnification is indicated by giving the Leitz numbers of the eye-piece 

 and objective used. The figures have then been reduced one half in 

 making the plates. 



Fig. 5. — Gastrula, showing shape and arrangement of ectodermal cells and 

 beginning of cilia rows. 3 and 7. 



Figs. 6 and 7. — Successive stages in the process of escaping from the egg 

 membranes. 3 and 3. 



Fig. 8. — Gastrula just escaped from the membranes, thirty-eight hours old. 

 Cilia and flagella drawn the exact length seen. 1 and 7. 



Fig. 9. — Larva forty-eight hours old, first stage in formation of apical 

 muscle. ';fl and 7 ; tube drawn 60 mm. 



Fig. 10. — Same, fifty-four hours old, second stage; side view. 



Fig. 11. — Same, ninety-six hours old, third stage. Zeiss camera lucida, 

 magnified 575 diams. 



Fig. 12. — Same, 108 hours old, fourth stage; end view, land 7; tube 

 drawn 60 mm. 



Fig. 13. — Same, 120 hours old, fifth stage ; side view, showing apical muscle 

 attached to dorsal wall of oesophagus. 



Fig. 14. — Same, six days old, sixth stage, showing apical muscle divided 

 and extending down on either side of the intestine to fasten to the oral surface 

 anterior to the mouth. This figure shows also interparietal, post-cesophageal, 

 and circumoral muscles. 



Fig. 15. — Same, ten days old, seventh stage. Apical muscle fully de- 

 veloped ; post-oesophageal muscle forming a triangular sheet ; radiating lappet 

 muscles well developed. 



Fig. 16.— Same, twelve days old; surface view showing parietal muscles. 



Fig. 17. — Transverse interparietal muscle attached to apical plate, from 

 larva six days old. 1 and 7. 



Fig. 18. — Eggs taken from ripe ovary and examined before they have 

 touched any water. 3 and 3. 



Fig. 19. — Same after immersion in salt water three minutes. 



Fig. 20.— a ripe, freshly laid ovum, unfertilised, showing membranes and 

 attachment protuberance. 1 and 7. 



