because they have no skeleton.
An arthropod is an invertebrate animal having an exoskeleton (external skeleton), a segmented body, and jointed appendages. Arthropods are members of the Phylum Arthropoda (from Greek ἄρθρον...
arthropods
all are invertibrate but thats all i know thats why i asked this question
Vertebrates have a spine/backbone and mollusks and arthropods don't (they are invertebrates). See related questions below for more detail.