EXPLANATION OF PLATE T^^[ .—Continued. 



occupied by the mouth. Ed. Ectoderm. End. Eudoderm. Mes. Meso- 

 derm. 



Fig. 9. — Lateral surface view of a somewhat older specimen. Letters 

 as in Fig. 8. 



Fig. 10. — A specimen still further advanced in development, seen from 

 the pallial or dorsal surface, so as to show the shell-gland. The super- 

 ficial cells of this region are drawn as they are brought to view by the 

 action of dilute acetic acid. Bv. Dome of the velum area. Vr. Ridge of 

 the velum. 



Fig. 11. — The same specimen before treatment with acetic acid, viewed 

 from the pedal or ventral surface by transmitted light with Hartnack's Oc. 

 iv, Obj. 8. Fr. Ridge of the velum. Sm. Site subsecpiently occupied by 

 the mouth. Bl. Blastopore. Ae. Archenteron. Ed. Ectoderm. E)id. 

 Endoderm. Mes. Mesoderm. Pi. Pedicle of invagination or rectal 

 peduncle. 



Fig, 12. — Lateral view of the same specimen, similarly viewed as a 

 transparent object. I)v. Dome of the velum area. Vr. Marginal ridge of 

 the velum. Sm. Site subsequently occupied by the mouth or stomodseal 

 invagination. F. Foot. Bl. Blastopore. Shgl. Shell-gland. Pi. Pedicle 

 of invagination. 



Fig. 13. — Lateral superficial view of a specimen further advanced in 

 size and developmeni^ representing the Veliger stage. Dv. Dome of the 

 velum area. Vr. Marginal ridge of the velum. M. Mouth or orifice of 

 the stomodseal invagination. F. Foot. An. Anus coinciding witii the 

 blastopore. Shgl. Shell-gland. 



Fig. 14. - The same specimen seen as a transparent object in median 

 dorso-ventral vertical section, Ed. Ectoderm. Mes. Mesoderm. End. 

 Endoderm. Ae. Archenteron. Ph. Stomodseum, the so-called pharynx, 

 being the invagination of the ectoderm to form the most anterior portion 

 of the alimentary canal. Other letters as in the preceding figure. 



Fig. 15. — Surface view from the anterior pole of an embryo intermediate 

 in age between that represented in Fig. 12 and that represented in Fig. 13. 

 The embryo sometimes appears in this position when swimming. Compare 

 this Journal, Vol. XIV, New Series, PI. XVII, fig. 6, for a similar view of 

 the Veliger of Limnseus. 



Fig. 16. — The same embryo as that of Figs. 13 and 14, seen from the 

 pedal or ventral surface. Dv. Dome of the velum area. Vr. Marginal 

 ridge of the velum. M. Mouth. 



