The nudibranch, a shell-less snail of tropical waters, maintains an unusual symbiotic relationship with the unicelled plants called zooxanthellae. Juvenile
nudibranchs, foraging on corals and similar organisms, ingest algae-bearing
polyps. Algae thus _57___ the nudibranch. Almost immediately, rapid
development of the algae ensues in the nudibranch's intestinal tract. As this
occurs, the formerly colorless nudibranch gradually develops colored patterns
underneath its translucent skin. Its reliance on nourishment from external
sources declines and eventually ceases as it increasingly derives nourishment
from metabolic products of the algae. Thereafter, nudibranchs exhibit a type of
self-contained agriculture: algae living in their intestinal tract produce, via
photosynthesis, sugars and other nutrients fully sufficient to sustain them. No
other _58 is required.
57 A. kill B. enter C. avoid D. follow E. threatened 58. A. home B. direction C. name D. problem E. food