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In the comparative physiology of photoreception by the Protista and
the invertebrates two aspects are emphasized: (1) the diversity of
visual processes in these groups and (2) their bearing upon general
mechanisms of photoreception. Invertebrates have evolved a far
greater variety of adaptations than vertebrates modifications
aiding survival in the remarkably different biotopes they occupy.
The number of species in itself suggests this multiformity; each of
them has peculiarities of its own, in morphology as well as in
physiology and behavior. But these special adaptations are
variations on a few great themes. Although the catalogue of
invertebrate species is immense, the literature concerning them
nearly rivals it in extent-even if one considers only that fraction
dealing with visual physiology. Taxonomy proceeds by grouping the
species, categorizing them in genera, families, orders, and
progressively larger units. Similarly, comparative physiology aims
at an analogous, more or less compre hensive, classification. This
Part A of Volume VII/6, like Part B that follows it, emphasizes the
broad questions that concern groups larger than the individual
species; in some cases these questions have general applicability.
The middle course between approaches that are too specialized and
those that are too general is often elusive, but here we attempt to
follow it. The vast number of special adaptations-probably, as we
have said, as large as the number of species-is beyond the range
even of a handbook."
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