In animals such as annelids and molluscs, organs associate to form functional systems, each concerned with a specific physiological function. This pattern is called which level of organisation?

Introduction / Context:

Animal bodies show increasing complexity of organisation from simple to advanced groups. At lower levels such as sponges and cnidarians, cells may be loosely arranged or form tissues, while in more advanced animals, organs combine to form systems. Recognising the different levels of organisation helps in understanding animal diversity and evolution. This question focuses on animals like annelids and molluscs and asks which level of organisation describes their pattern, where organs work together as systems for specific physiological functions.

Given Data / Assumptions:

• The animals mentioned include annelids and molluscs.
• In these animals, individual organs associate to carry out functions.
• These associations perform specific physiological roles such as digestion, circulation, and excretion.
• We must identify the appropriate organisational level term.
• We assume standard zoology classification of levels such as cellular, tissue, organ, and organ system.

Concept / Approach:

The simplest multicellular animals like sponges have cellular level organisation, where cells are loosely arranged. Cnidarians and ctenophores show tissue level organisation. In flatworms, organs begin to form, leading to organ level organisation. In annelids, arthropods, molluscs, and chordates, organs do not work alone but are grouped into organ systems such as the digestive system, circulatory system, and nervous system. Each system is made of multiple organs and has a specific physiological function. This pattern is called organ system level of organisation. Open or closed circulatory terms refer only to types of circulatory systems, and coelom refers to body cavity, not the organisational level as a whole.

Step-by-Step Solution:

Step 1: Recall the four main levels of organisation in animals: cellular, tissue, organ, and organ system. Step 2: Note that animals like annelids and molluscs have distinct structures such as hearts, kidneys, and digestive tracts. Step 3: Recognise that these organs are combined into systems like the circulatory system and excretory system, each with a defined role. Step 4: Match this pattern, where organs form systems, to the term organ system level of organisation. Step 5: Select organ system level of organisation as the correct answer.

Verification / Alternative check:

Zoology textbooks often present a table where examples are given for each level of organisation. Sponges are listed under cellular, cnidarians under tissue, flatworms under organ, and annelids onward under organ system. The descriptions for annelids and molluscs highlight the presence of well developed organ systems. Diagrams of earthworms and snails label multiple organ systems working together, confirming that the correct term for their organisational pattern is organ system level.

Why Other Options Are Wrong:

Option A, Organ level of organisation, applies more appropriately to flatworms, where organs are present but integrated systems are still less developed. Option B and Option C, open or closed circulatory level, describe variants of circulatory systems but are not names for overall body organisation. Option D, Coelom based level of organisation, refers to the presence or absence of a body cavity and does not directly describe how organs combine into systems.

Common Pitfalls:

Students may confuse organ level with organ system level, especially if they have not carefully noted examples for each. Others may focus on the presence of a coelom in annelids and select an option that mentions coelom rather than organisational level. To avoid such errors, learners should remember that the key phrase in the question is organs associated to form functional systems, and this is the hallmark of organ system level organisation.

Final Answer:

This pattern in animals like annelids and molluscs is called the organ system level of organisation.