BIOLOGIYA MORYA, 2015, Vol. 41, No. 2, pp. 81-91 |
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Secondary mouth and its phylogenetic significance Saint Petersburg State University, St. Petersburg 199034 E-mail: The present paper discusses the evolutionary relations between the blastopore and openings of the digestive tract in Bilateria, which have been often considered to be of 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 precursor 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 have 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), without relation to the blastopore. Sometimes, the same result is achieved by amphistomy - 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, thus leading to deuterostomy. 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 in ontogeny. The hypotheses by Mamkaev (1991), Martindale, Hejnol (2009), and Martin-Duran et al. (2012) are analyzed. It is proposed to divide Bilateria into three major groups: Acoelomata, Trochozoa, and Enterocoelia. Key words: blastopore, mouth, anus, protostomy, deuterostomy, amphistomy. |
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