Problem 71 - Objective Biology for NEET

Question

Platyhelminthes are called flat worms because (a) They are triploblastic (b) They are without coelom (c) They have organ level of organization (d) Their body is dorsoventrally flattened

Step-by-Step Solution

Answer

The correct answer is (d). Platyhelminthes are called flat worms because their body is dorsoventrally flattened.

Step 1: Analyze all given options

The first step is to understand all the given options: (a) Triploblastic means an organism has three primary germ cell layers, the ectoderm, mesoderm, and endoderm. (b) Absence of coelom means the organism doesn't have a main body cavity. (c) Having an organ level of organization means that the organism's cells form tissues, which then form organs. (d) A dorsoventrally flattened body means the organism's body is flat from its dorsal (back) side to its ventral (belly) side.

Step 2: Match the options with common name 'Flat worms'

The common name 'flat worms' suggests that these creatures have a flattened body structure. Using this clue, (d) Their body is dorsoventrally flattened seems to fit the statement best. It's this characteristic that gives them their common name, not their cellular organization, presence/absence of a coelom, or level of body organization.

Step 3: Confirm the answer

Upon reviewing options and logical matching, it can be confirmed that option (d) 'Their body is dorsoventrally flattened' is the correct choice giving them the name 'Flat worms'. This physical attribute of Platyhelminthes accurately explains their common name.

Key Concepts

Dorsoventrally Flattened Body

Understanding the unique body structure of plathelminthes is pivotal to grasping why these organisms are commonly known as flatworms. Their distinctive shape is described as a dorsoventrally flattened body. This means that their anatomy is compressed from the dorsal, or back, side down to the ventral, or belly, side, which gives them a pancake-like appearance when viewed from the side.

This flattened shape has biological advantages. It increases the surface area to volume ratio, which aids diffusion, allowing efficient exchange of gases and nutrients directly across their body surface. This adaptation is also beneficial for living in narrow spaces within their environments, such as under rocks or within the layers of sediment at the bottom of a body of water.

Triploblastic Organisms

In the biological context, triploblastic organisms designate those species which, during embryonic development, form three primary layers: the ectoderm, mesoderm, and endoderm. Platyhelminthes fit into this category as they have these three cell layers that further differentiate into various tissues and organs as they mature.

Each layer has its specialized role. The ectoderm develops into the nervous system and the skin, the mesoderm becomes the muscles and reproductive system, and the endoderm forms the lining of their digestive tract. This triploblastic characteristic represents a significant evolutionary advancement from diploblastic organisms, which only have two tissue layers, and is central to the complexity of these organisms' body structures.

Organ Level of Organization

When we mention an organ level of organization, we refer to a complexity in the biological system where tissues group together to form organs, each with specialized functions. Platyhelminthes exhibit this level of biological organization. Their cells form tissues like muscle, nerve, and digestive tissues, which in turn group together to create organs.

An organ level of organization is more advanced than a tissue level, where the tissues do not combine to form organs, or a cellular level, where cells do not form tissues. For Platyhelminthes, this organizational structure allows for more specialized functions such as digestion and reproduction, albeit in a simpler form compared to higher organisms that have systems of organs working in unison.

Absence of Coelom

A key feature in the anatomy of Platyhelminthes is the absence of coelom. A coelom is a fluid-filled body cavity that is completely lined with tissue derived from the mesoderm, which acts as a buffer and provides space for the development and suspension of the organism's internal organs.

In contrast, Platyhelminthes are acoelomates—meaning their body does not feature this cavity. Instead, the space between the organs is filled with a tissue called mesenchyme or parenchyma. This characteristic is crucial for understanding the simplicity of their structure relative to other triploblastic organisms, as it affects how their organs are arranged and how they function within the limited anatomical space. The absence of a coelom is one reason why platyhelminths' bodies are flat.