Molluscs are the largest marine phylum, comprising about 23% of all marine organisms. Numerous molluscs can also live in freshwater and terrestrial habitats. They are highly diverse, not just in size but even in anatomical structure, but also in behaviour and habitat. The phylum is divided into seven or eight taxonomic classes, among which two are entirely extinct. Cephalopod molluscs, such as cuttlefish, squid, and octopuses, are advanced neurologically of all invertebrates. The largest known invertebrate species is either the giant squid or the colossal squid. The gastropods are the most that molluscs account for about 80% of the total classified species.
The Phylum Mollusca Consists of Eight Classes, They are:
The class Monoplacophora was discovered in 1977.
The worm-like animals found in class Aplacophora or solenogasters of the deep sea.
Another worm-like animal of class Caudofoveata.
The class Polyplacophora is also known as chitons.
The class Pelecypoda also known as bivalves.
The class Gastropoda is also known as snails.
The class Scaphopoda also known as tusk shells.
The class Cephalopoda includes among others squid and the octopus.
Chitons are marine molluscs that are of varying size and found in the class Polyplacophora which is formerly known as Amphineura. About 940 chitons are extant and 430 fossil species of chitons are recognized so far. These are sometimes also known as sea cradles or "coat-of-mail shells'', or more formally they are also known as loricates, polyplacophorans, and Polyplacophora. Chiton meaning is as follows: it is a bilateral marine mollusc made up of a dorsal shell that is made up of calcareous plates.
Chitons have a shell that is composed of eight separate shell plates or valves. These plates or valves overlap slightly at the front and back edges of the chitons and yet they articulate well with each other. Because of this reason, the shell of the chitons provides protection at the same time permits the flexible upward locomotion over uneven surfaces whenever required. Sometimes even this shell allows the animals to curl up to form a ball when dislodged from rocks. The shell plates are encircled by a skirt-like structure known as a girdle.
Chitons habitat is found worldwide, from the cold waters to the tropics. They live on hard surfaces, such as on or under rocks, or in rock crevices. Some of the species live quite high in the intertidal zone and they are not exposed to the air and light for longer periods. Most of these species inhabit the intertidal or subtidal zones and they do not extend beyond the photic zone, but few of these species can live in deep water with a depth of 6,000 m.
Chitons are fully marine animals. In contrast to the bivalves, which are capable of adapting to the brackish water and fresh water, and the gastropods are able to make the successful transitions to freshwater and terrestrial environments. Similar to most of the species of the saltwater limpets, several other species of a chiton can exhibit homing behaviours such as journeying to feed and then returning to the exact spot to which they previously inhabited. The method that is used by chitons to perform such behavioural actions has been investigated to some extent but those methods remain unknown.
One of the theories has the chitons that remember the topographic profile of the region. They are able to guide themselves back to their home by the physical knowledge they have about the rocks and visual input from their numerous primitive eyespots. The sea snail Nerita textilis can deposit a mucus trail as it moves. It has a chemoreceptive organ that is able to detect and guide the snail to get back to its home site. It is unclear that the chiton homing functions in the same way. These may leave the chemical cues along with the rock surfaces, with their olfactory senses these can detect the way to the home. Furthermore, older trails may also be detected that are provided with further stimulus for the chiton to find their home.
The radular teeth of chitons are made up of magnetite, and iron crystals. Within these substances, it is involved in the magnetoception, it is the ability to sense the polarity and the inclination of the magnetic field of the Earth. Experimental work has suggested that chitons can detect and respond to magnetism.
Chiton Animal - Morphology
Shell: All chitons bear a protective dorsal shell and it is divided into eight articulating aragonite valves that are embedded in the tough muscular girdle that helps to surround the chiton's body. Compared with the single or two-piece shells of other molluscs, this arrangement of shells allows the chitons to roll into a protective ball when dislodged. These can cling tightly to irregular surfaces. In some of the species, the valves are reduced or are covered by a structure called girdle tissue. These valves consist of a variety of colours, more patterns, they are smooth and are sculptured. Most of the anterior plate is crescent-shaped; it is known as the cephalic plate. The most posterior plate is known as the anal plate sometimes it is called the "tail plate", even though the chitons do not have tails.
