Morphology of salp chain communication

Abstract

The plaque consists on one side of a small button of modified epithelial cells (plate 1, 5 and 6) and on the other of a small group of sensory cells, which are less conspicuous than the button cells. At the light microscope level, the button cells can be seen to form a sort of small button or mushroom protuding from the epithelium of the plaque (plate 1, 11 and 12; figure 2) into the test material separating the two epithelial layers. Opposite this button are a group of sensory cells, the axons of which pass to the brain shrouded in the bases of the epithelial cells (plate 1, 8 and 9). The arrangement of the button cells and opposing sensory cells is a constant one, so that, on a single zooid, one of the pair of plaques joining it to an adjacent zooid has a group of sensory cells, the other (the non-innervated plaque), a group of button cells. The plaques at the outer ends of the horns are innervated, as are those near the horn bases, while those on the ventral surface and at the inner end of the horns are not innervated, and have the button cells. In consequence of this, it is a simple matter to separate one or other of the plaques joining two zooids to investigate their physiological properties (Anderson & Bone 1980). Fedele (1920) first described the sensory cells of the attachment plaques, but he supposed that they occurred at both sides in each plaque. In fact, they lie only on one side of the plaque, as described above, and this asymmetry has important consequences for the way in which the system operates. Apart from the specialized region of the plaque, where these cells are found, the plaque is symmetrical, and the two epithelial layers are some 5 μm apart, separated by test material (plate 1, 7). The epithelial cells are flattened (like those of the outer epithelium outside the plaques), and are some 2–5 pm thick (plate 2, 1). They contain an extensive smooth endoplasmic reticulum, mitochondria with tubular cristae, flattened nuclei, and occasional vesicles of various sizes