243
Polyplacophora
ticides in most of the developed countries, concentrations in all environmental compartments have decreased. In tropical areas, however, where these
compounds are still applied in antimalaria programs (DDT) and to fight
swarms of locusts (dieldrin), environmental contamination still occurs.
PCBs are a group of chlorinated aromatic compounds, produced synthetically by chlorinating the biphenyl skeleton. Theoretically, 209 different
chlorobiphenyls can exist based on the possible distribution of Cl atoms in
the two rings of the biphenyls. The amount of chlorine in PCBs ranges from
ca. 18 to 71 %, and each technical mixture contains at least 37 biphenyls with
different degrees of chlorination (congeners), detectable with capillary gas
chromatography with electron-capture detection. Their application was primarily in the field of noncombustible lubricators and isolation oils in electrical transformers, but also as additions to paint and no-carbon-required
paper. Since 1945, total world production has been about 1.2 10 6 ton y-I.
Since 1980, PCBs have gradually been banned, but because these compounds
are very persistent, their effect on the environment will persist for many
decades. In 1988,31 % of the total production was dispersed in the environment, while 65 % is still in use, or otherwise potentially able to reach the
environment. Polychlors are hydrophobic compounds and therefore accumulate mainly in lipids of aquatic organisms, either directly from water or
through food. In aquatic organisms equilibria are attained between concentrations in these lipids and water with distribution coefficients of about 10 4 to 10 6 •
Accumulation along the food chain is not evident. This is different for sea
mammals and sea birds, which lack a system for exchange with seawater,
and therefore may accumulate much higher concentrations. Metabolic conversion of polychlors is possible for DDTs and some low-chlorinated PCBs.
polynya open area between the polar sea ice fields. -7ice.
polyp general name for individual soft-bodied animals of polypoid shape, i.e.,
of hollow cylindrical shape, with central mouth opening, surrounded by tentacles, standing on a foot. Polypoid individuals occur in the Cnidaria, as sessile
vegetative life form of the Cnidaria, in corals, and in Bryozoa.
polyphagous thriving on many different food types.
Polyplacophora (chitons) class of the phylum Mollusca. Mollusks with a flattened,
oval body and a strong foot, to crawl and to adhere strongly to hard substrate, and a heavy mantle with a dorsal shell, typically divided into eight
overlapping shell plates. There are some 500 species, many living in rocky
intertidal areas, where they overtide, clinging to rocks and stones. They feed
by scraping algae, etc. from the substrate with the radula. (Figure see p. 244)
Polyplacophora
ticides in most of the developed countries, concentrations in all environmental compartments have decreased. In tropical areas, however, where these
compounds are still applied in antimalaria programs (DDT) and to fight
swarms of locusts (dieldrin), environmental contamination still occurs.
PCBs are a group of chlorinated aromatic compounds, produced synthetically by chlorinating the biphenyl skeleton. Theoretically, 209 different
chlorobiphenyls can exist based on the possible distribution of Cl atoms in
the two rings of the biphenyls. The amount of chlorine in PCBs ranges from
ca. 18 to 71 %, and each technical mixture contains at least 37 biphenyls with
different degrees of chlorination (congeners), detectable with capillary gas
chromatography with electron-capture detection. Their application was primarily in the field of noncombustible lubricators and isolation oils in electrical transformers, but also as additions to paint and no-carbon-required
paper. Since 1945, total world production has been about 1.2 10 6 ton y-I.
Since 1980, PCBs have gradually been banned, but because these compounds
are very persistent, their effect on the environment will persist for many
decades. In 1988,31 % of the total production was dispersed in the environment, while 65 % is still in use, or otherwise potentially able to reach the
environment. Polychlors are hydrophobic compounds and therefore accumulate mainly in lipids of aquatic organisms, either directly from water or
through food. In aquatic organisms equilibria are attained between concentrations in these lipids and water with distribution coefficients of about 10 4 to 10 6 •
Accumulation along the food chain is not evident. This is different for sea
mammals and sea birds, which lack a system for exchange with seawater,
and therefore may accumulate much higher concentrations. Metabolic conversion of polychlors is possible for DDTs and some low-chlorinated PCBs.
polynya open area between the polar sea ice fields. -7ice.
polyp general name for individual soft-bodied animals of polypoid shape, i.e.,
of hollow cylindrical shape, with central mouth opening, surrounded by tentacles, standing on a foot. Polypoid individuals occur in the Cnidaria, as sessile
vegetative life form of the Cnidaria, in corals, and in Bryozoa.
polyphagous thriving on many different food types.
Polyplacophora (chitons) class of the phylum Mollusca. Mollusks with a flattened,
oval body and a strong foot, to crawl and to adhere strongly to hard substrate, and a heavy mantle with a dorsal shell, typically divided into eight
overlapping shell plates. There are some 500 species, many living in rocky
intertidal areas, where they overtide, clinging to rocks and stones. They feed
by scraping algae, etc. from the substrate with the radula. (Figure see p. 244)