The inner layer of each of the six intermediate plates is produced anteriorly as an articulating flange, called the articulamentum. This inner layer can also be produced in the form of notched insertion plates laterally. These plates function as an attachment to the valve plates with respect to the soft body. A similar series of these insertion plates may be attached to the convex posterior border of the anal plate or the convex anterior border of the cephalic plate. The sculpture of these valves is in one of the taxonomic characteristics, along with the speculation or granulation of the girdle.
After a chiton dies, these individual valves can make up the shell as eight parts that come apart because the girdle is no longer holding these shells together. These plates sometimes wash up in the drifts that occur on the beaches. The individual shell plates that are on the chiton due to their shape are sometimes known as "butterfly shells".
Girdle Ornament: The girdle may be ornamented with scales or spicules that are like shell plates. These are mineralized with the aragonite although a different mineralization process operates on the spicules to that in the teeth or shells. This process seems to be quite simple in comparison to other shell tissue. In some taxa groups, the crystal structure of these deposited minerals can closely resemble the disordered nature of the crystals that form inorganically.
The protein component of these scales and sclerites is minuscule in the comparison with the other biomineralized structures, whereas the total proportion of the matrix is higher than that of the mollusk shells. This implies that the polysaccharides can make up the bulk of the matrix. The girdle spines often bear a length that has parallel striations. The wide form of these girdle ornaments can suggest that it serves a secondary role. The chitons are capable of surviving perfectly well without these ornaments. Camouflage or defense are the two likely functions that are performed by these ornaments.
Spicules are secreted by cells that do not express "engrailed", but these cells are surrounded by engrailed-expressing cells. These neighbouring cells can secrete an organic pellicle on the outside of the spicule that is in the developing stage. The cells whose aragonite mineral is deposited by the central cell the subsequent division of these central cells allows the larger spines to be secreted in a certain kind of taxa. The organic pellicule that is found in most of the class Polyplacophora is unusual in the class Aplacophora. Developmentally, sclerite-secreting cells arise from pre-trochal and post-trochal cells.
Internal Anatomy: The girdle is often ornamented with bristles, spicules, hairy tufts, spikes, or snake-like scales. The majority of these body features are a snail-like foot, but that has no head or other soft parts beyond the girdle that are visible from the dorsal side. The mantle cavity consists of a narrow channel on each side lying in between the body and the girdle. Water can enter the cavity through openings that are present on either side of the mouth. Then the water flows along the channel to a second exhalant or the opening and closing to the anus. Multiple gills can hang down into the mantle cavity along the part or all of the lateral pallial groove. Each of these consists of a central axis with a number of flattened filaments through which absorption of oxygen takes place.
The chitons consist of a three-chambered heart that is located towards the hind end of the animal. Each of the two auricles that are present can collect the blood from the gills on one side. While the muscular ventricle pumps the blood through the aorta and around the body.
The excretory system consists of two nephridia, that are connected to the pericardial cavity around the heart. The excreta can be removed through a pore that opens near the rear of the mantle cavity. These consist of a single gonad that is located in front of the heart and they can release the gametes through a pair of pores that are present just in front of those used for the process of excretion.
The mouth is located on the underside of the animal and contains a structure called a radula; it is a tongue-like structure that has numerous rows of teeth, each consisting of 17 teeth. The teeth are coated with the minerals of magnetite that is a hard ferric or ferrous oxide mineral. The radula is used to scrape microscopic algae off the substratum. The mouth cavity itself is lined within the chitin and it is associated with a pair of salivary glands. There are two sacs that can open from the back of the mouth, among these sacs one containing the radula, and the other containing a protrusible sensory subgranular organ that is pressed against the substratum to taste for food.
