Question

Which one of these is not a cephalopod?
A) Octopus
B) Sea squirt
C) Nautilus
D) Sea squid

Answer 1

Cephalopods are the members of the class Cephalopoda. They all are the marine animals.The study of cephalopods is a branch of malacology.

Complete answer :

Cephalopods are characterized by the bilateral symmetry, a prominent head, a set of arms, or tentacles.
Cephalopods are found in the oceans of the Earth because they cannot tolerate the fresh water.
The members of cephalopods include Octopus, Sea squid, Cuttlefish, Nautilus, etc.

Characteristic features of Cephalopods:

Nervous system – Cephalopods are widely regarded as the most intelligent invertebrates among all the invertebrates. They have well developed senses and large brains. The nervous system of cephalopods is most complex among all the invertebrates.
Senses – Cephalopods have advanced vision. They have a variety of chemical sense organs.
Behaviour – Cephalopods are the social creatures.
Hearing – Cephalopods detect sound by using their statocysts.
Circulatory system – Cephalopods are the only molluscs who have a closed circulatory system.
Locomotion – Cephalopods move by jet propulsion. It is a very energy consuming process.
Excretory system – Cephalopods possess a single pair of large nephridia which produce nitrogenous waste and excrete it out from the body.
Therefore, from the above discussion it has been clear that Sea squirt is not a cephalopod.

Hence, option B is the correct answer.

Note: Cephalopods are a diverse group of species but they have a short life span. There are some cephalopods which are extinct. The extinct cephalopods include ammonites, belemnites and nautiloids except Nautilus which is a living species. Cephalopods have the closest affinity with the ancestors of class Gastropoda.