EXPLANATION OF PLATE VI. 
Fig. 1. Transverse section of lower valve, exhibiting a series of cavities 
formed by the oyster in its attempts to cover over the various 
patches of mud collected by the worms. The entrances to 
some of the cavities may be seen on the 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 inwards. 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. 
