Invertebrate Zoology: A Study Guide - T. A. Dauda 2014

Multicellular Animals
Cnidarians
Class Anthozoa

Coral polyps inhabit exclusively marine environments, leading a sessile lifestyle. The majority are colonial forms, though solitary species such as sea anemones are also common. Their length ranges from a few millimeters to several tens of millimeters, occasionally reaching up to 1 m. Coral polyps exist exclusively in polypoid form, lacking a medusa stage and generational alternation. While their general body plan is similar to that of hydrozoans, they exhibit A number of evolutionary advancements. Specifically, their digestive cavity is partitioned by septa into numerous chambers (6, 8, 12, etc.), and they possess an ectodermal Pharynx.

Most coral polyps develop an external (ectodermal) Skeleton. In stony (madreporarian) corals, this skeleton Supports vast colonies that build reefs and coral islands. A significant portion of limestone rock is composed of the fossilized skeletons of extinct madreporarian corals.

Madreporarian corals are colonial organisms that thrive primarily in tropical seas at depths of up to 50 m. Their survival depends on an optimal combination of abiotic environmental factors: clean Water, a sufficiently high salinity level, and temperatures no lower than +20°C. This restricts their distribution to the Pacific and Indian Oceans and the Red Sea, where coral formations such as fringing reefs, barrier reefs, and coral islands, or atolls, are widespread.

Coral formations develop through the gradual die-off of the lower PARTS OF THE colony, upon the limestone framework of which new branches grow. Fringing reefs are formed by coral activity in the tidal zone close to the shore. Barrier reefs are typically massive structures located at a considerable distance from the coast. For instance, the Great Barrier Reef along the eastern coast of Australia stretches for 1400 km.

An atoll is a ring-shaped coral reef rising slightly above the sea surface. Enclosed within the ring is a body of seawater known as a lagoon. Winds and ocean currents deposit plant seeds onto atolls, eventually transforming them into lush, blossoming islets.

Several hypotheses exist regarding THE ORIGIN OF atolls. Charles Darwin, for example, proposed that atolls evolved from fringing reefs that originally surrounded volcanic islands, as the seafloor gradually subsided. When the island fully submerges, a shallow lagoon forms in place of the land, and the fringing reef transforms into an atoll. In recent times, Darwin's hypothesis has received scientific validation.

Review and Self-Assessment Questions:

1. What are the common characteristics of cnidarians?

2. What is the internal and External Structure of the freshwater hydra?

3. How does the hydra feed?

4. How does the hydra reproduce under various environmental conditions?

5. What structural and life-cycle features characterize marine hydrozoan polyps?

6. What is the Anatomical Structure of coral polyps?

7. What is The Essence of Charles Darwin's hypothesis regarding the origin of atolls?



Last update: 13/08/2026

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