Hint: Cnidarians represent a more dynamic degree of organisation than Porifera does. They have outer and inner tissue layers that are sandwiched with a non-cellular mesoglea. Cnidarians have a well-formed digestive system and conduct extracellular digestion. Cnidocyte is a specialised cell for the transmission of prey toxins as well as for alert against predators.
Complete Answer: - Phylum Cnidaria comprises types as diverse as jellyfish, hydra, sea anemones and corals. Cnidarians are radially or radially symmetrical, a general form of symmetry believed to be primitive for eumetazoans. - They have reached the degree of organisation of tissues in which certain related cells are associated into groups or aggregations called tissues, but true organs do not exist. Cnidarian bodies have two or even three layers. - Some cells are organised into two basic nerve nets, one epidermal and the other gastrodermal, which help to coordinate muscle and sensory functions. - Cnidarians have two specific body types, medusa and polyp. Polyp reproduction is by asexual budding (polyps) or sexual creation of gametes (medusae, some polyps). Cnidarian people may be monoecious or dioecious. - The outcome of sexual reproduction is a planula larva, ciliated and free-swimming.If the collar cells and spicules define the characteristics of Phylum Porifera, the nematocysts define the cnidarians. - These tiny organelles, compared to Hickman's cocked arms, are both highly efficient for the capture of prey and extremely effective for predators. Each of them contains a coiled, tubular thread, which can bear barbs and is sometimes poisoned.
Thus the correct answer is option(B) Tissue level.
Note: The exact relationship between the various cnidarian groups is uncertain. Among the hypotheses suggested on the evolution of phylum Cnidaria, the majority treat radial symmetry and the degree of tissue organisation as proof that the group is primitive.