Both planarians and nemertean worms are invertebrates that capture prey using protrusible tubes. Specifically, planarians use their pharynx and nemerteans use their proboscis to push out and retract a tube that can capture prey, with both tubes having a similar function. While their digestive systems differ in other ways, such as planarians lacking a complete digestive tract, their mouths and protrusible prey-capturing tubes are analogous structures that demonstrate their shared predatory behavior.

1. How is the mouth of planaria similar to the proboscis of Nemertean worms?
How is the mouth of planaria similar to the proboscis of Nemertean worms?
Solution
Both nemerteans and planarians are invertebrates. Planarians are tubellarian flat worms and
nemerteans have been called ribbon worms. Both nemerteans and planarians are acoelomates.
Nemerteans have complete digestive tract and circulatory system like coelomates, but some
similarities exist with planarians. Planarians have been mostly carnivorous and they do not have
digestive tract and circulatory system like nemerteans and other coelomates. Eventhough
nemerteans are the primitive organisms with closed circulatory system with blood vessels
structurally but functionally it looks like planarians. Planarians and nemerteans both have
ciliated epidermis, sense organs such as light sensitive pigment cups. Planarians have single
opening digestive tract with mouth and blind ended flame bulbs for excretion through diffusion.
But in nemerteans, this digestive tube have both ends possessing openings, with mouth for food
taking and blind ended tube develops into end for excretion. Reproduction through fragmentation
could be seen both in planarians and nemerteans. Nemerteans also have the name proboscis
worms because of distinctive feature of proboscis. Nemerteans use the proboscis to capture the
prey, immobilize the prey. Proboscis is a long, hollow, sticky, muscular tube which can be
moved inside out. The proboscis produces toxic chemical to kill the prey. Like the nemerteans
have proboscis to capture the prey, the planarians also have tube like muscular pharynx, which is
the first part of digestive tube of planarians. The planarians like the proboscis of nemerteans can
push its pharynx outward to catch the prey and can take back its pharynx inside once it catches
its prey through the mouth. Thus the pharynx of planarians and proboscis of nemerteans have
similar features.