II. Read the following text paying attention to the highlighted words. Explain or interpret the contextual meaning of the underlined phrases

The plant way of life can be characterized as the utilization of simple inorganic molecules from the environment by a usually sessile organism relying on the direct energy of sunlight. An animal can be viewed as an organism that requires a variety of complex organic molecules as sources of energy and obtains these molecules by the active expenditure of energy either to move through the environment, or to move the environment to it. The cells of animals are differentiated primarily in terms of their internal structure. Lacking rigid cell walls, animals, unlike plants, are unable to utilize high osmotic pressures to regulate the intake of materials from the environment. Instead, they have evolved elaborate mechanisms for regulating their internal environments. Active seeking for food has also given strong selective advantage to structures that provide more detailed information about the environment and to structures able to receive and coordinate this information. Consequently most animals are behaviorally more complex than plants.

Animals are believed to have arisen evolutionary from ancestral Protista by a process of division of labour among cells. Within the ancestral colonies of cells some cells began to differentiate into somatic types and others into gametes and gamete precursors. Once this step was taken, it was possible for the units to become increasingly differentiated as specialized types of cells, all the while improving their coordination with other cells as working groups. These groups of cells made up even larger and more complex organisms: animals.

The multicellular animal form of organization has arisen from the protists at least three times. The sponges (phylum Porifera), coelenterates (phyla Cnidaria and Ctenophora), and flatworms (phylum Platyhelmintes) each probably represent separate phylogenetic lines. Some zoologists believe that two other small and obscure phyla (Placozoa and Mesozoa) also evolved independently from the protists. The other 28 animal phyla are generally thought to have been derived from a flatworm or a flatworm-like ancestor.

Over 1 million living species of animals are now known to science, and estimates of the true number range up to 15 million or more. These species have arisen over more than 1 billion years of evolution.

Traits that figure prominently in current classifications of animals are body symmetry (radial or bilateral), body cavities (none, pseudocoel, coelom), embryological development (protostomes or deuterostomes), segmentation and type of skeleton. The radially symmetrical phyla (Cnidaria and Ctenophora) have no well developed tissues or organs, dead-end digestive tracts, and no body cavities. The Platyhelminthes, believed to be the ancestors of most animal phyla, also lack body cavities and have dead-end digestive tracts, but they are bilaterally symmetric and have heads where sense organs are concentrated. All other phyla except the Nemerteans have some type of body cavity. A fluid-filled body cavity functions as a hydrostatic skeleton and provides an incompressible medium in which internal organs are suspended. More precise control over locomotion and other physiological functions is found among animals whose bodies are segmented (annelids and arthropods).

Very different types of body plans are found among the mollusks, which are built around a large muscular foot, a mantle, which secretes the shell in those species that have one, and a large gill; and among the echinoderms, which have secondarily adopted radial symmetry and an unusual water vascular system which is in direct contact with the surrounding sea water. The water vascular system functions in locomotion, respiration and excretion. The chordate body plan is based upon an internal skeleton, gills and a dorsal, hollow nervous system. Different phyla have radiated at different times during the evolution of life on Earth, but the representatives of most groups that have ever lived still survive today.

Currently, the most diverse phylum is arthropoda, with nearly 1 million described species and many more undescribed ones. The vertebrates – the subphylum to which humans belong – are the ecologically dominant and conspicuous animals in both aquatic and terrestrial environments.

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IV. Discuss the poem from the previous exercise. What do you feel when you plant a tree? How important are plants in our lives? Try to add one more verse to the poem	\|	Vocabulary notes

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