Which of the following animal groups does not have tissues derived from mesoderm?
a. Annelids
b. Echinoderms
c. Cnidarians
d. Flatworms

Verified step by step guidance

1. Understand the concept of germ layers: In animal development, three primary germ layers form during embryogenesis—ectoderm, mesoderm, and endoderm. These layers give rise to different tissues and organs in the body.
2. Identify the role of the mesoderm: The mesoderm is the middle germ layer that develops into muscles, bones, the circulatory system, and other internal structures. Animals with mesoderm-derived tissues are considered triploblastic.
3. Recognize the difference between diploblastic and triploblastic animals: Diploblastic animals have only two germ layers (ectoderm and endoderm), while triploblastic animals have all three germ layers (ectoderm, mesoderm, and endoderm).
4. Analyze the animal groups listed: Annelids, echinoderms, and flatworms are triploblastic, meaning they have tissues derived from the mesoderm. Cnidarians, however, are diploblastic and lack a mesoderm layer.
5. Conclude that cnidarians do not have tissues derived from the mesoderm, as they only possess ectoderm and endoderm layers.

Key Concepts

Germ Layers

Germ layers are the primary layers of cells in an embryo that develop into different tissues and organs. In triploblastic organisms, there are three germ layers: ectoderm, mesoderm, and endoderm. The mesoderm is particularly important as it gives rise to muscles, circulatory systems, and other internal structures. Understanding the role of these layers is crucial for identifying which animal groups possess or lack specific tissue types.

Mesoderm

The mesoderm is the middle germ layer found in triploblastic organisms, situated between the ectoderm and endoderm. It is responsible for forming various structures, including muscles, bones, and the circulatory system. Animals that lack mesoderm, such as cnidarians, are classified as diploblastic, having only ectoderm and endoderm. Recognizing the presence or absence of mesoderm helps in categorizing animal groups based on their developmental biology.

Diploblastic vs. Triploblastic

Diploblastic organisms have two germ layers (ectoderm and endoderm), while triploblastic organisms possess three layers, including mesoderm. This distinction is fundamental in animal classification and affects the complexity of body structures. Cnidarians, such as jellyfish and corals, are examples of diploblastic animals, whereas annelids, echinoderms, and flatworms are triploblastic. Understanding these classifications aids in answering questions about tissue development in various animal groups.