72 CEYLON PEARL OYSTER REPORT. 



Fig. 4. Ventral part of visceral mass of same species, from the side, showing the visceral lobe (v.l.) more 



prominent and more distinctly separated than in fig. 2 ; nat. size. 

 5. Palmate anal funnel of M. margaritifera (Linn.) ; enlarged. 



,, 6. Marginal processes from ventral part of velum; s.h., sensory tufts on tips of branches. 

 7. Similar processes from another individual. 

 8. Simpler processes of the pallia! edge typical forms. 

 9. Some variations in the longer processes of the pallial edge a, from near exhalent orifice ; 



b, c, d, common forms ; e, bifurcate form, unusual all these processes x 4. 

 ,, 10. Semi-diagrammatic view of posterior aspect of living pearl oyster, showing the usual circular 



appearance of exhalent orifice ; enlarged ; Mg.pall., edge of mantle lobe ; Pall./,, pallial fold ; 

 Vel.pr., velar processes interdigitating. 

 ,, 11. An abnormal anal funnel, the tip being bifurcate (compare fig. 3, A); enlarged. 

 12. An abnormal foot (from South-east Cheval), the tip being bifurcate. The lateral branch is non- 

 functional, its groove having no connection with the byssal gutter and pit (compare fig. 18). 

 ,, 13. Dissection showing foot (ventral surface) lying at rest within the mantle cavity ; three byssal 



fibres are present; nat. size. 

 ,, 14. Lateral view of the foot at rest; nat. size. 

 ,, 15. Foot extended in the act of secreting a new byssal fibre; the locomotor region and the byssal 



disc-pit are pressed against the rock a; By. p., byssus pore. 

 ,, 16. Foot retracted from rock to reveal the new byssal fibre; By.d.p., the disc-pit in which the 



attachment disc of the fibre was moulded the lips have partially closed. 

 17. Diagram showing the relative positions of the parts during and after the secretion of a byssal fibre. 

 ,, 18. Ventral surface of foot when, the old byssus having been cast off, the animal is crawling with 



tip of foot searching for a new place of attachment. 

 ,, 19. The same as seen through a glass plate over which the animal is crawling by means of its 



flattened locomotor region ; By.p., byssus pore ; By.d.p.; byssus disc-pit ; By.t., byssus groove ; 



Ped.loc, distal locomotor region. 

 ,, 20. The same when attached by byssus, and elongated for the purpose of forming a fourth byssal 



fibre (compare with fig. 15). 

 ,, 21. Semi-diagrammatic transverse sections through foot; a and b, through the distal locomotor 



region ; e and d, through the proximal secretory region ; x, locomotor surface ; By.t., byssal 



groove ; the shaded part is secretory, the unshaded part mainly muscular (compare figs. 3 and 4 



on Plate VIII.). All these figures of foot are about natural size. 

 22. Sagittal section through foot and byssal organ; By., byssus ; By.p.', pleated surface of byssal 



gland; By.p.gl., glandular tissue of byssal gland ; By.t , byssal groove; My., muscular tissue 



of foot ; Ped.loc,., locomotor distal third of foot. 

 ,, 23. The byssus : A, an entire byssus which was sloughed oft" by the oyster, showing the numerous 



fibres uniting in a common root lodged in the byssal pouch ; B, one side of root of same byssus 



enlarged ; C, reverse side of same root, showing furrows and ridges in dendritic form moulded 



in the pleated surface of the byssal gland the two sides of the root diverge in some cases to 



form two separate masses of rootlets; D, distal end of byssal fibre, showing the disc of 



attachment, a. 

 24. Antero-ventral view of visceral mass, looking towards pericardium to show relative positions of 



the retractor and adductor muscles ; nat. size. 

 25. Dissection from right side to show the course of the retractor muscle from its origin (Bet.) in the 



shell to its insertion (Or.) at the base of the foot. The visceral mass is shaded dark. The 



right mantle lobe is cut away along line a ; nat. size. 

 26. Dissection to show the course of the anterior (Lena.) and the posterior (Lev.p.) levator muscles, 



and the manner of their attachment to the base of the foot (F.). 



