Principle of main body components and phylogenetic relations between coelomate phyla: 2. levels of coelom organization and phylogeny of Protostomia

摘要

This is a continuation of the first communication, in which the principle of main body components and the closely associated concept of the evolutionary levels of coelom organization were elaborated for purposes of the phylogeny of coelomates. The mollusks are at the proarchicoelomate level of organization; they have small metacoel (pericard) and larger mesocoel (commonly playing the part of a gonocoel); no acrocoel (first coelom) is present (very likely primarily). The archicoelomate level is shared by Sipunculida (the only protostomian phylum with developed ventro-postanal body extension) and Tentaculata (=Podaxonia), where I also include the pleuropygian "brachiopods" with chitin-phosphate shells. The representatives of the last group are similar to phoronids in the presence of ventral body projection, testified by the presence of lateral mesenteries. The protomocoelomate level is occupied by annelids primarily lacking growth zone (e.g., Hirudinea), as well as by Echiurida (judging from the segmentation pattern in males) and Pogonophora. More precise differentiation between the annelid groups of this level needs additional exact information on the presence or primary absence of the growth zone in these groups. The relationship between Protostomia and Pogonophora is proved by the presence, in some species of the latter of trochophore-type larvae and by characteristic opisthosomal somites. Recent data made it possible to attribute Nemertini to the same level, since their rhynchocoel, gonads, and lateral "blood vessels" are considered to be coelomic formations, while the multitude of gonads and gonoducts testifies to body segmentation in their ancestors. The tomocoelomate level involves Annelida and Arthropoda, both with present or secondarilylost growth zone. A list of 8 holapomorphic characters for arthropods is given, suggesting their monophyletic origin. The author's views are illustrated by the accompanying phylogenetic scheme.