What is 'degeneration' as described in the text?

'Degeneration' as described in the text refers to the simplification of the structure of animals that have adapted to a parasitic or sessile lifestyle. The example given is parasitic worms, which lose certain organs like a developed digestive system and nervous system as their structure simplifies in response to their mode of life.

Degeneration is the simplification of an organism's structure due to adaptation to specific lifestyles like parasitism or being sessile.