ИСТИНА

The gill slits are the mysterious structures special only for two phyla of Deuterostomia. It is difficult to image from the evolutionary viewpoint the origin of numerous paired openings leading from the gut to the exterior. We studied the branchial apparatus and the gill skeleton in acorn worm Saccoglossus mereschkowskii by methods of the 3D-reconstruction. In Saccoglossus mereschkowskii (Harrimaniidae), slits of the first pair communicate straight with the pharynx, and the collar coelomoducts run into them. The first pair of the gill slits communicates with exterior, not straight, but through the branchial sacs of the second pair of the gill slits. We suggest the following hypothesis of the metameric gill slits origin. The ancestral organisms had the metameric coelomoducts leading both to the gut and to the exterior. The gut openings functioned as excretory enteronephridia. The exterior pores functioned as gonoducts during the reproduction. The coelomoducts remain in the collar segment, but the exterior pores are reduced here. In the anterior part of the trunk, the coelomoducts are reduced, but both openings remain. Therefore the gut communicates with the exterior. In the posterior part of the trunk, the enteronephridia are reduced while the metameric gonoducts remain. This hypothesis explains the origin of metameric gill slits on the base of prior existed metameric coelomoducts.