Problem 256 - Objective Biology for NEET

Question

Assertion: Digestive system of platyhelminthes is incomplete. Reason: They have single opening to outside of the body, serve as both mouth as well as anus.

Step-by-Step Solution

Answer

The assertion that the digestive system of platyhelminthes is incomplete is correct. This is because it only has one opening to the outside of the body, which serves both as a mouth for food intake and an anus for waste elimination.

Step 1: Understanding a Complete Digestive System

A complete digestive system is one that has two separate openings – one for intake of food (mouth), and the other for elimination of waste (anus). This allows for a one-way flow of food through the system, from ingestion to digestion to absorption, and finally elimination.

Step 2: Understanding the Digestive System of Platyhelminthes

Platyhelminthes, or flatworms, have what is known as an incomplete digestive system. This is because they have only one opening to the outside of their body — this single opening serves both as a mouth for intake of food, and as an anus for elimination of waste.

Step 3: Examining the Assertion and Reason

Given this information, the assertion is correct: the digestive system of platyhelminthes is incomplete. The reason provided is also correct: platyhelminthes have a single opening that serves both as a mouth and an anus. This means that the flow of food and waste is not one-way, as it is in a complete digestive system.

Key Concepts

Incomplete Digestive System

Many organisms, including humans, have what is known as a complete digestive system. This means that food travels in one direction, from the mouth to the anus, passing through various organs for digestion and absorption. In contrast, the platyhelminthes, or flatworms, have an incomplete digestive system. This is because they possess only a single opening for their digestive process. This solitary opening serves dual functions:

• It acts as the mouth, where food is ingested into the body.
• It also serves as the anus, where waste is expelled from the body.

Without a distinct exit point for waste, their digestive processes differ significantly from those in more evolved organisms with complete systems.

One-way Flow of Food

In organisms with a complete digestive system, there is a clear one-way flow of food. This means that food is ingested through the mouth, travels through the digestive tract, and waste is eventually expelled through the anus. Each part of the digestive tract plays a specialized role, enhancing the efficiency of digestion. However, this is not the case for platyhelminthes. The lack of a separate exit opening in flatworms leads to a circular or bidirectional flow of food and waste. Instead of a one-way journey:

• Food enters and waste exits through the same opening.
• The digestive process is halted when it's time for waste removal, before any further ingestion can occur.

This setup can be less efficient and limits the organism's ability to continuously process food.

Flatworms

Flatworms, belonging to the phylum Platyhelminthes, are fascinating creatures with unique biological traits. One of their defining features is their flat, bilaterally symmetrical body. This body structure not only affects their movement but also their internal systems. Key characteristics of flatworms include:

• An incomplete digestive system, as previously discussed, with a single opening serving dual purposes.
• A lack of specialized circulatory and respiratory systems, with gases exchanged directly through their skin due to their flat shape.

Despite these simpler systems, flatworms are incredibly adaptable and inhabit a range of environments, from marine to freshwater and even on land. Their simplicity illustrates early evolutionary stages of more complex organisms.