Which characteristic is associated with cnidarians?

Cnidarians, which include organisms such as jellyfish, corals, and sea anemones, are recognized for their distinctive stinging tentacles. These tentacles are equipped with specialized cells called cnidocytes that contain stinging structures known as nematocysts. When prey comes into contact with these tentacles, the nematocysts can fire, delivering a sting that helps in capturing prey and serving as a defense mechanism. This characteristic is fundamental to the survival and feeding strategies of cnidarians.

In contrast, the other options do not accurately describe cnidarians. For instance, they do not primarily have a single layer of cells; instead, cnidarians possess two main tissue layers known as the ectoderm and endoderm separated by a gelatinous substance called mesoglea. While some cnidarians exhibit a degree of asymmetry, many have a radial symmetry, particularly when viewed in their typical body plan. Finally, cnidarians do not have a solid exterior shell like mollusks; instead, their structure is more gelatinous and flexible without a hard outer casing. Thus, the presence of stinging tentacles is a defining and critical characteristic of cnidarians.