Cnidaria Digestive System
Cnidaria is a phylum containing over 10,000 marine organisms. They are characterized by Cnidocytes located on the tentacles of polyps. Polyps are organisms ranging in size from an entire jellyfish, which is considered an individual polyp, to the hundreds of tiny polyps functioning on the skeletal structure of coral. The cnidocytes located on the tentacles of the polyps are small cells containing a harpoon-like structures which capture food or paralyzes and capture the organism's prey. The Cnidarian then proceeds to begin digestion of its captured food throught the tentacles and pulls the food into the mouth to complete digestion and absorption. This is a common feature of all Cnidarians, hence the name Cnidaria, coming from cnidocytes.
Chrysaora fuscescens (Pacific Sea Nettle)
The Pacific Sea Nettle is a species of jelly fish native to the pacific ocean. Like other members of Phylum Cnidaria, the Pacific Sea Nettle is a carnivore feeding on zooplankton, crustaceans, and small fish among other small marine organisms. The Pacific Sea nettle is capable of limited movement by pushing wate with the use of its bell head, yet they usually capture prey by floating in large groups and using their long tentacles as nets.
The tentacles of the Sea Nettle contain small cells known as Cnidocytes containing organelles called nematocysts, with chemical detectors that detect prey coming into contact with the tentacles. Once the prey is near, the Cnidocytes launch a barbed harpoon-like structure into their prey, releasing a paralyzing toxin. Once the prey is caught in this net of tentacles, the frilled tentacles known as oral arms begin to digest the prey as it is slowly moved up toward the mouth, located on the underside of the bell.
Food is carried by the oral arms into the mouth, which then brings the food directly into the stomach to complete digestion and absorption. Once the waste is change into fecal matter, the waste is released through the mouth again, which in the Pacific Sea Nettle, serves the purpose of both mouth and anus at the same time.
Dendrogyra cylindricus (Pillar Coral)
In the case of corals, coral is not so much one single organism as a skeletal structure for hundreds of small organisms attached to the coral known as polyps. The polyps on a Pillar Coral feed on organisms ranging from microscopic plankton to very small fish. The polyps can be seen periodically on the coral while feeding, giving the coral a 'furry' appearance. The polyps, similarly to the tentacles of a the Sea Nettle and other jellyfish, make use of Cnidocytes to to paralyze and capture their prey.
Once the Cnidocytes paralyze the prey, the short tentacles on the polyps take the food into the mouth. The mouth leads directly to the stomach where digestive enzymes break it down into basic substances to be absorbed. The waste is transformed into fecal matter and is expelled through the mouth, just as in jellyfish.
The polyps themselves are individual multi celled organisms and the coral serves as a joint skeletal structure. The polyps cannot move to their prey and need to wait for prey to come into contact with the coral directly. When not feeding, the polyps can retract their tentacles into the mouth area.
Since polyps are individual organisms,the coral itself technically has no single digestive system, but hundreds of independent organisms with their own individual digestives systems permanently attached to each other.
Pennatula phosphorea (Common Sea Pen)
Similarly to corals, sea pens are large connected colonies of multiple individual organisms known as polyps. In the case of the sea pen, the polyps are divided into three groups according to function. At first, a single individual polyp burrows itself into the sediment and sprouts a longer stalk from which the other polyps form.These are divided into polyps responsible for feeding, reproduction, and water intake structures. The central stalk has a calcium carbonate support structure to maintain some rigidity. Polyps can uproot and transplant at ay time, often positioning themselves in water currents to collect plankton, the main part of their diet.
The feeding polyps on the Common sea Pen do not differ much from those on coral. They utilize similar tentacles containing Cnidocytes in order to capture their food. These tentacles then take the food into the mouth which leads directly into the stomach. Digestive enzymes break down the food and absorb the necessary nutrients and chemicals. The waste is the regurgitated through the mouth. The polyps on the Common Sea Pen are much smaller than those on coral as they are meant for mostly algae. The polyps on a sea pen also function more as individuals working as a unit rather than the individuals in coral.