Following an examination by transmission and scanning electron microscopy of the latero-frontal tracts on the gills of
Nucula sulcata, Ostrea edulis
and
Chlamys varia
, it is suggested that only two types of structure are involved, namely compound eu-latero-frontal cirri and pro-laterofrontal cilia; the terms ‘anomalous latero-frontal’, ‘para-latero-frontal’ and ‘micro-latero-frontal’ should be dropped. Each latero-frontal tract of
N. sulcata
consists of a row of compound eu-latero-frontal cirri and four rows of pro-latero-frontal cilia. Each cirrus is borne by a single cell and consists of some 20 pairs of cilia arranged in two parallel, alternating rows. Individual cilia leave the shaft of the cirrus at regular intervals on each side but there is no stiffening element present in the region of the bend. Each latero-frontal tract on the plicate, heterorhabdic gill of
O. edulis
consists of a single row of compound cirri and two alternating rows of pro-latero-frontal cilia. The cirri of the principal filaments are spaced 1.5- 2.0 pm apart and consist of 10 or 11 pairs of cilia. Those on the ordinary filaments forming the crests of the plicae are spaced 2.5 μm apart and consist of but 6 or 7 pairs of cilia. As in
N. sulcata
, the individual cilia bend to either side of the main axis of each cirrus but, unlike those of
N. sulcata
, a small stiffening element is present in the immediate region of the bend. It is concluded th at the Ostreidae should not be grouped with the Microciliobranchia. The latero-frontal tracts of the plicate, heterorhabdic gills of
C. varia
consist of a single row of pro-latero-frontal cilia only. In both
O. edulis
and
C. varia
, mucous glands and sensory ciliary tufts occur mainly along the frontal faces of the ordinary filaments forming the crests of the plicae; the glands of
O. edulis
appear to contain a neutral mueoprotein and those of
C. varia
an acid mucopolysaccharide. The principal filaments of
C. varia
are capable of marked changes in form with consequent effects on the nature of the plication of the gill. When few particles are presented to the gill the principal filaments are U-shaped in section and form a pronounced gutter at the base of the grooves between adjacent plicae. It is suggested that collection of particles for possible ingestion is by way of water currents which flow dorsally in the U-shaped principal filaments rather than by any straining effect by the latero-frontal tracts. Correlated with this method of collection the frontal cilia of the principal filaments are arranged in three well defined tracts, all of which beat dorsally. When the concentration of particles presented to the gill is increased, the principal filaments temporarily lose their U-shape and, in extreme cases, become T-shaped in section; the current flow referred to above thus breaks down. Increased amounts of mucus are secreted by the glands associated with the ordinary filaments and mucus, plus entangled particles, are carried ventrally to the free margins of the demibranchs by the frontal tracts of the ordinary filaments. It is suggested that the collection of particles by way of water currents, as proposed for the gill of
C. varia
, has been developed in those bivalves in which the latero-frontal tracts consist of pro-latero-frontal cilia only, that is the Microciliobranchia excluding the Ostreidae. In those lamellibranch bivalves in which the latero-frontal tracts consist of compound eu-latero-frontal cirri in addition to pro-latero-frontal cilia, the collection of particles is achieved primarily by the straining action of the cirri and the retained material is transported by direct ciliary action rather than in suspension by water currents.
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