KEY TO PLATE VI 



Figs. 1-9. Sections through full-grown oyster hirvav 1-8, transverse; 

 9, median sagittal. All the transverse sections are looked at from behind, 

 so that dorsal is above and left is on the observer's left. Their position 

 should be located on fig. 9 (or by comparison with PI. V, figs. 30, 32). 

 l^^asily recognizable parts are generally indicated where they first occur 

 and sometimes not afterwards. 



Fig. 1. Section across the anterior end (6th of a series of 32 sections): 

 ad, anterior adductor muscle; v, velum; m, mouth; s, shell; mn, border 

 of mantle. 



Fig. 2. Second section behind the preceding in the same series: /, tip 

 of foot. 



Fig. 3. Section of another series (slanting downwards and forwards): 

 st, stomach; oe, oesophagus; I, liver; no, supra-ccsophageal nerve com- 

 missure; e, pigmentary eye-spot; p, rudiment of palps; v, velum. (Above 

 the velum on a level with the eye-spot is a crumpled layer of chitin extend- 

 ing crosswise; above the eye, next to the mantle, is a retractor muscle 

 of the velum.) 



Fig. 4. Section of same series as 1 and 2 (14th of the series, slanting 

 slightly down and liack. On account of the asymmetry of the larva, and 

 also on account of the difficulty of orientation the two sides are not quite 

 alike.) hi, hinge ligament; st, stomach; p, rudiment of palps above (in 

 front of) the cesophagus; e, eye-spot; rig, right gill (what becomes the 

 right inner hemibranch of the adult gill). The left gill is larger than the 

 right and its filaments were slanting so that three w^ere cut across; np, 

 infra-oesophageal or pedal commissure; /, foot; ot, otocyst (right one 

 also showing) ; ch, chitin in the crease between velum and body. 



Fig. 5. Section 17 of the series; I, liver; b, blood vessel; bd, duct of 

 the byssus gland in the heel of the foot (in Fig. 4 it is cut across and in each 

 side are some follicles). Part of the stomach, the a?sophagus near where 

 it enters the stomach, one eye-spot and the gills are also shown. 



Fig. 6. Section 23 of the series: h heart containing blood-cells; vg, 

 visceral ganglion; pd, posterior adductor muscle. Stomach and lobe 

 of liver showing. 



Fig. 7. Section 16 of same series as Fig. 3. pd, posterior adductor 

 muscle; vg, visceral ganglia; /, heel of foot; g, gills of opposite sides 

 uniting l^ehind foot. 



Fig. 8. Second section behind preceding: sb. supra-bi-auchial chamber; 

 be, branchial chamber. 



Fig. 9. Median sagittal section through larva. The velum and foot 

 are withdrawn tightly up against the body and partly turned sideways. 



