Question

the name of phylum cnidaria originated from the fact that organisms in this phylum have cnidocytes. what are cnidocytes?
stinging cells used for protection and hunting prey
a sac that fills with water and used for propulsion

Explanation:

Cnidocytes are specialized cells unique to Cnidaria. They contain nematocysts, which can be discharged. When triggered (e.g., by contact with prey or a threat), the nematocyst inside the cnidocyte fires out. This can inject toxins (for hunting prey, like immobilizing small fish or zooplankton) or be used as a defense mechanism against predators. The description "stinging cells used for protection and hunting prey" aligns with this biological function. The other option about a water - filled sac for propulsion refers more to structures like the hydrostatic skeleton in some organisms (not cnidocytes).

Answer:

stinging cells used for protection and hunting prey