Mollusks are the second largest group of animals in number of species (about 100,000 species), being supplanted only by arthropods.
They present a morphological disparity without comparison among the other phyla of animals, bringing together the family snails (reptantes), oysters and clams (sessile) and squid and octopuses (free-natants), as well as little-known forms such as chitons, elephant tooth shells (Scaphopoda) and vermiform species (Caudofoveata and Solenogastres).
Mollusks have invaded almost every environment; It is often said that there are just no clams flying. They occur from the abyssal pits to the highest mountains; from Antarctic glaciers to torrid deserts.
Several groups of bivalves and gastropods came out of the sea and invaded freshwater and, in the case of gastropods, the terrestrial environment. There are predatory (even vertebrate) molluscs, herbivores, ecto and endoparasites, filterers, diners, sessiles, vile, pelagic, neustonic, etc. In certain environments they represent large biomass and may be important in nutrient recycling.
Evidence of man's contact with mollusks goes back to prehistoric times. Mollusk shells are part of archaeological deposits, including, here in Brazil, the "sambaquis".
The mollusks served as food and their shells were used as ornaments and for making cutting utensils, abrasions etc.
There are reports of many cultures in which shells were used as coins or even display of power and wisdom. Even today mollusks are extremely important in the economy of many countries as a source of protein rich food, being collected directly from nature or even cultivated. In many countries, even a pearl and mother-of-pearl adornment industry is possible. They are of medical and health interest because many species are vectors of diseases, while others apparently can be used to control them.
Clams are animals. triblastics, hairy and protostomies. They present the soft, non-segmented body with bilateral symmetry. The head occupies the anterior position, where the mouth opens, entrance of the digestive tract. Many sensory structures are also located in the head, such as the eyes. Chemical sensors are also present in the mollusks and allow one to foresee the approach of natural enemies, when the mollusk quickly closes its shell, placing itself protected.
The foot is the most developed muscle structure of mollusks.. With it, they can move, dig, swim or capture their prey. The rest of the organs are in the visceral mass.
In it are the digestive, excretory, nervous and reproductive systems. Around the visceral mass is the mantle, responsible for the production of the shell.
Organization of a bivalve, note the presence of the inhaling and exhaling siphon.
Between the visceral mass and the mantle, there is a chamber called mantle cavity. In aquatic mollusks, this cavity is occupied by the water that bathes the gills; in terrestrial, it is full of air and richly vascularized, functioning as a gas exchange organ, analogous to a lung.
Organization of a gastropod
A striking feature of most mollusks is the shell presence. It is a limestone carapace, which guarantees good protection to the animal. In slugs and octopuses it is absent; in the squid it is small and internal.
Mollusks are complete enterozoans (which have digestive cavities). Many of them have a grating structure called radula. With it, they can scrape pieces of food, breaking them into small portions. Food digestion takes place almost entirely within the digestive tract (extracellular digestion). Some macromolecules only complete their fragmentation within the gut lining cells (intracellular digestion).
THE radula It is a structure that is located at the base of the mouth of the clam.
Most mollusks have open or lacunar circulatory system, in which blood is driven by the heart, passes through some vessels and then reaches gaps between the various tissues, where it circulates slowly, under low pressure, leaving nutrients and oxygen, and collecting carbon dioxide and other metabolic waste.These gaps are the hemoceles. Cephalopods are an exception because they have a closed circulatory system.
In the cellomatic cavity the nephrids, the excretory structures. Through the internal opening of nephrids (the nephrostome), penetrate substances present in the blood and in the cellomatic fluid. In some mollusks, such as cephalopods, nephrids are quite clustered, forming a primitive "kidney".
In almost all mollusks, the mantle membrane is vascularized and allows gaseous exchange between blood and water to occur. In terrestrial molluscs such as the garden snail (Helix sp.), the mantle cavity is full of air and behaves like a lung. It is therefore a particular form of pulmonary respiration. In aquatic mollusks, there are slides richly irrigated by blood vessels in the mantle that form the gills of these animals. Therefore, among the mollusks we can find lung breathing and gill breathing.
O nervous system of the clam is ganglion, with three parts of nerve ganglia from which nerves depart to the various parts of the body. The cephalopods have a large cerebroid ganglion, similar to the vertebrate brain which allows the execution of highly elaborate activities.
The movement of most representatives is slow due to the muscular foot. Those who are fast, such as squids and octopuses, move around thanks to the expulsion of water jets coming out of a siphon. Many, however, are attached to the substrate, such as oysters and shellfish in adulthood.
Reproduction of molluscs is sexual and in most group representatives the fertilization is internal and crossed. The garden snail, for example, is monoecious. In copulation, two individuals approach and touch their genital pores, through which they fertilize each other. The eggs develop and, upon hatching, release new individuals without passing through the larval phase (direct development).
In cephalopods, the male carries a package of sperm that is introduced into the female's mantle cavity for fertilization. After fertilization, thousands of eggs with gelatinous shell are released. Females of many species lay their eggs in protected places, under rocks, inside caves, etc. Some octopus females even take care of eggs by "aerating" them with water jets expelled from the siphon. Development is straightforward, without larva. Most of the pups that are born will serve as food for several predators. Few octopuses and squids reach adulthood, as the parent's death coincides with the birth of the young.
Mollusks and the Environment