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The article focuses on the anterior and posterior filter valves. Two basic types of mucous secreting epithelia are found associated with the structures. The first type, here called the pitted type, is found on the roof of the pharynx immediately above the posterior filter valve in two bands lying over the filter slit between the valves. The second type referred to as the ridged type is found on the underside of the anterior filter value and forms an integral part of the valve. The pitted type epithelium consists basically of clusters of columnar secretory cells arranged so that their free ends converge to form a secretory pit.