Question

Which of the following animals lack alimentary canals (complete digestive systems)?
A.Earthworm
B.Jellyfish
C.Insect
D.Fish

Answer 1

Hint:A complete alimentary canal includes a rostral and caudal opening.
In incomplete digestive system,digestion is mainly intracellular.It can be extracellular and also inside the gastrovascular cavity wherein the digestive enzymes are secreted by the endothelial cells

Complete step by step answer:
-Jellyfish is a coelenterate and is a medusa (motile coelenterate). It lacks an alimentary canal. -Digestion is mainly intracellular in it. It can be extracellular and also inside the gastrovascular cavity wherein the digestive enzymes are secreted by the endothelial cells
-The alimentary/ gastrointestinal tract (GIT) is the tract from the mouth (rostral/ cranial opening) to the anus (caudal opening) which includes all the organs of the digestive system in humans and other animals. The food taken in through the mouth is digested to extract the nutrients and absorb energy. The waste is then expelled as feces.
-An incomplete digestive system consists of a digestive cavity (gastrocoel in the jellyfish) with one opening. The single opening serves as both the mouth and the anus. A complete digestive system consists of a digestive tract with two openings (mouth and anus).
-Primitive animals including sponges, roundworms, coelenterates, ctenophores, etc. have an incomplete digestive tract.

Hence option B is correct.

Additional Information:
They are multicellular organisms that exhibit tissue grade of the body organization
They are diploblastic organisms with a non-cellular layer that is known as the mesoglea in between the ectoderm and the endoderm
They are radially symmetrical
They are marine or freshwater organisms
They can be solitary or live in colonies. Each individual is known as a zooid
These organisms exist in two morphological forms that include polyps and medusa

Note:
-Polyps contain an exoskeleton and an endoskeleton. The skeletons are composed of calcium carbonate.
-Respiration and excretion occur through the process of simple diffusion.
-The circulatory system is rudimentary or absent.
-Asexual reproduction is seen in polyps, through budding and sexual reproduction is seen in medusa form, through gametic.