Question

Observe the following diagrams of invertebrate embryos illustrating the characteristics of the body plan.

Select the correct sequence which corresponds to the Phyla represented with $I,II,III,IV$ and $V$ .

• A.

  I	II	III	IV	V
  Cnidaria	Platyhelminthes	Annelida	Nematoda	Arthro- poda

• B.

  I	II	III	IV	V
  Cnidaria	Platyhelminthes	Nematoda	Arthro-poda	Annelida

• C.

  I	II	III	IV	V
  Nematoda	Arthropoda	Platyhelminthes	Cnidaria	Annelida

• D.

  I	II	III	IV	V
  Annelida	Cnidaria	Arthropoda	Platyhelminthes	Nematoda

Answer 1

Answer: (B)

Organ systems in various groups of animals have multiple patterns of complexities. For instance, the digestive system in Platyhelminthes has an only single opening to the outside of the body that serves as both mouth and anus and is hence known as incomplete. A complete gastrointestinal system has two openings, mouth and anus. Similarly, the
circulatory system is also of two types: (i) open type during which the blood is pumped out of the heart, and therefore the cells and tissues are directly bathed in it and (ii) closed type in which the blood is circulated through a series of vessels of varying diameters (arteries, veins and capillaries) The embryo of animal I show two germinal layers, ectoderm and mesoderm. This animal is diploblastic; it can be a poriferan or cnidarian. The embryos of all other animals have three germinal layers, the ectoderm, mesoderm and endoderm. These animals are triploblastic animals. They can be from Phylum Platyhelminthes to Phylum Mollusca. Embryos I and II haven't any coelom, Ill has pseudocoelom and IV, and V have a true coelom. The coelom is a fluid-filled cavity that forms the main body cavity of animals. It is formed by splitting of the mesoderm. Therefore, embryos I and II are of poriferans, cnidarians, ctenophores or Platyhelminthes; embryos III is of nemathelminthes; whereas embryos IV and V are of annelids, arthropods or molluscs. Embryos of V shows internal divisions or segmentation within the body.
Embryo V, thus, is annelidan. And the incomplete or blind gut is found in cnidarians and Platyhelminthes. So, we are able to conclude, embryo I is of cnidaria, embryo II is of Platyhelminthes, embryo III is of nemathelminthes, embryo IV is of arthropoda and embryo V is of Annelida.