Question

Tube-within-a-tube body plan is shown by
a. Coelenterates
b. Platyhelminthes
c. Aschelminthes (Nemathelminthes)
d. Porifera

Answer 1

Hint: We are familiar with the word phylum i.e. a level of classification that lies between the kingdom and the class. Phylum Coelenterates mainly live in saltwater and are radially symmetrical. The Platyhelminthes are also known as flatworms. Phylum Aschelminthes have bilateral symmetry and are also known as roundworms. Their digestive system is complete. Phylum Porifera: Their body wall has many pores and are supported by a skeleton of silica.

Complete answer:
> Option (a) Coelenterates and option (b) Platyhelminthes both show blind sac body plan i.e. there is only one opening of the digestive system or the digestive system is tube like means the food is taken and excreted out from the same opening.

> Option (d) Porifera- they show aggregate plan i.e. they consist of clusters of cells.
Aschelminthes are also called Nementhelminthes – means roundworms. This phylum was built by Gegenbaur. These animals possess a digestive canal with two openings such as mouth: at the anterior part and anus: at the posterior part.

Their body cavity is known as pseudocoelomate. A digestive canal is present in the body cavity for this reason it is known as tube-within-a-tube body plan. Hence, they have a complete digestive system.

Hence, the correct answer is option (c) i.e. Aschelminthes or Nemathelminthes.
They have organ grade of organization (i.e. when all the organs work together to perform any function).
Their body wall is made of three layers Cuticle, Epidermis and the Muscle layer and their excretory system is H shaped and the excretory substance is mainly ammonia. Nerve ring and nerve cord is present in Roundworms.
These organisms show sexual dimorphism i.e. male and female can be easily differentiated. Males are small in length and their tail is curved whereas the females are long and straight and they fertilize internally.

Note: Phylum Annelida also shows tube-within-a-tube body plan. Aschelminthes are round and Platyhelminthes are flat. Platyhelminthes are acoelomate whereas Aschelminthes are pseudocoelomate. Here, pseudocoelomate means false cavity. Platyhelminthes and Coelenterates do not have a complete digestive system.