Problem 17 - Campbell Biology Concepts & Connections

Question

Compare the structure of a planarian (a flatworm) and an earthworm with regard to the following: digestive tract, body cavity, and segmentation.

Step-by-Step Solution

Answer

Planarians have a simple, unsegmented body with a single-opening digestive tract and no body cavity, while earthworms have a segmented body with a tube-like digestive system and a true body cavity.

Step 1 - Digestive Tract

The planarian has a simple digestive system with a single opening that serves as both mouth and anus. Its digestive tract is branched, distribution nutrients throughout its body. In contrast, the earthworm has a more complex, tube-like digestive system with two separate openings: a mouth and an anus.

Step 2 - Body Cavity

Planarians lack a true body cavity, making them acoelomates. Their internal structure is filled with parenchyma cells. Earthworms, however, are coelomates and have a true body cavity (coelom) that is lined with mesoderm on both sides, allowing for better organ differentiation and development.

Step 3 - Segmentation

The planarian's body is unsegmented and has a simple structure. Earthworms, on the other hand, exhibit true segmentation, called metamerism, where their bodies are divided into many similar segments (metameres), each containing repeated organs and structures.

Key Concepts

Digestive Tract Structure

Understanding the different structures of digestive tracts is essential. In planarians, the digestive system is quite simple. They have a single opening that works as both a mouth and an anus, which limits the efficiency of food digestion and waste expulsion. This opening leads to a branched digestive tract that helps distribute nutrients throughout the planarian's body.

Earthworms, on the other hand, have a more advanced digestive system. They have a complete, tubular digestive tract with two distinct openings: a mouth for ingestion and an anus for excretion. This setup allows for a more efficient process of digestion, absorption, and waste elimination, enhancing their ability to process food.

The key points to remember:

• Planarians: Single opening, branched tract
• Earthworms: Two openings, tube-like system

Body Cavity Types

Body cavities play a vital role in the structure and function of animals. Planarians are classified as acoelomates, meaning they lack a true body cavity. Instead, their body is filled with a type of tissue called parenchyma, which supports their organs.

Earthworms belong to the category of coelomates. They possess a true body cavity, known as a coelom, which is lined with mesoderm on both sides. This cavity is filled with fluid and allows for better organ development and separation. It also provides a hydrostatic skeleton that helps the earthworm with movement.

Key points to note:

• Planarians: Acoelomate, parenchyma-filled body
• Earthworms: Coelomate, true body cavity

Segmentation in Animals

Segmentation offers structural and functional advantages in animals. Planarians do not have segmented bodies. Their body structure is relatively simple and unsegmented.

Earthworms, conversely, exhibit true segmentation, known as metamerism. Their bodies are divided into multiple segments called metameres. Each segment contains repeated organs and structures, such as a pair of nephridia for excretion. This segmentation allows for improved mobility and more efficient organ function.

Important points to remember:

• Planarians: Unsegmented body
• Earthworms: Segmented body, metamerism