The mollusk's feeding system starts with a rasping "tongue", the radula. The complex digestive system uses mucus and microscopic, muscle-powered "hairs" called cilia. The generalized mollusk has two paired nerve cords, or three in bivalves. The brain, in species that have one, circles the oesophagus. 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. All produce eggs, from which may emerge trochophore larvae, more complex veliger larvae, or miniature adults.
A striking feature of molluscs is the use of the same organ for multiple functions. For example, the heart and nephridia ("kidneys") are important parts of the reproductive system, as well as the circulatory and excretory systems. In bivalves, the gills both "breathe" and produce a water current in the mantle cavity: this is important for excretion and reproduction. In reproduction, molluscs may change gender to accommodate the other breeding partner.
Good evidence exists for the appearance of gastropods, cephalopods and bivalves in the Cambrian period, 541 to 485.4 million years ago (mya). Before that, the evolutionary history of molluscs' emergence from the ancestral Lophotrochozoa is still unclear.
Mollusk species can also represent hazards or pests for human activities. The bite of the blue-ringed octopus is often fatal, and that of "Octopus apollyon" causes inflammation that can last for over a month. Stings from a few species of large tropical cone shells can also kill: their venom have become important tools in neurological research. Schistosomiasis (also known as bilharzia, bilharziosis or snail fever) is transmitted to humans via water snail hosts, and affects about 200 million people. Snails and slugs can also be serious agricultural pests, and accidental or deliberate introduction of some snail species into new environments has seriously damaged some ecosystems.
Because mollusks have so many different shapes, many textbooks start the subject of molluscan anatomy by describing what is called an "archi-mollusc", "hypothetical generalized mollusc", or "hypothetical ancestral mollusc" ("HAM") to illustrate the most common features in the phylum. The picture is rather similar to modern monoplacophorans: some think it may resemble very early mollusks.
The generalized mollusk is bilaterally symmetrical and has a single, "limpet-like" shell on top. The shell is secreted by a mantle covering the upper surface. The underside consists of a single muscular "foot". The visceral mass, or visceropallium, is the soft, nonmuscular metabolic region of the mollusc. It contains the body organs.
Classes of molluscs:
It has become clear that the fossil taxon Helcionelloida does "not" belong to the class Gastropoda. It is now a separate class in the Mollusca. Parkhaev (2006, 2007) created the class Helcionelloida, whose members were previously treated as "Paleozoic mollusks of uncertain systematic position" by Bouchet & Rocroi.
Sponge
A sponge is a member of the phylum Porifera. It is a simple animal with many cells, but no mouth, muscles, heart or brain. It is sessile: it cannot move from place to place the way most animals can. A sponge is an animal that grows in one spot like most plants do. Nevertheless, sponges are quite successful.
The basic body plan is a jelly-like layer sandwiched between two thin layers of cells. Their bodies are full of pores and channels allowing water to circulate through them. Most of them feed on bacteria and other microorganisms. A few of them eat tiny crustaceans.
There are more than 10,000 species of sponge. Most sponges live in the ocean. A few live in fresh water. All sponges take in water through pores (little holes) in their bodies. The water goes out through a big tube in the center. Most sponges filter (take out) little bits of food from the water going through their bodies. Animals that get food this way are called filter feeders.
Sponges are the oldest animals with fossil evidence (from ~635 million years ago).
Sponges are simple animals. A few things make them different from other animals.
There are four classes of sponges. The difference between these kinds is in how their skeleton is made.
Sponges are sessile, they live in one place, where they are "fixed" to the ground. A few sponges can change their position, they can move at speeds of between 1 mm and 4 mm a day. They do this like amoebae. A few species can contract their whole bodies. Many can close their openings/holes.
A few species live in waters where there is very little food available. They have therefore changed, and became predators. They eat small crustaceans and other small animals. Most of these sponges belong to the family Cladorhizidae, but a few members of the Guitarridae and Esperiopsidae are also carnivores. In most cases little is known about how they actually capture prey. Some species are thought to use either sticky threads or hooked spicules. Most carnivorous sponges live in deep waters, up to , and the development of deep-ocean exploration techniques is expected to lead to the discovery of several more. However one species has been found in Mediterranean caves at depths of 17-23m alongside the more usual filter feeding sponges. The cave-dwelling predators capture crustaceans under long by entangling them with fine threads, digest them by enveloping them with further threads over the course of a few days, and then return to their normal shape; there is no evidence that they use venom.
Most known carnivorous sponges have completely lost the water flow system and choanocytes. However the genus "Chondrocladia" uses a highly modified water flow system to inflate balloon-like structures that are used for capturing prey.
Sponges usually reproduce when little pieces break off. If such a piece has the right types of cells it can grow to become a new sponge. A few sponges can also use budding. With budding a small sponge grows on the parent; when it is done growing, it simply "falls off". When the conditions are bad, some sponges can also grow lumps of unspecialised cells. These will not develop until the conditions improve again. They can then either make a new sponge, or they can use the skeleton of the parent sponge (that died).
Most sponges reproduce sexually. They can make sperm cells that are released into the water. These are either captured by another sponge, and are then transported to the egg cells of the parent. This is known as "viviparous". Both cells are joined to form larvae, which can swim off to find a good place to settle.
The other way, known as "oviparous" is that both sperm cells and egg cells are released into the water. These then combine outside the sponges.
A report in 1997 described how bottlenose dolphins in Shark Bay used sponges as tools: A dolphin may attach a marine sponge to its rostrum (snoutlike projection). It is thought the sponge is used to protect the rostrum when the dolphin is looking for food in the sandy sea bottom but scientists have not confirmed this. The behaviour, known as "sponging", has only been seen in this bay, and is almost exclusively shown by females. A study in 2005 concluded that mothers teach the behaviour to their daughters, and that all the sponge-users are closely related. This suggests that it is a fairly recent innovation. Dolphins also use sponges as an exfoliator, rubbing their skin against the sponge attached to the rock to get rid of their old/dry skin.