Do crabs & starfish are molluscs ?

Question

That's b'coz i think they are not.

Answer 1

You are correct. Neither crabs or starfish are molluscs.

Crabs are Arthropods - possessing a hard exoskeleton and jointed legs. Other arthropods include the extinct trilobites, spiders, scorpions, lobsters and all insects.

Starfish are Echinoderms - with radial symmetry and tube feet. Other echinoderms include crinoids, sea cucumbers and sea urchins.

Answer 2

Crabs are crustaceans which are in the phylum Arthropoda while starfish is in the phylum Echinodermata. So, neither a crab nor a starfish is a mollusc.

Answer 3

There is nothing common in them as crabs are crustaceans belonging to Phylum Arthropoda. A starfish is an echinoderm, belonging to class steroidea. Mollusks belong to Phylum Mollusca. The only underlying factor which runs through them is they are all invertebrates. Another sidelight is that crabs are related to crayfish, one of the mollusks is a cuttle fish, and starfish is a starfish. None of them are true fish.

Answer 4

Echinoderms are all invertebrates since they do not have a back bone but many of them have calcareous skeleton inside their bodies. Their soft body is covered with a tough skin which is either prickly, warty or spiny. The name Echinoderm is derived from the Greek echinos and derma meaning hedgehog skin. As well as this spiny skin they also have a special internal water system which allows them to alter the pressure within different parts of their bodies. For example they can extend or contract their tube-like feet which are used to pull them along but are also used for food collection and gas exchange.



Starfish are of course not fish but one of the Echinoderms and they have hundreds of tiny tube feet with suckers on the ends. These they use to move around but also to open the shells of their prey such as oysters and mussels. They are able to prise open the shells with these tube feet and then push their stomach out of their body and into the shell where it digests the mollusc before absorbing it.



The most familiar starfish around the Argyll islands is the Common Starfish, Asterias rubens, which is occasionally found in enormous groups up to a kilometre across. These aggregations move slowly along the coast eating crustaceans and bivalves as they go. The Common starfish is quite a large starfish, growing to 50 cm and they are found on all types of shore below the tide line. Most starfish have 5 arms but some are found with 6 or more.



The Common Sun Star, Crossater papposus, is similar to the starfish but has 8 – 12 narrow arms around a disc-like body. It is carnivorous and in fact its favourite food is the common starfish!







Brittlestars also have a central disc-like body but this is tiny compared to their five long, spiny arms. Their internal skeleton is embedded beneath their skin and arranged in plates down the arms giving a jointed appearance. When attacked the brittlestars drop the ends of their arms off and they can regenerate them later. Most of them are scavengers and use their tube feet to collect debris from the sea bed.



Another Echinoderm nis the beautiful sea urchin. Their brightly coloured spherical shells are often found washed up along the high tide line. They feed on algae especially kelp and have a mouth surrounde by 5 hard plates which is called the Aristotle's Lantern. The spines of the Common Sea Urchin, Echinus esculentus, are often red with violet tips and when broken off they reveal a deeper red or purple shell or “test” covered with white spine bosses. The spines grow from these bosses and work on a ball and socket arrangement allowing plenty of movement so that the spines can be used to move the urchin along and also to remove debris and for defense.

Answer 5

crabs comes under arthropods
starfish comes under molluscs

Answer 6

starfish is an echinoderm
crab is a crustacean