The pair of characteristics that one must use to classify a fish is the presence of jaw and skeleton type. Agnatha fishes are also called jawless fishes and they lack bony skeletons. Cartilaginous fishes are composed of cartilage skeletons. Bony fishes have bony skeleton structures.
The answer in the space provided is anaerobic respiration.
It is because this allows of having to utilize other substance than oxygen such
as nitrate or sulfate to be their electron acceptors in which is for organisms
that undergoes respiration.
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The Cell, the fundamental structural unit of all living organisms. Some cells are complete organisms, such as the unicellular bacteria and protozoa, others, such as nerve, liver, and muscle cells, are specialized components of multicellular organisms. In another words, without cells we wouldn't be able to live or function correctly.