Cluster sampling See sampling

Cnidaria A large phylum of aquatic, mostly marine invertebrates - the most primitive of the multicellular animals. Cnidarians are radially symmetrical and diploblastic, the body wall having two layers separated by a layer of jelly (mesoglea) and enclosing the body cavity (coelen-teron). The single opening (mouth) is surrounded by a circle of tentacles, which are used for food capture and defense and may bear stinging cells (cnidoblasts). Two structural forms include the sedentary polyp (e.g. Hydra, sea anemones), and the colonial corals and the mobile medusa (jellyfish); either or both forms occur in the life cycle. See also Anthozoa.

coagulation The clumping of particles into larger particles. It is an important stage in the processing of sewage, in which small light suspended particles that will not otherwise settle out are first clumped together using chemical coagulants such as aluminum sulfate to form lumps called flocs. This process is called flocculation. After coagulation, the suspension is transferred to a sedimentation tank, and the (now larger) particles will settle out.

coal A carbonaceous deposit formed from the remains of fossil plants. Most of the world's coal deposits formed about 300 million years ago during the Carboniferous (Pennsylvanian) period, under warm tropical or subtropical climates when vast swamp forests flourished. The stages in the formation of so-called humic or woody coals pass through partially decomposed vegetable matter such as peat, through lignite, subbituminous coal, semianthracite, to anthracite. During this process, the percentage of carbon increases and volatile components and moisture are gradually eliminated. Sapropelic coals are derived from algae, spores, and finely divided plant material. See Carboniferous; fossil fuel.

COD See chemical oxygen demand.

codon A group of three nucleotide bases in a messenger RNA (mRNA) molecule that codes for a specific amino acid or signals the beginning or end of the message (start and stop codons). Since four different bases are found in nucleic acids there are 64 (4 X 4 X 4) possible triplet combinations. The arrangement of codons along the mRNA molecule constitutes the genetic code. When synthesis of a given protein is necessary the segment of DNA with the appropriate base sequences is transcribed into messenger RNA (see transcription).

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