Animals in which the body cells are organised into three distinct embryonic germ layers (ectoderm, mesoderm and endoderm) are collectively called ________.

Introduction / Context:
This animal biology question deals with the concept of germ layers formed during embryonic development. The number of germ layers in an embryo is a key feature used to classify animals as diploblastic or triploblastic. Knowing the correct term for animals that possess three germ layers is important for understanding animal body plans and comparative anatomy.

Given Data / Assumptions:
- The question mentions three embryonic layers: ectoderm, mesoderm, and endoderm. - It asks for the collective term used for animals with all three layers. - Some options list germ layer names, and some list classification terms. - Standard embryology terminology is assumed.

Concept / Approach:
During embryonic development in animals, layers of cells differentiate to form the adult body tissues. Animals with only two principal layers, ectoderm and endoderm, are called diploblastic and include phyla like Coelenterata. Animals that have three germ layers, ectoderm, mesoderm, and endoderm, are called triploblastic. The mesoderm contributes to muscles, circulatory system, and many internal organs. Ectoderm and endoderm are names of germ layers, not of groups of animals. Therefore, the correct term for animals with three germ layers is triploblastic.

Step-by-Step Solution:
Step 1: Identify the three layers mentioned in the question: ectoderm, mesoderm, and endoderm. Step 2: Recall that animals with only two layers, ectoderm and endoderm, are described as diploblastic. Step 3: Remember that when a third layer, mesoderm, is present between ectoderm and endoderm, the animals are called triploblastic. Step 4: Note that ectoderm and endoderm themselves are names of individual layers, not group names for animals. Step 5: Compare the options and recognise that triploblastic is the only term that correctly describes animals with three germ layers. Step 6: Conclude that animals with ectoderm, mesoderm, and endoderm are triploblastic animals.

Verification / Alternative check:
Textbooks introduce a comparison table: diploblastic animals such as cnidarians have two layers, while triploblastic animals, including Platyhelminthes, Annelida, Arthropoda, Mollusca, Echinodermata, and Chordata, have three layers. The word triploblastic appears repeatedly alongside these phyla. Ectoderm, mesoderm, and endoderm are always listed as layer names, not as categories of animals. This consistent usage confirms that triploblastic is the correct classification term for animals with three germ layers.

Why Other Options Are Wrong:
Diploblastic: This term is used for animals with only two germ layers, lacking mesoderm, so it cannot describe animals that have all three layers. Ectoderm: This is the outermost germ layer that forms skin and nervous system structures, not the name for the whole group of animals. Endoderm: This is the innermost germ layer that forms the lining of the gut and related organs, again not the classification name for animals with three layers.

Common Pitfalls:
A frequent confusion arises from the similar looking terms diploblastic and triploblastic. Students may misread the prefix tri and choose the wrong one if they are not careful. Another pitfall is to mistake ectoderm or endoderm as group names because they are frequently mentioned in embryology sections. To avoid errors, focus on the prefixes diplo meaning two and triplo meaning three in these classification terms and remember that these words always refer to number of germ layers.

Final Answer:
Animals that possess three germ layers are called Triploblastic animals.