Question

Which of the following statements aptly describes the phylum Annelida?
A) A group of animals in which the space between the ectoderm and endoderm is simply filled with mesenchymal mesodermal cells.
B) A group of animals with long, flat bodies without true segmentation, that lives in water or moist soil or are parasitic invertebrates.
C) A group of animals that have an elongated, cylindrical and unsegmented body.
D) A group of animals that have an elongated body that is composed of partitions called metameres or annulations.

Answer 1

The phylum Annelida belongs to the group of organisms which have little rings in their body. The body of these organisms is segmented and have a body cavity. These organisms are bilaterally symmetrical. An example of Annelida is earthworm.

Complete answer:
- The body of this organism is elongated, cylindrical and divided into segments called the septa. (Statement B and C incorrect)
- The body wall is made up of thin and moist non-cellular cuticle, single layered epidermis and circular longitudinal muscles. These muscles help in locomotion.
- These organisms have organ and tissue level of organisation.
- The organisms exhibit the presence of true coelom or body cavity.
-They are triploblastic. It means that they have an endoderm or the inner layer, mesoderm or the middle layer and ectoderm which is the outer protective layer.
- The respiration or exchange of gases takes place through most skin.
- They have a complete circulatory system with a body consisting of capillaries and blood vessels.
- The digestive system comprises the mouth and anus.
- The nephridia are the excretory organs present in these organisms.

Thus, the correct answer is option ‘A’.

Note: The metameric segmentation is the characteristic feature of Annelida and Arthropoda. The segmented body helps the organism in locomotion. It also protects the sensitive organs from damage. This allows the organism to perform complex activities. Earthworms, leeches and polychaetes belong to the phylum Annelida.