Further Studies on the Latero-Frontal Tracts of Bivalves

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