Jellyfish (Class: Scyphozoa)
Occurence - Worldwide; in marine plankton, a few on seaweed or the sea bed.
With bodies that can be over 2m across and tentacles that may extend many metres, jellyfish are among the largest animals found in plankton. A few species, however, attach themselves to seaweeds, and some spend much of their time on the sea bed. A jellyfish’s body, or “bell”, is shaped like an inverted saucer or bowl, and consists mainly of a thick layer of jelly-like mesoglea sandwiched between the outer and inner cell layers. The mouth in the centre of the underside opens into a simple gut, and stinging tentacles hang down from the bell margin. These simple animals can have complex life cycles. Some (but not all) begin life as small polyps on the sea bed. These split into a stack of tiny medusae, or baby jellyfish, which drift away to start life on their own, growing into sexually reproducing adults. Although jellyfish can swim by contracting their muscles, they make little headway against even a gentle current, and are often washed up on beaches after storms. Most are carnivorous, catching prey with their tentacles. This class includes species such as the moon jelly and lion’s mane jellyfish.
(Side note: I will possibly list more jellyfish species in the future, especially if they are requested, but right now I’m leaving it at a very broad description)