Question

Polyp phase is absent in
A) Hydra
B) Aurelia
C) Physalia
D) Obelia

Answer 1

Hint:-Phylum Cnidaria consists of various aquatic species and they are abundant in marine waters. They are non-motile or freely floating invertebrate organisms. All species that belong to this phylum have special cells called cnidoblasts/ nematocysts. Those with cnidoblasts can sting other organisms. The toxins released by these cells can immobilize the prey or an attacker. The species of this phylum show two forms- the polyp form and the medusa form.

Complete Answer:-The polyp form looks more tubular with tentacles and a mouth at one end. Both polyp and medusa have two tissue layers- ectoderm and endoderm. A jelly-like substance called mesoglea is found in the middle of two layers. Both forms also have a cavity called coelenteron where digestion occurs. A reproductive tissue and a budding tissue are also found in polyp forms. Polyps are non-motile forms. Polyps usually undergo asexual reproduction through budding. It is, however, able to change to medusa form.
Medusa form has a bell-shaped body and they are motile forms. They usually reproduce sexually, therefore, only have reproductive tissue present in its body. Some of them can take up the polyp form. This alternation of sexual and asexual phases seen in some Cnidarian species is called metagenesis. Some Cnidarian species never change forms.
Hydra is found only in polyp form and it reproduces asexually. During extremely unfavourable conditions Hydra may undergo sexual reproduction to again produce polyp forms. Aurelia is a scyphozoan and the polyp phase is usually absent. They are mainly found in bell-shaped medusa form. Physalia and Obelia have both polyp and medusa forms and they alternate between both forms during their life cycle.

The correct option is (B). Polyp phase is absent in Aurelia.

Note:- Corals are invertebrate non-motile polyps that belong to the phylum Cnidaria. They are of economic and environmental importance. The underwater ecosystem, coral reef, is abundant in diverse organisms. Pollution and global warming is a threat to this ecosystem. Almost half the coral reefs are dead and the restoration of this ecology is of at most importance.