The unique embryology of Chaetognatha

Chartognaths are doubtlessly coelomate Bilateria, but after doubting the re lationship of Chaetognatha and Deuterostomia there is no phylogenetic basis for considering the embryology of chaetognaths as a kind of enterocoely, o n the contrary it differs considerably from an enterocoely. The pattern of the embryonic development and especially of the stages deter mining the further formations or the body organisation are outlined. This d evelopmental pattern is characterised by a blastocoel which soon after gast rulation disappears - hence the mesoderm cannot grow between ectoderm and e ntoderm - and by an entoderm which is so closely connected to the mesoderm that it does not even persist as a primitive gut: A part of the entoderm de velops into epithelial folds being anlagen of coelom walls as well as gut a nlagen; the other part of the entoderm differentiates directly to coelom wa ll anlagen. All the embryonic organs grow in the cavity enclosed by the ori ginal entoderm. I propose to term this unique mode of embryonic development heterocoely and place it beside enterocoely and metamerism.