JOURNAL OF MICROSCOPICAL SCIENCE. 



DESCRIPTION OF PLATE XXXI, 



Illustrating Prof. Haddou's " Notes on the Development 

 of Mollusca.'^ 



The outlines of all Ihe figures were traced with the camera lucida. 



a., anus ; au., auditory iuvolution ; 01.^ blastopore ; c. e., ciliated epi- 

 thelium of foot ; e, eye ; e/J., epiblast ; e. s., egg-shell ; ft., foot ; hy., 

 hypoblast; i., intestine; i. o. c, infra-oral circlet of cilia; /., liver (with 

 remainder of yolk) ; m., mesoblast ; m. c, mantle-cavity, within whicli the 

 velum can be retracted ; mth., mouth ; a;., oesophagus ; op., tem])orary 

 operculum; opt., optic involution; of,, otocyst; /;,, pigment; p. c, 

 polar cells ; p. g., pedal ganglion ; p. k., provisional kidney ; r., rectum ; 

 r. m., retractor muscle ; s., shell : s. g., supra-oesophageal ganglion : st., 

 stomach ; v., velum ; y., yolk ; i/. c, yolk-cells. 



Fig.]. Mysia viridis. Figs. 2 — 5. Fiona sp. ? Figs. 6 — 11, Janthina 

 fragtlis. Figs. 12 — 14. Purpura lapillus. Figs. 15 — 17. Murex erinaceus? 



Fig. 1. — Gastrula formation, with elongated blastopore, x 320. 



Fig. 2. — Gastrula, blastopore still slit-like, x 320. Compare Lan- 

 kester, ' Phil. Trans.,' 1875, Plate 9, fig. 5, and Plate 10, fig. 2. 



Fig. 3. — Young veliger, with boat shaped larval-shell, showing the two 

 cilia (.r) in the centre of the velum. Note the temporary larval shell and 

 operculum. 



Fig. 4, — Oral view of velum of an older embryo, showing the velar 

 groove with the superior motor cilia and the infra-oral circlet of cilia 

 above the mouth. The two central cilia {x) are seen in the distance, x G30. 



Fig. 5. — Three-quarter aboral view of an old veliger, with ovoid shell, 

 showing the velar groove and central pair of cilia. 



Fig. 6.— Ovum, with the four primitive epiblast cells segmenting off 

 from the four primary segmentation spheres. 



Fig. 7. — A similar ovum with the process completed. 



Fig. 8. — The succeeding stage. 



Fig. 9. — Solid blastula with an epiblastic cap. 



Fig. 10. — The cap of epiblast cells growing round the large yolk cells, 

 thus forming a gastrula by epibole — at the junction of the epiblast with the 

 yolk cells, i.e. the lip of the blastopore — are two mesoderm cells {ni). 



Fig. 11. — Right-side view of a well-developed veliger, with all the larval 

 organs ; aboral to the ciliated aims («.) is the large violet-coloured pigment 

 mass. The shell itself is of a light madder tint. Note the temporary 

 operculum (o/j.). Compare Lankester, ' Phil. Trans.,' 1875, Plate 8, fig. 28. 



Fig. 12. — Left-side view of a young veliger with rudimentary shell (*.), 

 showing the surface organs, x 90. 



Fig. 13. — Transverse section through anterior end of above, showing 

 the paired patches of thickened and proliferating epiblast in the velum and 

 foot. X 90. 



Fig. 14. — Enlarged view of a portion of above, showing the formation 

 of the supra-oesophageal ganglion, x 320. 



Fig. IS.^Transverse section through the foot, showing the origin of 

 the pedal ganglion by the side of the already formed otocyst. Above the 

 ciliated epithelium of the foot is a section of a portion of the oesophagus. 



Fig. 16. — Section through the optic involution ; the black pigment of 

 the retina is already being formed at the fundus. 



Fig. 17. — Section through the auditory involution. 



