What are Basal Metazoans?

Metazoa is a general term for all animals except protozoa (metazoan sub-world). The general term for all multicellular animal species in the animal kingdom except for the protozoa. (The sponge animal is a lateral animal. The true metazoan starts from the coelenterate.)

Metazoans

Metazoa can be divided into 4 corresponding levels according to their morphology:

Real

In metazoans, only metazoans and lateral animals (spongees) are Eumetazoa. It includes groups from the coelenterate (Cnidaria and Tadpole jellyfish) to the spine. animal. Although the metazoan is also a multicellular animal, it is not just a group of cells, which are arranged into two germ layers or three germ layers according to cells, also known as germ layer animals (De Keimblattie-re). In addition, because these cells are differentiated into tissues of various shapes in terms of morphology and function, they are also called tissue animals (De Histozoa, Gewebetiere) (except for mesozoic and sponge animals). Germ layer and tissue are differentiated to form various organs, so metazoans generally maintain the system of higher animals. Related to this, the individual occurrence of metazoans is also generally complex. Single-celled eggs undergo cleavage to form a germ layer that develops into an embryo. While undergoing tissue differentiation and forming a certain morphology, some also need to undergo extremely complicated metamorphosis before forming adults. Although asexual reproduction is sometimes performed, sexual reproduction is predominant.

There is no doubt about the ancestors of protozoa from protozoa, but there are many discussions about what kind of protozoa are their ancestors. Can be roughly divided:

(1) the result of cell differentiation of multinucleated protozoa;

(2) From the old theory, it is thought that it evolved from a flagellate population such as agglomerans. Of the two doctrines, the argument for the former is generally considered to be inadequate [Hadzi's doctrine is more noticeable than that of G. de Beer], and more are in favor of the latter. The latter's argument: think that this group structure is the germ of the metazoan without change; the individuals in the group constitute structural changes according to the polar axis; differentiate into trophic and reproductive members; in the process of forming the group, Especially similar to the occurrence of spongiform individuals; the male germ cells of metazoans maintain the morphology of flagellates and so on.

Based on these materials, E.Hackel reasoned that the ancestors of metazoans and intestinal ancestors appeared at the same time. E. Metchnikoff (1886) argues that floating wave larvae develop from coelenterate larvae. According to the opinions of many scholars, the second theory is considered to be close to the real situation.

The true metazoan is related to the inner cavity of the body. It is divided into coelenterate animals with coelenterate and coelenterate animals with body cavity. Coelenterate animals are further divided into protozoan and true cochlear [H.E. Ziegler, 1889]. In addition, some people have divided them into two groups: old and new animals [C. Grobben, 1908]. In addition, there are two types of end cell stem and intestinal body cavity stem related to mesoderm or body cavity formation. The lower animals in the telomeres are equivalent to the protozoa, and the other higher species and intestinal luminal stems are all true luminal. Consistent. If both species are higher animals, somites are visible. The above opinions are of great significance to the relationship between metaphysical systems of metazoans.

This makes it easy to distinguish between vertebrates and invertebrates. Therefore, the phylogenetic relationship between chordate animals (protozoans) and vertebrates, including invertebrates, is attracting attention.