what are the exceptions in this chapter?

Halogen containing organic compounds are relatively rare in terrestrial plants and animals. The thyroid hormones T₃and T₄are exceptions; as is fluoroacetate, the toxic agent in the South African shrubDichapetalum cymosum, known as "gifblaar". However, the halogen rich environment of the ocean has produced many interesting natural products incorporating large amounts of halogen. Some examples are shown below.
The ocean is the largest known source for atmospheric methyl bromide and methyl iodide. Furthermore, the ocean is also estimated to supply 10-20% of atmospheric methyl chloride, with other significant contributions coming from biomass burning, salt marshes and wood-rotting fungi. Many subsequent chemical and biological processes produce poly-halogenated methanes.

Thank You
Ruchi
Askiitians Faculty