Question

The first true coelomates are (a) Nereis (b) Centipede (c) \(\mathrm{Crab}\) (d) Wuchereria

Short answer

The first true coelomates are Nereis.

Step-by-step solution

Understanding the Concept of Coelomates

Coelomates are organisms that have a body cavity enclosed by a mesoderm, or the intermediate layer of cells. This allows the internal organs to function independently of the outer body wall. The evolution of the coelomate body structure has allowed for greater size and complexity in organisms.

Identifying the organisms

Nereis is a type of marine polychaete worm, Centipede is a type of arthropod belonging to the class Chilopoda, and Crab is a decapod crustacean. Wuchereria is a genus of parasitic roundworm (a Nematode). Among these, Nematodes do not have true coeloms, and among the Arthropods and Annelids, Annelids evolved earlier.

Choosing the right organism

As Nereis (a type of polychaete worm) belongs to the Annelids, and Annelids were the first major group of organisms to have a true coelom, the correct answer is Nereis.

Key concepts

Evolution of Coelom

The evolution of the coelom was a significant milestone in the history of life on Earth. The coelom is a fluid-filled body cavity that lies within the mesoderm and provides space for internal organs and systems to grow and operate independently from the body wall. This advancement allowed for the development of more complex organisms with specialized organs.

In simple terms, think of the coelom as a room for organs to move without bumping into each other, providing them with the 'personal space' necessary to perform more efficiently. Among the earliest coelomates were annelids, like the marine polychaete worm Nereis referred to in our exercise. The presence of a coelom contributed to the diversification of animal body plans during the Cambrian explosion, leading to the variety of life forms we see today.

Body Cavity Structure

The body cavity, or coelom, is structured in a manner that it is completely lined with a tissue called mesoderm. There are other forms of body cavities in animals, but the coelom is unique because it is not part of the digestive tract or the outer body wall.

Think of it as a 'true' body cavity, as other non-coelomates, like nematodes, have a pseudo body cavity, which is not entirely lined with mesodermal tissue. This distinction is important because it defines the way an organism's internal systems are organized and protected, influencing their levels of functionality and complexity. For example, the true coelom in annelids allows for a more advanced circulatory system than in nematodes.

Animal Kingdom Classification

Classification in the animal kingdom relies heavily on body structure and developmental patterns. In general, animals can be divided into three broad categories according to their body cavity type: acoelomates without a coelom, pseudocoelomates with a body cavity partially lined with mesoderm, and coelomates with a fully lined coelom.

Within these categories, animals are further classified based on characteristics such as symmetry, segmentation, and embryological development. Acoelomates, like flatworms, have no body cavity. Pseudocoelomates, like the roundworms from our exercise question, have an imperfectly lined body cavity. Lastly, coelomates, such as mollusks, annelids, arthropods, and vertebrates, possess a true coelom. Each classification level has evolved to fill different niches in the environment, showcasing the diversity of life and evolutionary adaptation.