In vivo studies of development of the main functional systems in the heteronemertean pilidium larva

There is performed in vivo morphological study of the White Sea heteronemertines belonging to the type of Pilidium pyramidale (conussoidale). Based on the layer-by-layer microshooting with subsequent computer processing, development of the pilidium digestive, nervous, and muscle systems is described from the stage following at once the gastrula till the premetamorphosis larva. Peculiarities of structural organization of the main functional systems are revealed depending on the larva size and the stage of formation of imaginal discs. It is first shown that even in the not completely formed pilidium, neurons are located not only in envelopes and walls of the digestive tract, but also in the depth of cupola along the central muscle retractor. Their processes are distributed between the main body parts and organs by seeming to perform connections of the apical organ and central muscle retractor with the digestive tract, blades, and the nerve plexus of the cupola wall. In the digestive tract between pharynx and stomach in the formed pilidium, the sphincter is first revealed. It has been shown that in the course of larva development, the non-orderly arranged and poorly developed muscle fibers gradually form in the blade the fan-like, whereas in the cupola wall, the net-like structure.