Abstract
The morphology of gill slits in the appendicularian Oikopleura gracilis (Oikopleuridae) has been studied using light, scanning, and transmission-electron microscopy. The gill slits are a pair of tube-shaped outgrowths of the single-layered flat epithelium of the ventro-lateral pharynx wall. Each gill slit, in its middle part, has a thickening in the form of a ciliated epithelium, which is referred to as a ciliary ring, or stigma. It is formed by cells of two types: multiciliated cells, arranged in four parallel rows, and two rows of higher parietal cells, which form a kind of pocket for the first type of cells. The gill-slit epithelium contacts the single-layered epidermis of the pharyngo-branchial and digestive compartments of the trunk; thus, the pharyngeal cavity is connected with the external environment through the gill slits. The issue of the oligomerization of gill slits in appendicularians is discussed in the context of the hypothesis of their neotenic origin from the ancestral larvae of ascidians.
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