Cilia can help to pull the food through the mouth through a stream of mucus and the esophagus. It is partially digested by enzymes that are produced from a pair of large pharyngeal glands. The esophagus can open up into a stomach, where enzymes from a digestive gland complete the breakdown of the food. Nutrients are absorbed through the linings that are found inside the stomach and the first part of the intestine. The intestine is divided into two parts with the help of a sphincter, where the latter part is being highly coiled and the functioning can compact the waste matter into fecal pellets. The anus opens just behind the foot.
Chitons can lack a clearly demarcated head where their nervous system resembles a dispersed ladder. No true ganglia are present whereas in other mollusks although a ring of dense neural tissue occurs around the esophagus. From this ring, nerves branch forwards to innervate the mouth and subradula, while two pairs of main nerve cords run back through the body. One pair, the pedal cords, innervate the foot, while the palio visceral cords innervate the mantle and remaining internal organs. Some species bear an array of tentacles in front of the head.
Senses: The primary sense organs of chitons are the subgranular organ and a large number of unique organs called aesthetes. The aesthetes consist of light-sensitive cells just below the surface of the shell, although they are not capable of true vision. In some cases, however, they are modified to form ocelli, with a cluster of individual photoreceptor cells lying beneath a small aragonite-based lens. Each lens can form clear images and is composed of relatively large, highly crystallographically aligned grains to minimize light scattering.
An individual chiton may have thousands of such ocelli. These aragonite-based eyes make them capable of true vision through research continues as to the extent of their visual acuity. It is known that they can differentiate between a predator's shadow and changes in light caused by clouds. An evolutionary trade-off has led to a compromise between the eyes and the shell; as the size and complexity of the eyes increase, the mechanical performance of their shells decreases, and vice versa.
A relatively good fossil record of chiton shells exists, but ocelli are only present in those dating to ten million years ago or younger; this would make the ocelli, whose precise function is unclear, likely the most recent eyes to evolve. Although chitons lack osphradia, statocysts, and other sensory organs common to other mollusks, they do have numerous tactile nerve endings, especially on the girdle and within the mantle cavity. The order Lepidopleurida also has a pigmented sensory organ called the Schwabe organ, but its function remains unknown. However, chitons lack a cerebral ganglion.
Chiton - Reproduction and Life Cycle
A chiton creeps along slowly on a muscular foot. It has considerable power of adhesion and can cling to rocks very powerfully, like a limpet. Chitons are generally herbivorous grazers, though some of them are omnivorous and some are carnivorous. They can eat diatoms, algae, bryozoans, barnacles, and sometimes bacteria by scraping the substrates found on rocks with their well-developed radulae.
A few of the species of the chitons are predatory in nature such as the small western Pacific species Placiphorella velata. These predatory chitons have an enlarged anterior girdle. They can catch some other small invertebrates, such as shrimp. Possibly they can even catch small fishes, by holding the enlarged and hood-like front end girdle up off the surface, and then clamping down on unsuspecting, shelter-seeking prey.
Chitons have separate sexes, and fertilization is usually external. The male releases sperm into the water, while the female releases eggs either individually, or in a long string. In most cases, fertilization takes place either in the surrounding water or in the mantle cavity of the female. Some species brood the eggs within the mantle cavity, and the species Callistochiton viviparous even retain them within the ovary and give birth to live young, an example of ovoviviparity.
The egg has a tough spiny coat and usually hatches to release a free-swimming trochophore larva, typical of many other mollusk groups. In a few cases, the trochophore remains within the egg which hatches to produce a miniature adult. Unlike most other molluscs, there is no intermediate stage, or veliger, between the trochophore and the adult. Instead, a segmented shell gland forms on one side of the larva, and a foot forms on the opposite side. When the larva is ready to become an adult, the body elongates, and the shell gland secretes the plates of the shell. Unlike the fully grown adult, the larva has a pair of simple eyes, although these may remain for some time in the immature adult.