Cnidaria
Cnidaria generally referred to as jellyfish, are an entirely aquatic phylum. They are filled with a jelly like substance called mesoglea. This is where we derive the name jellyfish. The phylum contains more than 10,000 species with many similar features. They all have one main body structure called the bell similar to water filled balloons as well as tentacles hanging from the main bell structure.
Animals in the phylum like Cubozoa also known as Box Jellyfish, Hydrozoa, and Siphonophorae best known for the poisonous Portugese Man O' War have the exact same basic muscular systems. Their muscular systems are not incredibly complex considering their basic body shape. In order for movement they rely on a something like a jet propulsion system. This works by in taking water through something like a rim in the bottom of the bell. Once the water has been taken in the muscles squeeze the water out. These muscles are usually only one-cell thick and not very strong. The Jellyfish do not have much of a chance against strong currents and are forced to move along with them. Their locomotion only allows them to move within the current in general directions but not against it.
Animals in the phylum like Cubozoa also known as Box Jellyfish, Hydrozoa, and Siphonophorae best known for the poisonous Portugese Man O' War have the exact same basic muscular systems. Their muscular systems are not incredibly complex considering their basic body shape. In order for movement they rely on a something like a jet propulsion system. This works by in taking water through something like a rim in the bottom of the bell. Once the water has been taken in the muscles squeeze the water out. These muscles are usually only one-cell thick and not very strong. The Jellyfish do not have much of a chance against strong currents and are forced to move along with them. Their locomotion only allows them to move within the current in general directions but not against it.