Principle of main body components and phylogenetic relations between coelomate phyla: 1. main body components, evolution of coelom, and phylogeny of Deuterostomia

摘要

Principle of the main body components and the associated concept of the evolutionary levels of coelom organization are briefly outlined. These views make it possible to cut significantly the number of the existing hypotheses concerning the phylogenetic relations between the coelomate phyla. Myzostomida and Dinophilida are regarded as lower coelomates (eocoelomate level). The coelomates had separated into Proto- and Deuterostomia at the proarchicoelomate level, now shared by Mollusca and Testicardines. Further evolution of the coelom proceeded independently in Proto- and Deuterostomia. A transition to the archicoelomate level occurred through growth of the metacoel (often becoming in the process the main body cavity) and the corresponding body part. This can be seen even in closely related groups, such as Testicardines and Crania. The author's hypothesis, postulating separation of the coelomate animals into Proto- and Deuterostomia as early as at the proarchicoelomate level, makes it possible to supplement their respective diagnoses by five additional pairs of characters: coelom formation; types of larvae; paired or unpaired acrocoels (first coeloms), and presence or absence of coelomoducts in these; degree of ventropostanal body extension; and shape of somites in segmented organisms. With the brachiopods (except Pleuropygia, i.e. brachiopods with chitin-phosphate shells) or, at least, Testicardines, which are organisms at the proarchicoelomate level, accepted as the lower deuterostomes, it is further possible to identify some other deuterostomian groups remaining at the archicoelomate level of organization. These are Graptolitha (including Pterobranchia) and Echinodermata. Further, it is also possible to identify protomocoelomate groups of segmented deuterostomes: Enteropneusta with their segmentation secondarily lost judging from the number of gill slits, clearly associated in Chordata with myosepta, Vestimentifera with obvious segmentation in the opisthosoma, Tunicata, and Chordata. An important feature of the body plan in Chordata is that their body is overturned and moving with neural side upwards, which has been proved by modern methods of developmental genetics.