Question

Cestoda is differentiated from other flatworms by the absence of:
A. Digestive system
B. Nervous system
C. Excretory system
D. Reproductive system

Answer 1

Hint: Cestoda or commonly called as tapeworms is the class of phylum Platyhelminthes of Kingdom Animalia. Most of these organisms live as an endoparasite on the host's body. Many of these are infectious as well.

Complete answer:
Cestoda is a class of flatworms called tapeworms. These can grow as long as 18 meters. These usually inhabit the gut of hosts such as humans and animals. These have characters like all other flatworms such as:
These have dorso-ventrally flattened body, these are bilaterally symmetrical, triploblastic (have three germ layers) and acoelomate (no body cavity present).
These possess organ level of organization and possess hooks and suckers for the derivation of food and nutrition from the host body.
Some specialized cells called flame cells are present for osmoregulation and excretion.
These do not have separate sexes, these are hermaphrodites.
They support internal fertilization and the young one goes through many larval stages to become an adult.
As an additional feature, cestodes or a tapeworm is that they do not have their digestive system. They lack the alimentary canal as they obtain their food and nutrition from their host’s gut and do not require breaking it down as it is already broken down into simpler forms. Also, their body is moist and contains hooks and suckers on it which anchors them to the intestinal lining and provides them with adequate amounts of nutrition required for their growth and survival.

Therefore the correct answer for the given question is the digestive system.

Note: The tapeworms may lack digestive tract but they have a complex reproductive ability and life cycle as they complete their life cycle in various hosts. They do not infect the host purposely, the host only gets infected only when they ingest an encysted intermediate or egg.