Fundamentals of Evolution - O.P. Korzh - 2006

Part III. PATHS OF LIFE EVOLUTION

Chapter 20. Main Stages in the Evolution of Multicellular Animals

20.2. Complexification of Animal Body Structure as a Necessary Stage in Their Evolution

The Development of Multicellular animals is directly linked to the differentiation of their body, which leads to the appearance of germ layers in ontogeny. Initially, the ectoderm and endoderm arise from them.

Sponges differ significantly from all other multicellular forms, lacking a Nervous System, Sensory Organs, and specialized Muscle Cells, with a body built of Connective Tissue (mesenchyme). Due to their specific appearance and Structure, they were long classified as so-called zoophytes — intermediate organisms between plants and animals.

The body cells of sponges are differentiated and tend to form Tissues. The mesenchyme contains various Cell types: archaeocytes, which are capable of giving rise to other cells; amebocytes, which perform a phagocytic function; Germ Cells, which are diffusely distributed throughout the mesenchyme; and choanocytes (collared flagellated cells), which capture food using a cytoplasmic collar, etc.

Since the layer of choanocytes develops from the larval integument and the amebocytes from its internal cells, the further development of sponges can be viewed as an inversion of the germ layers.

In other cases, the animal's body consists of two cell layers: an outer (protective) and an inner (“digestive”) layer.

Nerve Cells and certain other types are diffusely distributed between these layers. This type of Organization is characteristic of Coelenterates (Cnidarians). Their distinctive feature is radial body Symmetry, associated with their initially passive lifestyle or low mobility in the past, as well as the alternation of sexual and asexual generations.

Today, it is known that coelenterates were the dominant animal group during the Vendian period, featuring A large number of giant soft-bodied forms (over 0.5 m in diameter). It is likely that the gigantism of these representatives drove them into an evolutionary dead end, leading to their gradual replacement by smaller forms.

According to the views of most zoologists, The First stage in the evolution of coelenterates should be considered The formation of a polyp, which, under a sessile lifestyle and colony formation, increases their chances in the Struggle for Existence (jointly capturing and assimilating prey, defending against enemies, etc.). However, the attached lifestyle also has its drawbacks: offspring settle close to the parental forms, causing overpopulation and increased competition. Therefore, the formation of motile reproductive forms (medusae) can be viewed as a kind of aaromorphosis that contributed to the further flourishing of this group of organisms. Corals, which frequently form diverse coral reefs, are of great interest. Science still lacks a unified theory that satisfactorily explains this phenomenon. Coral reefs are sometimes compared to oases of life amid the low-productivity waters of the tropical ocean (where primary production exceeds surrounding areas by 100 times).

Ctenophores are close in their organization to coelenterates: they also have only two cell layers separated by mesoglea. Their specific organizational features include a combination of two symmetry types—biradial and octiradial—as well as the absence of generational alternation. During embryonic development, these animals exhibit remnants of mesoderm formation. Some species display bilateral body symmetry. It is possible that ctenophores separated from the common trunk with coelenterates even before the latter transitioned to a sessile lifestyle.

Flatworms have the simplest body structure among all triploblastic animals. They are the oldest animal group in which organs and Organ Systems formed from the mesoderm appear for the first time.

However, the bulk of the mesoderm in their body remains undifferentiated, forming an enveloping tissue—mesenchyme or parenchyma—which provides Internal Organs with support and protection, and also plays a significant role in metabolic processes.

There is no consensus regarding THE ORIGIN OF this group of animals. A. Lang links the origin of turbellarians, as the baseline group of flatworms, to crawling ctenophores (both groups move using cilia and possess a statocyst, and their digestive systems share structural features). According to The Theory of V.N. Beklemishev and others, turbellarians originate from planula-like ancestors (the planula being the larva of coelenterates). It must be acknowledged that both theories have their strengths and weaknesses.

Due to directional movement, a longitudinal axis forms in the body of flatworms (as in most other multicellular animals) that coincides with the direction of this movement. As a result of such transformations, the right and left halves of the body become bilaterally symmetrical, and the nerve centers shift to the anterior end of the body. The Nervous System in these organisms is built on an orthogonal plan. A Circulatory system is not yet present, as the overall body structure does not require it: all PARTS OF THE body are located close to sources of food and oxygen. Many forms of these animals possess a branched gut that permeates the entire body, which significantly facilitates the Digestion AND ABSORPTION of nutrients.

Reproductive organs, predominantly in hermaphroditic flatworms characterized by internal Fertilization and a copulatory apparatus, conversely exhibit the most complex structure in the animal world. Special adaptations of these

animals to a parasitic lifestyle include the formation of a complex life cycle involving a change of hosts (sometimes several); in trematodes, an alternation of sexual and asexual generations also occurs.