KEY TO PLATE VII 



Figs. 1-9. Transverse sections through spat of 1 mm. length (compare 

 PL II, fig. 5 and PI. V, fig. 33). This is a size suflficiently large to be recog- 

 nized with a lens. 



Fig. 1. Section 17 of a series of 53: 



c, cement, of the left valve; ul, upper lip of proliferating epithe- 

 lium; gf, gill filament of left inner hemibranch; he, branchial 

 chamber; Us, inter-lamellar space (Avater tube) opening later- 

 ally between the filaments by ostia; s, shell (right valve); mn; 

 thickened tentacular border of the mantle. 

 Fig. 2. Section 20: 



m, mouth; ul, upper lip and l^eginning of palp. 

 Fig. 3. Section 22: 



m, mouth; II, lower lip and beginning of [^alp; ab, anterior tip of 

 abdomen and shrivelled foot. The lips of the right side 

 (uppermost) open farthest back. 

 Fig. 4. Section 24: 



oe, oesophagus (mouth closed in on both sides); I, liver; rig, 

 right inner hemibranch ; lig, left inner hemil)ranch. 

 Fig. 5. Section 26: I, liver. 



Fig. 6. Section 31. The upper of the two lobes of the liver on the 

 left is narrowed at its middle, and in the crease between the two parts, 

 as also between the two lobes, lies a fold of the intestine; st, stomach; 

 i, origin of the intestine; k, kidney; vg, visceral ganglion. 



Fig. 7. Section 33: pel, posterior adductor muscle; she, supra- 

 branchial chamber. 



Fig. 8. Section 39: h, heart. 



Fig. 9. Section 45: m/?, folds of the mantle; r, rectum; a, anus. 



Figs. 10-12. Sections of 1-5 mm. spat, slanting downwards and for- 

 wards. The left valve was flattened against the shell on which it was 

 fixed. 



Fig. 10. Anterior (upper) section: I, liver; oe, oesophagus; m, roof 

 of mouth cavity. 



Fig. 11. Second section following, st, stomach; p, space between 

 outer (upper) and inner (lower) palp of right side. Similarly on the left. 



Fig. 12. Second section following. The palps become separate. 



Fig. 13. Transverse section through the mouth of a 2 mm. spat: p. 

 outer and inner palps of the right side. 



Fig. 14. Section of 2-5 mm. spat: sbs, supra-branchial slit leading 

 from the supra-branchial cavity on to the dorsal surface; rog, right outer 



