Give four reasons why a snail and an octopus are classified under the same phylum.

Explanation

The snail and octopus are soft bodied animals included in phylum mollusca. It is characterized by the following salient features :
1. The body of a molluscan has a muscular foot on the ventral side. On the dorsal side of the foot is the visceral hump containing the main digestive and circulatory systems. The visceral hump is covered by a mantle.
2. The mantle cavity outside the body and below the mantle has a number of feather like gills. Gills are the organs of respiration and excretion.
3. The anterior head region has sensory tentacles.
4. The mouth contains a file-like grasping organ for feeding.
The body organisation of the snail and octopus is more or less same except for few exceptions. These are :
(i) The octopus has lost the outer protective shell but it has eight arms and a developed brain and eyes.
(ii) The octopus has a mantle cavity modified into a device where water can be expelled with force.
(iii) The foot and head regions are combined and so they are termed as cephalopods.
(iv) Octopus is found in sea and snail is found on land.