Biology: Phylum Cnidaria, part 1

Tutoring biology, various phyla are handy to know about. The tutor mentions Phylum Cnidaria.

Cnidaria includes jellyfish but also sea anemones. As examples, jellyfish are in the “medusa” form, while anemones are in the “polyp” form. Polyps are set in place and wait for food to come to them, while medusae cruise the ocean for food.

The Cnidarian body plan surrounds a digestive sac; food is brought in, enzymes are released to digest it, and then the nutrients are absorbed by cells lining the sac. Wastes pass out from the sac.

Nutrients are passed from cell to cell or into the mesoglea, which is a nonliving layer of “jelly” between the cells lining the digestive sac and the external cells. Nutrients and other resources can pass through the mesoglea to other cells.

Around the mouth and on the tentacles, Cnidarians have stinging cells called cnidocytes: the stinger of each one is a nematocyst. It is triggered by touch, when it will stab the prey and inject toxin therein. The toxin can repel an enemy or stun prey. (When I was stung by a jellyfish as a kid, it felt like pins and needles.) The stinger has a barb and even a thread which can grab the prey.

Cnidarians have a limited nervous system, but enough to cause tissue around the mouth to bring in food and expel waste.

Some species of Cnidarians are permanent polyps; many start out as polyps, then break off to become medusae. The medusae produce eggs or sperm; the fertilized eggs will become polyps. Then, asexually, buds break off from the polyps to become medusae. Interestingly, each medusa is either male or female.

Source:

bio.libretexts.org