Question

The cross-section of the body of an invertebrate is given below. Identify the animal which has this body plan.

A. Cockroach
B. Roundworm
C. Planaria
D. Earthworm

Answer 1

Hint: Platyhelminthes are triploblastic and acoelomate invertebrates. The coelom is not found in these animals. Between the body wall and alimentary canal, mesenchyme tissue is found. This is also called parenchyma tissue. Planaria is a member of Platyhelminthes.

Step by step answer:
On the basis of the presence of coelom and type of coelom, animals are classified. In cross-section, body plan Platyhelminthes looks like this-

The outermost layer represents the body wall. The innermost layer represents the endoderm and the middle layer is called the mesoderm. In Platyhelminthes no coelom (cavity) is found. At the time of embryonic development, three germ layers are made from outside towards inside, these are called ectoderm, mesoderm, and endoderm.
Later ectoderm develops in the body wall. The endoderm forms an alimentary canal and the mesoderm forms body organs.
In Platyhelminthes, mesoderm is found in the form of mesenchyme tissue. Not any cavity is found between the body wall and the alimentary canal. So these are called acoelomate.
In aschelminthes a cavity is found between the body wall and alimentary canal but the mesoderm is spitted, so these are called pseudocoelomates.
In phylum Annelida, the true coelom is found. It has a complete lining of mesoderm.

Hence option C is correct.

Note: From Annelida, all higher invertebrates and chordates are coelomate. Animals of Porifera and Coelenterata are diploblastic or diploblastic.