ИСТИНА

Aplacophora is an aberrant group of mollusks with a vermiform body devoid of a shell and covered with a thick cuticle with calcareous spicules. Their coelom is reduced to small sizes and is represented by the pericardium and the gonad cavity, thus the hemocoels take on the supporting function. Circulatory system organized into the system of sinuses by diaphragms. The similar organization represents in chitons (Polyplacophora). Thus, the question of the homology of the diaphragms in these groups remains open. The general morphology and ultrastructure of the circulatory and gonopericardial system of Crystallophrisson nitens (Chaetodermatida) were studied by light and electronic microscopy and three-dimensional computer reconstruction. The gonopericardial system consist of gonad, paired gonopericardial ducts, pericardium, paired gonoducts 1 and gonoducts 2. A distinctive feature of the organization of the body cavity of the aculifera is the presence of the main sinuses (cerebral, visceral and ventral), dorso-ventral septum in the anterior part of the body, the horizontal diaphragm and the dorsal aorta. On the basis of these data, a general circulation scheme for aculifera: the ventral sinus carries blood from the cerebral sinus to the gills, the blood enters and oxygenated in the system of gill sinuses and then blood returns through the visceral sinus to the head. However, the heart of Aplacofora has several distinctive features: the atrium has a connection with the dorsal sinus, and the ventricle is completely closed.