Question

Which of the following Cnidarians lacks a medusoid stage?
A. Obelia
B. Aurelia
C. Physalia
D. Metridium

Answer 1

Hint: Like plants that have two stages in their lifecycle one is gamete producing haploid gametophyte and spore-producing diploid sporophyte. Cnidaria also exhibits two stages in its life cycle.

Complete answer:
All the options given above belong to Cnidarian. Cnidaria or Coelenterata are aquatic organisms and are mostly marine, except a few like Hydra, which are freshwater. They are solitary or colonial; both sessile and free-swimming forms are found. Cnidaria exhibit two basic body forms-polyps and medusae. Medusae is also known as jellyfish. Polyps reproduce medusae asexually and medusae form polyp sexually. The most primitive form of a cnidarian is anthozoans. Only Anthozoans that only exhibit one form throughout their life are Metridium. Metridium does not exhibit in the medusae or jellyfish stage.
So, the correct answer is (D).

Additional Information:
Cnidaria is the first multicellular animal from an evolution point of view which shows tissue level of organization. Cnidaria are diploblastic organisms. Diploblastic organisms mean they derive from two embryonic layers. Ectoderm and mesoderm are the two embryonic layers present in Cnidaria. They have body walls that consist of two layers of cells; the outer epidermis and inner gastrodermis separated by a gelatinous layer, called mesoglea. They show radial symmetry. In Coelenterata intra and extracellular digestion are present. In some coelenterates, the body is supported by a horny or calcareous exoskeleton or endoskeleton.

Note: In Cnidaria, polyps reproduce medusae asexually and medusae form polyp sexually. Such alternation of asexual and sexual phases in the life cycle of cnidaria is called metagenesis.