EXPLANATION OF PLATE VI. 



Fig. 1. Transverse section of lower valve, exhibiting a series of cavities 

 formed by the oystcjr in its attempts to cover over the various 

 patches of mnd collected by the worms. The entrances to 

 some of the cavities may be seen on tlie right hand side of the 

 figure (the anterior margin). 



„ 2. Section of upper valve showing two cavities, with the openings 

 also on the anterior margin. 



„ 3. Upper valve showing the extent of the mud patch collected by a 

 single worm, and the surface of the mud covered by a thin 

 uncalcified membrane. 



„ 4. Tubes erected by Polydora at the aperture of its tunnel. The 

 attenuated tentacles are seen protruding from the mouth of 

 one. Enlarged under a lens, after Prof. Mcintosh. 



„ 5. Upper valve showing an elevated nodule ; near its summit is the 

 tube of the worm projecting at right angles to that of the 

 nodule ; the latter is so situated that when the oyster closed 

 its valves there was no communication from without. 



,, G. Edge of an old shell, exhibiting the grooves made by the action 

 of the worms in moving in and out of tubes. The grooves only 

 exist at the margin, and disappear entirely inwai'ds. Slightly 

 enlarged. 



„ 7, 8, & 9. Sections of shells showing the openings of the tubes 

 occupied by worms. 



Fig, 7, first stage ; Fig. 8, second stage ; Fig. 9, third stage. 

 Enlarged three times. See page 48 



„ 10. Portion of a blister showing the inequalities on the inner surface. 

 See page 46. 



