Mollusks

Mollusks, also called molluscs, molluskans, and molluscans, are invertebrate animals in the 2nd-largest phylum Mollusca after the arthropods. Around 85,000 extant species of mollusks are recognized. The number of fossil species is estimated between 60,000 and 100,000 additional species. The proportion of undescribed species is very high. Many taxa remain poorly studied.

Mollusks are the largest marine phylum, comprising about 23% of all the named marine organisms. Numerous mollusks also live in freshwater and terrestrial habitats. They are highly diverse, not just in size and anatomical structure, but also in behavior and habitat. The phylum is typically divided into 7 or 8 taxonomic classes, of which 2 are entirely extinct. Cephalopod mollusks, such as squid, cuttlefish and octopuses, are among the most neurologically advanced of all invertebrates - and either the giant squid or the colossal squid is the largest known invertebrate species. The gastropods (snails and slugs) are by far the most numerous mollusks and account for 80% of the total classified species.

The 3 most universal features defining modern mollusks are a mantle with a significant cavity used for breathing and excretion, the presence of a radula (except for bivalves), and the structure of the nervous system. Other than these common elements, mollusks express great morphological diversity, so many textbooks base their descriptions on a "hypothetical ancestral mollusk". This has a single, "limpet-like" shell on top, which is made of proteins and chitin reinforced with calcium carbonate, and is secreted by a mantle covering the whole upper surface. The underside of the animal consists of a single muscular "foot". Although mollusks are coelomates, the coelom tends to be small. The main body cavity is a hemocoel through which blood circulates; as such, their circulatory systems are mainly open. The "generalized" mollusk's feeding system consists of a rasping "tongue", the radula, and a complex digestive system in which exuded mucus and microscopic, muscle-powered "hairs" called cilia play various important roles. The generalized mollusk has 2 paired nerve cords, or 3 in bivalves. The brain, in species that have it, encircles the esophagus. Most mollusks have eyes, and all have sensors to detect chemicals, vibrations, and touch. The simplest type of molluscan reproductive system relies on external fertilization, but more complex variations occur. Nearly all produce eggs, from which may emerge trochophore larvae, more complex veliger larvae, or miniature adults. The coelomic cavity is reduced. They have an open circulatory system and kidney-like organs for excretion.

Good evidence exists for the appearance of gastropods, cephalopods, and bivalves in the Cambrian period, 541–485.4 million years ago. However, the evolutionary history both of mollusks' emergence from the ancestral Lophotrochozoa and of their diversification into the well-known living and fossil forms are still subjects of vigorous debate among scientists.

This category contains all mollusks that have appeared on Wild Kratts.