Fine structure of muscle and other components of the trunk of Sagitta setosa (Chaetognath).

The histology of the Chaetognath's trunk is largely based upon works of Hertwig (1880) and Burfield (1927) and is revised here essentially on ultrastructural basis, in a study on Sagitta setosa. The trunk is composed of a thick multilayered epidermis, without cuticle, and contains an abundant nervous system. It is separated from the underlying general musculature by a "basement membrane" which is not crossed by nerves fibres in the region of the ventral ganglion. We have been mainly interested in the muscular tissue. It has no close contact with the body cavity but is surrounded by a thin monolayer epithelium. The general musculature is composed of two types of muscles. The first one, forming the primary musculature, is divided by mesenteries and lateral fields into four quadrants. It contains groups of fibres (C then A) alternating with other groups of fibres (B). The second one, constituting the secondary musculature, is divided into four bands. Both types of fibres are readily identifiable by their myofibrils. The fibres of primary musculature, in close contact with their membrane level, have no basement lamina except at the myoepidermic junctions.