EXPLANATION OF PLATE 4. 



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 valve there was no communication f i-om without. 



„ 6. 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 8. 



„ 10. Portion of a blister showing the inequalities on the inner sm-face. See page 6. 



