Key to Plate VI — Continued 



s^, stomach; no, supra-cesophageal nervous system; ad, anterior adductor 

 muscle; v, velum; mn, border of mantle; oe, oesophagus; /, foot; j)d, 

 posterior adductor muscle; h, heart. 



Figs. 10-15. Transverse sections from a series of 30 through the 

 youngest oyster spat (compare PI. II, fig. 1). This spat, caught on a 

 strip of glass, was actually smaller than the oldest larv* of the preceding 

 sections. It measured 53 (53 x 0-9= -30 mm.), while they measured 55. 

 There had not been the slightest growth of shell since the attachment, 

 but the left valve is thickened by byssus cement which is not to be found 

 on the right and is not dissolved av/ay by decalcification. 



Fig. 10. Section 2 of the series: s, shell; c, cement; ad, anterior 

 adductor muscle; mn, mantle. 



Fig. 11. Section 8 of the series: k, knolj or drop of cement; v, velum^ 

 still present in the fixed spat; m, mouth. 

 Fig. 12. Section 10 of the series: 



p, rudimentary palps, between velum and mouth; /, tip of foot. 

 Fig. 13. Section 14 of the series: 



st, stomach; I, liver; oe, oesophagus; g, gill; /, foot. 

 Fig. 14. Section 17 of the series: stomach, two lobes of liver, right 



and left gills, foot, mantle, and shell. 

 Fig. 15. Section 23 of the series: stomach, two lobes of liver, heart 

 with l^lood, visceral commissure, posterior undivided knob of 

 gill axis. 



Figs. 16-20. Sections 5, 8, 13, 20, 23 through a spat of length 60 

 (=•41 mm.). Compare PI. II, fig. 2; ep, high epithelium in region of 

 mouth; r, rectum. The velum is gone and the foot greatly reduced. 



Figs. 21-22. Sections 5 and 15 through spat of length 72 (= -49 mm.). 

 Compare PI. II, fig. 3. The mouth is twisted to the right (upwards). 



Fig. 23. Part of two sagittal sections of a spat 105 in length put 

 together. 



Figs. 24-26. Sections 8, 9, 10 of a spat 110 length (=-75 mm.). 

 Compare PI. II, fig. 4. The mouth is twisted to the right (upwards) 

 and shows the mode of origin of the upper and lower palps of that side. 

 The left gill (left inner hemibranch) is partly split into outer and inner 

 (lower and upper) lamelkr. In 26 the smaller right gill unites with the 

 larger left one. 



Figs. 27-28. Sections 5 and 6 of a spat of 115 units ( = «79 mm.). 

 The mouth and palps are not twisted out of position or grown upwards 

 towards the right side. The foot is shrunk to form only a muscular ridge 

 along the abdomen. The left valve of the shell is damaged, as often 

 happens in separating the spat from the object on which it is fastened. 



