Mollusks, also spelled as molluscs, are living entities characterized by the presence of a hard outer shell. This phylum encompasses some of the highly-diverse animal species in the world. Mollusks are invertebrate animals, meaning that they lack the vertebral column. More information on mollusk characteristics is given in the following paragraphs. As of date, more than 85,000 species of living mollusks are identified scientifically. Contributing to about 23 percent of all marine organisms, it is considered as the largest phylum of the marine world. Underneath the outer shell of mollusks lies a soft body that is prone to desiccation. In short, the shell serves as a protecting structure for these soft-bodied animals. Studying mollusk characteristics is interesting for avid zoologists and marine biologists, particularly because of the distinctive features exhibited by these organisms as a means of adaptation to their natural habitat. Classification The name mollusks is coined from the Latin word for soft body (Molluscus). The species of mollusks are grouped under the phylum Mollusca and superphylum Lophotrochozoa. This phylum is further classified into 7-9 classes, of which Gastropod is the largest group that comprises snails and slugs. Habitat Mollusks are found in highly diverse environment, ranging from high altitude mountains to the deep oceans. They are available in gardens, large trees, thick forests, ponds, streams, estuaries, continental shelves, and deserts. Thus, mollusks are nearly ubiquitous in occurrence. Locomotion Some truly aquatic mollusks are fast swimmers, while others are stationary animals that remain attached to ground rocks or plant parts. You will also find many of them inhabiting underground habitats by their burrowing habit. In addition, some species move from one place to another by crawling. Size The size of mollusks may range from a small, inconspicuous body (about 0.1 inches) to as large as 70 m in case of some squid. The giant squid, recorded as the largest invertebrate animal species, has a body length of approximately 12 m and tentacles measuring about 50 m. Likewise, the weight of microscopic mollusks is measured in mg, while an adult giant squid weighs about 270 kg. Examples The nudibranchs are a group of mollusks having remarkably vibrant colors. These colorful marine animals lost their outer shell during the process of evolution. Some of the commonly known examples of mollusks are snails, slugs, oysters, mussels, squid, octopus, etc. Common Characteristics Despite the diversities in the appearance and size of mollusks, there are some basic features that are shared by all species. They are, a soft limbless body (bilaterally symmetrical), absence of skeletons, and being cold blooded. The body can be separated into three basic parts, the head, mantle, and a muscular foot. It is the mantle that forms the shell and also increases it in size. Some species possess internal skeletons, examples of which are the cuttlefish and the octopus. Speaking more about animal facts on mollusks, the anatomy shows a gut with a mouth, basic digestive parts, and the anus at the end. They possess all major organ systems like the brain, an open heart, kidneys, and gills, but lack cavities. The food habit of mollusks varies in accordance with the species. Reproduction takes place by means of sexual methods. This was a brief overview of mollusk characteristics. Some species have strikingly beautiful shells, which are collected for decorative purposes. Also, pearls are some of the most valuable jewels harvested from oysters. Marine biologists are sure of the occurrence of many other mollusk species in various habitats, especially in the vast seas and oceans. So, it is quite likely that the number of mollusks will increase with the passage of time.