Key to Plate VII — Continued 



hemibranch. The filaments are cut lengthwise or nearly so in contra- 

 distinction to Fig. 13 where they are cut across. 



Fig. 15. Section of 3mm. spat. Similar to last Ijut with rudiment 

 of log, left outer hemibranch. Both halves of Ijoth gills are now repre- 

 sented. 



Fig. 16. Section of 3' 5 mm. spat just l)ehind the mouth and oeso- 

 phagus but through the palps (compare Fig. 12). 



Fig. 17. Section just in front of the adductor muscle of a 4 mm. spat: 

 r, rectum; h, heart; pd, adductor muscle; k, kidney; .s^, descending 

 portion from stomach (compare Fig. 7). 



Fig. 18. Section 199 from a series of 307 of a 10-5 mm. spat (the 

 extracted soft parts measured over 7 mm. long and 4 mm. deep) ; r, 

 rectum; h, portion of heart; pd, anterior fibres of adductor; sV-, descend- 

 ing loop from stomach; k, kidney; k^, nephropore. 



Fig. 19. Section 254 from a series of 537 (I stopped sectioning 

 after passing beyond the visceral ganglion) through a 21 mm. spat of 

 which the soft parts measured 14 mm. long and 9 deep, .sf^ descending 

 loop from stomach; i, ascending portion; r, rectum; I, liver follicles; a, 

 aorta; t,t, minute empty follicles of the gonad (testis) which would be 

 unrecognizable but for comparison with more mature stages; d, duct 

 (gonaduct) which runs forwards but does not connect at this stage with 

 the gonad. 



Fig. 20. Section through the same region of a 32 mm. (nearly 1^ inch) 

 oyster (the soft parts measured 24 x 14 mm.). The follicles of the testis 

 are distended with millions of spermatozoa, the whole gonad staining 

 deeply and standing out in contrast to the rest of the oyster. 



