Multicellular animals

Both the G- and V-agents have the same physiological action on humans. They are potent inhibitors of the enzyme acetylcholinesterase (AChE), which is required for the function of many nerves and muscles in nearly every multicellular animal. Normally, AChE prevents the accumulation of acetylcholine after its release in the nervous system. Acetylcholine plays a vital role in stimulating voluntary muscles and nerve endings of the autonomic nervous system and many structures within the CNS. Thus, nerve agents that are cholinesterase inhibitors permit acetylcholine to accumulate at those sites, mimicking the effects of a massive release of acetylcholine. The major effects will be on skeletal muscles, parasympathetic end organs, and the CNS. 

Poriferans (sponges) are considered as the simplest multicellular animals. The vast majority of the sponge species are marine. Unlike most... 

From the environmental point of view, the three principal routes of entry of xenobiot-ics into the human body are percutaneous, respiratory, and oral. In multicellular animals, the extracellular space is filled with interstitial fluid. Thus, regardless of how a compound enters the body (with the exception of intravenous administration), it enters interstitial fluid after penetrating the initial cellular barrier (such as skin, intestinal mucosa, or the lining of the respiratory tract). From the interstitial fluid, the compound penetrates the capillaries and enters the bloodstream, which distributes it throughout the body. 

Several mechanisms have evolved to prevent this catastrophe. In bacteria and plants, the plasma membrane is surrounded by a nonexpandable cell wall of sufficient rigidity and strength to resist osmotic pressure and prevent osmotic lysis. Certain freshwater protists that live in a highly hypotonic medium have an organelle (contractile vacuole) that pumps water out of the cell. In multicellular animals, blood plasma and interstitial fluid (the extracellular fluid of tissues) are maintained at an osmolarity close to that of the cytosol. The high concentration of albumin and other proteins in blood plasma contributes to its osmolarity. Cells also actively pump out ions such as Na+ into the interstitial fluid to stay in osmotic balance with their surroundings. 

In this section, we will consider only a few biochemical and other aspects of multicellular animals or Metazoa. The sudden appearance of a large number of Metazoans about 0.5 x 109 years ago113 114 may have been an outcome of the appearance of split genes (see Section B, 1). As a result of gene duplication the coding pieces of split genes, the exons, could be moved to new locations in a chromosome where they could have become fused with other pieces of DNA to form entirely new genes.115... 

Sponges are the most primitive multicellular animals (Metazoa), whose bodies do not yet have true tissues and organs. The phylum Spongia is subdivided into three classes Calcarea, Hexactinellida, and Demospongia. Data on glycolipids exist only for the last class. 

Most multicellular animals possess a nervous system. In every case the function of the nervous system is to receive information about the external and internal environment, integrate the information, and then coordinate a response appropriate to the... 

Mechanoreception is the term used to describe the process that transmits the informational content of an extracellular mechanical stimulus to a receptor cell. Mechanotransduction is the term used to describe the process that transforms the mechanical stimulus content into an intra-cellular signal. The term mechanosensory is employed to mean both mechanoreception and mechanotransduction. Additional processes of inter-cellular transmission of transduced signals are required at tissue, organ and organismal structural levels. The mechanosensing process(es) of a cell enables it to sense the presence of, and to respond to, extrinsic physical loadings. This property is widespread in uni- and multicellular animals [54, 101, 53, 74, 73, 36] plants [201, 65] and bacteria [152], Tissue sensibility is a property of a connected set of cells and it is accomplished by the intracellular processes of mechanoreception and mechanotransduction. 

The study of differentiation in multicellular animals is beset by problems arising from the complex and poorly understood interactions of the various tissues. These interactions can be limited or eliminated in cell culture and a number of systems are described below which are leading to an understanding of the mechanisms of cell and tissue differentiation. This chapter supplements material presented earlier, especially in Chapter 6. 

The second model illustrates the point that more than one kind of memory can be responsible for the reconstruction from incomplete information that takes place during the (epigenetic) formation of an organism. Barbieri proposes that two kinds of memory are in fact responsible for the development of multicellular animals - one for the earlier stages, the other for the later ones. He shows how the existence of these two kinds of memory might account for the pattern of macroevolution, notably the Cambrian explosion. 

Metazoa is a kingdom to which all multicellular animals belong whose cells are organized into tissues with specific functions and which are controlled by a nervous system. 

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