660-630 million years ago
Cnidarians do not possess organs or systems, but they possess a number
of significant characteristics of the digestive systems of higher animals.
They possess a mouth and a gastrovascular cavity. The food which will
be digested can be trapped by mucus and moved to the gastrovascular cavity
by the action of cilia. Digestive enzymes are secreted into cavity, mostly
proteinases from endodermal gland cells. Muscle cells surround the gastrovascular
cavity and may be present in multiple layers. Some cnidarians have a pharynx.
Some ctenophores possess anal canals which means that indigestible material
does not need to exit the body through the mouth, which is the condition
of most cnidarians. In cnidarians, extracellular enzymes don't break down
the food completely-small pieces undergo phagocytosis and are digested
further intercellularly (Hickman 185, Fretter). Hydra possess 2 kinds
of gland cells in their digestive tracts for the production of mucus and
enzymes. The use of extracellular digestion by primitive animals probably
introduced a selective advantage for greater differentiation of the gastrointestinal
tract. Although extracellular digestion can aid intracellular digestion,
eventually it requires a control of the pH in the area of digestion, the
ability to mix food and enzymes, and the separation of the digestive area
into separate compartments to function well (Beklemishev). Cnidarians
can digest most types of biomolecules but most cannot digest starches
(Hickman, 137; Hyman). Cnidarian digestive enzymes include a trypsin-like
digestive enzyme that functions in an alkaline environment (Hyman, 393).