This note examines the way the pharyngeal feeding filter is constructed within the salp endostyle. Three salp species (Pegea confoederata, Thalia democratica and Salpa fusiformis) have been examined: results from each are essentially similar. The endostyle is a deep ventral groove in the pharynx, divided by a line of long basal cilia. These are flanked by gland cells which secrete the ‘mucus” which forms one of the sets of filaments of the filter. Beside the openings of this basal row of gland cells (marked by bumps of curious ‘mucus’ rodlets), there is a fence of two adjacent rows of cilia. The first of these two rows, (nearest the basal mid-line), whose tips lie next to the rodlet bumps, are regularly spaced 1.0 μm apart along the endostyle, whilst the cilia of the second row are at around half this spacing. Passing upwards in the endostylar groove, this first ciliary fence is followed by a ciliated zone, and then a second row of glandular cells is linked to a second ciliary fence with a single row of cilia only.
Filaments derived from the first set of gland cells pass obliquely over the ciliated zone, and continue over the second ciliary fence. The cilia of the second fence are often surrounded by very striking borders or curtains of rodlets which appear to dissolve, run down the cilia and pass off the tips as filaments, when they are joined by the first set of filaments to form a net with an elongate rhomboidal mesh. As this passes upwards, the mesh changes to become more or less square by the time it comes to overlie a striking asymmetrical band of cilia arranged in columns, when it is now composed of two sets of filaments running vertically and horizontally, spaced some 1.3 μm apart.
Various aspects of the formation of this filter net are considered.