CILIARY MECHANISMS OF LAMELLIBRANCHS 701 
the ventral wall of the mantle space, as described for several 
forms. 
There is much difference in individuals of the same species in 
the strength or activity of any tract, even where external con- 
ditions apparently have been the same. No marked variations 
in the positions of tracts were noticed. 
The aid of the nervous system and of muscles is frequently 
necessary to the operation of cilia tracts, as in the exposure of 
hidden outgoing tracts by the spreading apart of palp folds, the 
apposition of ventral margin and folds, of palps and gills, gills 
and mantle, and gills and visceral mass. Besides this there are 
violent expelling contortions of gills, palps, and mantle edge, 
sudden contractions of adductor muscles, and other similar 
movements. 
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