Question

The following are coelomates except (true coelom) (a) Annelida (b) Platyhelminthes (c) Mollusca (d) Chordata

Short answer

The phylum that does not belong to coelomates is Platyhelminthes.

Step-by-step solution

Understanding of Coelomates

Coelomates are organisms possessing a true coelom. A coelom is a body cavity completely surrounded by mesoderm tissue.

Classifying the Given Examples

The given phyla are Annelida, Platyhelminthes, Mollusca, and Chordata. All, except for Platyhelminthes, are coelomates.

Identifying the Non-coelomate

Platyhelminthes, also known as flatworms, do not have a coelom (body cavity), which makes them the only non-coelomate in the given list.

Key concepts

True Coelom

In biology, understanding the concept of a true coelom is fundamental for grasping the complexity of animal body plans. A true coelom is essentially a fluid-filled body cavity that is entirely lined by tissue derived from mesoderm, one of the three primary tissue layers formed during embryonic development. This internal cavity, which houses the internal organs, is a distinguishing feature of coelomates and provides several advantages: it allows for greater body flexibility, enables the growth of larger organs, and acts as a hydrostatic skeleton to support body structure.

When we examine animals such as annelids (segmented worms), molluscs (like snails and squids), and chordates (which includes humans), they show complexity and organization associated with the presence of this true coelom. It offers them an evolutionary edge, allowing for more sophisticated organ systems and hence, more complex and diverse life forms. Understanding this key feature helps us recognize how different animals have evolved and adapted over time.

Phyla Classification

Classification into phyla represents a significant step in organizing the vast diversity of life forms. In biological taxonomy, a phylum is a principal category that groups together organisms based on fundamental body structures, developmental processes, and genetic relationships. There are several different phyla in the animal kingdom, each representing a major type of body plan.

For instance, the phylum Annelida includes segmented worms, which are coelomates with bodies divided into ringlike segments. Mollusca, another phylum, encapsulates a wide array of animals, including octopuses and clams, all of which possess a true coelom. Chordata, perhaps the most familiar phylum, contains animals with a dorsal nerve cord, such as mammals, birds, and fish. When studying various life forms, understanding their phyla classification helps us decipher their evolutionary histories and relationships to each other, allowing us to appreciate the complex tapestry of life on Earth.

Non-coelomate Organisms

Contrasting with true coelomates are the non-coelomate organisms, which lack a true coelom or body cavity. These organisms have a body plan that is more straightforward and less compartmentalized than that of coelomates. One prominent group of non-coelomate organisms is the Platyhelminthes, also known as flatworms. These creatures do not possess an enclosed, fluid-filled body cavity; instead, their organs are embedded directly within the solid tissue of their body.

Non-coelomates are often simpler in form and less versatile in movement compared to coelomates, partly due to the absence of the cushioning and space provided by a coelom. They represent an early stage in the evolutionary development of complex multicellular organisms. Studying these simpler organisms gives insight into the basics of animal structure and function and can illuminate the early stages of animal evolution.