Mollusca Phylum Characteristics Structure and Examples

Molluscs (also known as molluscs) are the second-largest phylum of invertebrate animals after Arthropoda. They are smooth-bodied, bilaterally symmetrical, stratified, coelomate animals; usually shelled with a mantle, ventral foot, anterior head, and visceral dorsal mass.  

Molluscs are very diverse animals and make up a significant part of the world's ecosystem. They are the largest marine phylum, making up about 23% of all marine organisms. They are a highly diverse species, not only in size and anatomical structure but also in behavior and habitat, ranging from seawater to terrestrial habitats. Phylum Mollusca belongs to some animals you are familiar with, such as snails, octopuses, squids, oysters, clams, etc.

The phylum is usually divided into 8 taxonomic classes, two of which are extinct. Cephalopod molluscs, like squid, cuttlefish, and octopuses, are the most neurologically advanced of all invertebrates. The most universal characteristics defining modern molluscs are a mantle with a significant cavity used for breathing and excretion, the presence of a radula, and the organization of the nervous system. Did you know Molluscs can produce pearls with shells? Some of these are also consumed by humans in different parts of the world. Let us educate ourselves on this phylum of the Kingdom of Animals.

Mollusca Examples: 

Molluscs include mussels, scallops, oysters, periwinkles, whelks, squid, clams, snails, and octopus. 

Mollusca Characteristics

• They are mostly marine in water, few freshwater and some terrestrial form.

• They can be found inside other animals, as secret parasites.

• They range in size from giant squids and clams to small snails, one mm long.

• They have at least two radula and mantle characters, which are not found elsewhere.

• The body is soft, unsegmented, bilateral symmetrical, coelomates, triploblastic (except in Monoplacophora).

• Body organization is tissue-systems grade.

• The body has head, foot, mantle and visceral mass.

• The body is covered with often ciliated one-layered epidermis.

• The body is commonly protected by one or more pieces of exoskeletal calcareous shell secreted by the mantle.

• Except in pelecypoda and scaphopoda, the head is distinct, containing the mouth , eyes, tentacles and other sense organs.

• The ventral body is converted into a muscular plough-like surface, the foot which is modified in different ways for creeping, burrowing and swimming.

• Mantle or pallium is a fold of a wall of the body that leaves the main body, mantle cavity, within itself.

• Cavity of the body is hemocoel. The coelom is reduced and characterized by pericardial cavity, gonadial cavity. 

• Organ rasping, usually occurring radula or in pelecypoda.

• Except in cephalopods, the circulatory system is open type.

• There are numerous gills or ctenidia in the respiratory organs usually provided with osphradium at the base. In terrestrial forms the lung develops.

• Respiration in Mollusca is provided by gills or lungs, or both.

• Their respiratory pigments are haemocyanin;

• Excretion is achieved by paired metanephridia (kidney).

• The mollusca nervous system consists of paired prefrontal, pleural, pedal and visceral ganglia, along with longitudinal and transverse nervous connections. Usually ganglia form a circumentary ring.

• Sense organs are composed of skin, statocysts and touch, smell , and taste receptors.Sexes are usually separate (dioecious) but some are monoecious (hermaphroditic).

• Development through the trochophore stage called veliger larva is direct or with metamorphosis.

• The visceral mass, in its compact form, contains the vital organs of the body, taking the form of dorsal humps or dome.

These are the special features of Mollusca.

Classification of Mollusca

Phylum Mollusca is a very diverse group of mainly marine species (85,000 species), with a dramatic range of forms. Aplacophora, Monoplacophora, Polyplacophora, Bivalvia, Gastropoda, Cephalopoda, and Scaphopoda can be divided into seven groups.

Most animals belonging to this group of phylum have a calciferous shell. Their bodies are generally very soft and are covered by a hard exoskeleton. They can be found in the depths of the sea as well as in terrestrial regions, they range in size from twenty meters to one millimeter, to even a few millimeters. 

Class Aplacophora

Class Aplacophora ("bearing no plates") includes worm-like species found mainly in marine benthic habitats. Such species lack a calcareous shell but, on their epidermis, they possess aragonite spicules. They have a small cavity in their mantle and lack arms, tentacles and nephridia (excretory organs). 

Their habitat is exclusively marine, they burrow into the substrate in the water of more than 20m depth, and may reach densities of upto 4 - 5 square meters. Aplacophorans that belong in the subclass of Chaetodermomorpha are limited to by a minimum salinity of 28-30%. Aplacophorans are cylindrical, worm-like creatures, ranging from 1mm to 30cm in size. They don’t have an outer shell but spicules or scales secreted by the epidermis are embedded in the dorsal mantle. The radula, unlike a ribbon-like structure in other molluscs, is an expansion of the foregut epithelium. 

Aplacophora may be monoecious or dioecious with single or paired gonads. All gone pericardial ducts discharge gametes into the pericardial (heart) chamber. Gametes then travel to the mantle cavity, where they are expelled outside the body, via gametoducts. External fertilization is used by Chaetodermomorpha animals, but Neomeniomorpha animals are internally fertilized and sometimes brooded.

Class Monoplacophora 

Class members Monoplacophora ("bearing one plate") have a small, cap-like shell enclosing the body. The shell and underlying animal morphology can vary from circular to ovate. Among these species, there is a looped digestive system, several pairs of excretory glands, several gills and a number of gonads. Before the discovery of Neopilina galatheae in 1952, the monoplacophorans were considered extinct and known only from fossil records. Scientists have described almost two dozen species that exist today. 

