The present paper discusses the evolutionary relationships between the blastopore and openings of the digestive tract in Bilateria, which have often been though to have great phylogenetic significance. In the case of multipolar immigration as the original type of gastrulation, the blastopore is absent, although the mouth is formed. The blastopore appeared simultaneously with gastrulation by invagination and became a pecursor of the mouth opening. Initially (in Acoelomorpha), the gastric cavity had only one opening (the mouth), which was located at the posterior end of the body. Subsequently, various ontogenetic changes resulted in a forward shift of the mouth. Concurrently, the anal opening emerged at the posterior end of the body from a new invagination of the ectoderm (proctodeum), which was not related to the blastopore. The same result is sometimes achieved by amphistomy, viz., the separation of the blastopore into two holes. In some animals, the connection between the mouth opening and the blastopore began to weaken and a stomodeum began to form at the anterior end of the body; deuterostomy thus emerged. A secondary mouth is inherent not only in Deuterostomia, but also in Priapulida, Nematomorpha, some Nemertea, Crustacea, and other animal taxa; therefore, it is of little phylogenetic significance. In all likelihood, deuterostomy is only the most efficient way of forming a through digestive tract during ontogeny. The hypotheses by Mamkaev , Martindale and Hejnol , and Martin-Duran et al.  are analyzed. It is proposed to divide Bilateria into three major groups: Acoelomata, Trochozoa, and Enterocoelia.
Russian Journal of Marine Biology – Springer Journals
Published: May 27, 2015
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