4.2.9. Phylum – Echinodermata

These animals have an endoskeleton of calcareous ossicles and, hence, the name Echinodermata (Spiny bodied, Figure 4.14). All are marine with organ-system level of organisation. The adult echinoderms are radially symmetrical but larvae are bilaterally symmetrical. They are triploblastic and coelomate animals.

Echinoderms are a unique group of marine animals characterized by having a hard internal skeleton made of calcium carbonate. This structure is called an endoskeleton, which is made up of small plates known as ossicles. They display two different types of symmetry during their life cycle: adults typically exhibit radial symmetry, where the body plan can be divided into similar halves around a central axis, while the larvae show bilateral symmetry, where they can be divided into mirror-image halves along a single plane. Echinoderms are triploblastic and coelomate, meaning they have three germ layers and a body cavity lined by mesoderm.

Imagine a starfish that you might find at the beach. As an adult, the starfish has arms arranged around a central point, showcasing radial symmetry. However, when it was a larva, it was much more like a tiny fish, displaying bilateral symmetry. This change in symmetry is a fascinating aspect of their development.

Digestive system is complete with mouth on the lower (ventral) side and anus on the upper (dorsal) side. The most distinctive feature of echinoderms is the presence of water vascular system which helps in locomotion, capture and transport of food and respiration.

Echinoderms possess a complete digestive system, meaning they have a mouth and an anus, allowing for the efficient processing of food. The mouth is located on the underside of the body, while the anus is on the top side. A key characteristic of echinoderms is their unique water vascular system, which consists of a network of fluid-filled canals that help them in several important functions. This system is crucial for mobility, enabling echinoderms to move using tube feet powered by hydraulic pressure, and assisting in the capture and transport of food to the digestive system.

Think of the water vascular system as a set of hydraulic pumps. Just like a car lift that raises cars using hydraulic fluid, echinoderms use this system to extend and retract their tube feet, allowing them to crawl along the ocean floor or grip onto rocks. This amazing system also helps them eat by giving them the strength to pry open shells of bivalves they might prefer as food.

An excretory system is absent. Sexes are separate. Reproduction is sexual. Fertilisation is usually external. Development is indirect with free-swimming larva.

Unlike many other animal groups, echinoderms do not have a specialized excretory system; they can expel waste products directly into the surrounding water. They have separate sexes, meaning individual echinoderms are either male or female. Most echinoderms reproduce sexually, with fertilization typically occurring externally, where eggs and sperm are released into the water. This leads to indirect development, meaning after fertilization, the resulting larva is free-swimming and develops into the adult form over time.

Imagine you are at a beach during summer. You might see countless eggs and sperm released by starfish into the water, creating a cloud of tiny cells. This external fertilization is quite like sprinkling seeds over a garden and hoping some will take root and grow into flowers, or in this case, baby starfish. The larval stage is vital as it gives the young echinoderms time to develop before settling down and taking on the adult shape.

Examples: Asterias (Star fish), Echinus (Sea urchin), Antedon (Sea lily), Cucumaria (Sea cucumber) and Ophiura (Brittle star).

Several well-known examples of echinoderms include starfish, sea urchins, and sea cucumbers. Each of these represents different forms and adaptations within the phylum. Asterias, or starfish, are renowned for their unique shape and ability to regenerate lost arms. Echinus, the sea urchin, has a hard shell covered in spines, providing protection from predators. Meanwhile, Cucumaria, the sea cucumber, uniquely exhibits a soft body and a leathery outer skin, often found resting on the ocean floor.

Think of a vibrant underwater garden: the starfish can be seen prancing around the coral, while the urchins play their role as the spiky guards, and the sea cucumbers act like the cleanup crew, absorbing nutrients from the ocean floor. Each plays a vital part in their ecosystem, just like various roles in a community.

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