Tube within tube body plan is shown by

The correct option is A

Aschelminthes (Nemethelminthes)

The correct option is A

Explanation of the correct option:

1. The body of the aschelminthes is circular in cross-section, hence, it is called roundworms.
2. Roundworms have an organ-system level of body organization.
3. They are bilaterally symmetrical, triploblastic, and pseudocoelomate animals.
4. An excretory tube removes body waste from the body cavity through the excretory pore.
5. The tube within the tube body plan is seen in roundworms.
6. This plan calls for two openings: one for food to enter the body and another for waste to leave the body.

Explanation of the incorrect options:

Option B:

1. Platyhelminths are triploblastic, bilaterally symmetrical, dorsoventral, and acoelomate.
2. The body plan of Platyhelminthes is bilateral which means dividing the body into two equal halves. Example: Flatworms

Option C:

1. The members of the phylum Porifera are commonly known as sponges.
2. Asconoids are small and tubular. Water enters the Ostia in a large cavity called a Spongocoel.
3. The syconoid sponge is the same as the asconoid.
4. Leuconoid sponges have the most intricate design because not all compartments have flags. Water flowing through the flow channels is selectively pumped through the flow chambers and ejected through one of a series of vents.

Option D:

1. Coelenterates are aquatic animals, mainly marine animals.
2. The body form is radial symmetry, diploblastic and acoelomate (does not have a coelom/body cavity).
3. Example: Sea anemones and jellyfish.

Final answer: Tube within the tube body plan is shown by Aschelminthes (Nemethelminthes)