Which of the following is not a characteristic feature of fishes belonging to class Pisces?

Introduction / Context:

Fishes, grouped under class Pisces, share several typical characteristics related to their aquatic life, body structure, and internal organisation. Exam questions often ask which feature does not belong to a particular group, requiring students to identify an odd one out based on understanding of animal body plans. This question asks which listed feature is not a characteristic of fishes, focusing on knowledge of true coelom and pseudocoelom as well as basic fish anatomy.

Given Data / Assumptions:

• The group in focus is Pisces, the class containing fishes.
• We must identify the feature that is not characteristic of fishes.
• Options include scales, gill breathing, endoskeleton of bone or cartilage, presence of pseudocoelom, and streamlined body shape.
• We assume knowledge of body cavity types such as coelom and pseudocoelom.

Concept / Approach:

Typical features of fishes include an exoskeleton of scales, a streamlined body adapted for swimming, gills for respiration in water, and an internal skeleton made of bone (bony fishes) or cartilage (cartilaginous fishes). In terms of body cavity, vertebrates including fishes possess a true coelom, not a pseudocoelom. A pseudocoelom is a body cavity not fully lined by mesoderm and is characteristic of some invertebrate groups such as nematodes. Therefore, the presence of pseudocoelom is not a correct characteristic for Pisces.

Step-by-Step Solution:

Step 1: List the standard characteristics of fishes as taught in zoology chapters. Step 2: Confirm that scales, gills, endoskeleton, and streamlined shape all fit well with typical fish structure and function. Step 3: Recall that fishes are vertebrates and have a true coelom. Step 4: Recognise that pseudocoelom is a feature of certain invertebrates, not vertebrate fishes. Step 5: Conclude that Presence of pseudocoelom is the feature that does not belong to Pisces.

Verification / Alternative check:

Classification tables in textbooks place fishes in the phylum Chordata and state that chordates are coelomate animals with a true coelom. The word pseudocoelom appears in chapters discussing invertebrates like roundworms. Additionally, fish descriptions emphasise streamlined body, scales, and gills, and there is no mention of pseudocoelom as a defining feature. This separation in the content reinforces that pseudocoelom is not associated with fishes.

Why Other Options Are Wrong:

Option A, Exoskeleton of scales, is correct for most fishes, which have bony or placoid scales covering their body. Option B, Breathing through gills, is a distinctive fish feature allowing gas exchange in water. Option C, Endoskeleton of bone or cartilage, describes bony fishes and cartilaginous fishes. Option E, Streamlined body shape, is a common adaptation for efficient swimming. These features all correctly describe Pisces and therefore are not the odd ones out.

Common Pitfalls:

Students may be unfamiliar with the term pseudocoelom and feel tempted to ignore it in favour of more familiar sounding options. Others might incorrectly think that body cavity terms apply to all animals in the same way, without distinguishing vertebrates and invertebrates. A good practice is to revise the definitions: coelom for true body cavity in higher animals like fishes, amphibians, reptiles, birds, and mammals; pseudocoelom for certain invertebrates like nematodes; and acoelom for animals without a body cavity.

Final Answer:

The feature that is not characteristic of fishes of class Pisces is Presence of pseudocoelom.