Question

Which of the following animal phyla does not possess a coelom?
(a) Platyhelminthes
(b) Annelida
(c) Mollusca
(d) Echinodermata

Answer 1

The animal phyla that does not possess a coelom is generally known as flatworms, these invertebrate species are unsegmented worms with bilateral symmetry. In freshwater environments, some flatworms are free- living and widely found.

Complete step by step answer:
An acoelomate is an animal with a body cavity that does not exist. Acoelomates have a triploblastic body plan, meaning that three primary embryonic cells (germ cell) layers grow their tissues and organs. These tissue layers are the endoderm, mesoderm, and the outer layer or ectoderm. In these three layers, different tissues and organs grow.
The kingdom of Animalia and the phylum Platyhelminthes, constitute the of acoelomates.
The body of Platyhelminthes is solid between the outer surface and the cavity of the digestive system. The first invertebrates to have a third germ layer, the mesoderm, was believed to be Platyhelminthes.
So, the correct answer is, ‘(a) Platyhelminthes’.

Additional information: Some species do not have a coelom or may have a false (pseudocoelom) coelom. Animals with coeloms are referred to as coelomates, and acoelomates are called those without coeloms.Coelomate species have a body cavity called a coelom with a complete lining called a mesoderm- derived peritoneum. Coelomates are mainly bilateral animals, including all vertebrates.
Pseudocoelomate species have a pseudocoelom, which is a fluid- filled body cavity (literally "false cavity") . The fluid- filled body cavity of these animals is partly lined by tissue derived from mesoderm. Therefore, even though organs are kept loosely in place, they are not as well arranged as in a coelomate. The roundworm is an example of a Pseudocoelomate.
There are subtypes of Coelom as well:
- Schizocoelom: develops from mesoderm splitting found in annelids, arthropods, and molluscs.
- Haemocoelomic: decreased true coelom and blood- filled cavity from arthropod to mollusca.
- Enterocoely: evolves from the embryonic gut wall discovered from Echinodermata chordata

Note: Coelom, situated between the intestinal canal and the body wall, is the largest body cavity in most mammals. Platyhelminthes are either scavengers or predators; many are parasites that feed on their hosts' tissues. Platyhelminthes are classified into four classes: Turbellaria, aquatic free- living organisms; Monogenea, fish ectoparasites; Trematoda, human and other organisms internal parasites; and Cestoda (tapeworms), which are vertebrate internal parasites.