The majority of them reside at extreme depths and are all marine. Monoplacophorans are little limpet-like creatures with a single cap-like shell. They resemble metamerically structured animals like annelids and arthropods since a lot of their organs (nephridia, heart, etc.) are serially repeated. It is unclear if this likeness suggests a close link between monoplacophorans and phyla that demonstrate real metamerism. 

Class Polyplacophora

Animals in the Polyplacophora class ("bearing several plates") are generally referred to as "chitons" and carry an eight-plated armour-like dorsal shell. These animals have a long, ventral foot that is adapted for suction to rocks and other substrates, and a mantle in the shape of a girdle that extends beyond the shell. Calcareous spines may be present on the girdle to provide predator defense. Chitons survive in cold water, warm water and tropics all over the world. Many species of chiton inhabit intertidal or subtidal areas, and do not extend beyond the photic zone. Some species live quite high in the intertidal zone, and have long periods of exposure to air and light.

Class Bivalvia

Bivalvia is a class of freshwater and aquatic molluscs with laterally compressed bodies enclosed in two hinged sections by a shell. Bivalves include clams, oysters, mussels, scallops and many other shell families. Many are filter feeders and have neither head nor radula. The gills have evolved into ctenidia, organs adapted to feed and breathe. Some bivalves bury themselves on the seabed in sediments, while others lie on the seafloor or attach to rocks or other hard surfaces.

A bivalve's shell is composed of calcium carbonate, and consists of two parts called valves, typically identical. They are linked together along one edge by a flexible ligament that forms the hinge in combination with the interlocking "teeth" on each of the valves.

Class Gastropoda

Animals in the Gastropoda class ("stomach foot") include well-known mollusks, such as snails, slugs, conchs, sea hares, and marine butterflies. Gastropoda contains both shell-bearing species and reduced-shell species. These species are asymmetric and usually have a coiled shell. Shells may be planospiral (like a wounded garden hose), commonly seen in snails in the garden, or conispiral (like a spiral staircase), commonly seen in marine conches.

The visceral mass in the shelled species displays torsion at the center of the foot along the perpendicular axis, which is the main feature of this category, along with a foot that is adapted to crawl. Many gastropods have a head that includes tentacles, eyes and a style. The digestive system utilizes a complex radula and assists in the digestion of food. In some gastropods the eyes may be absent.

Class Cephalopoda

Class Cephalopoda (animals with the "head foot") comprises octopi, squids, cuttlefish and nautilus. Cephalopods are a class of shell-bearing animals, as well as a reduced shell mollusc. They exhibit vibrant coloring, usually seen in squids and octopi used for camouflage purposes. Both species in this class are carnivorous predators, and on the anterior end have beak-like jaws.

Both cephalopods demonstrate the presence, along with eyes, of a very well developed nervous system and a closed circulatory system. The foot is lobed and converted into tentacles and a funnel which is used as the locomotion mode. Locomotion in cephalopods is enabled by ejecting a propulsive stream of water ("jet" propulsion).

Cephalopods, such as squids and octopi, also develop sepia or a dark ink that is squirted on a predator to help getaway quickly. Suckers are present at octopi and squid on tentacles. Ctenidia is enclosed in a large mantle cavity that is serviced by blood vessels, each with its own nucleus. The mantle contains siphonophores, which facilitate water exchange.

Class Scaphopoda

Members of the Scaphopoda class ("boat feet") are known colloquially as "tusk shells" or "tooth shells" as is evident when studying Dentalium, one of the few remaining genera of scaphopods. Scaphopods are normally submerged in sand with water visible from the anterior mouth. Such species wear a single conical shell, which has two open ends. The head is small, and protrudes from the shell's back end. Such species do not have skin, but they have a radula, as well as a foot that has been turned into tentacles with a bulbous end called captaculae. Captaculae exist for catching and handling prey. Among these species, ctenidia are absent.

Economic Importance of Mollusca

Teredo, the shipworm burrows into wooden structures steeped in the sea, causing severe damage to ships, piers and wharves. Yet molluscs are a great source of human food in different parts of the world, consuming millions of clams, oysters, scallops in China, Japan, Malaya, Europe and America, oysters being considered a delicacy.

In Europe, other bivalves, octopuses and cuttlefishes furnish large amounts of food. Shells of freshwater mussels are used in the pearl button industry in all parts of the world, they are made of the nacreous shell layer, no other material is used for washing as these buttons.

Oyster shells are mixed with tar to create roads in America and lime from these shells is used for the formulation of their egg shells in feeding poultry. Lime is used in buildings too.

Many freshwater clams and marine oysters generate pearls, but the most ecologically valuable pearls are produced by pearl oysters.Pinctada margaritifera and Pinctada mertensi which live along the coasts of China, India, Sri Lanka and Japan in the warmer parts of the Indian and Pacific Oceans. A pearl is made when a small foreign object lodges between the shell and the mantle, such as a particle of sand or a parasite.

Key Points:

• Aplacophora, Monoplacophora, Polyplacophora, Bivalvia, Gastropoda, Cephalopoda and Scaphopoda can be segregated into seven classes. These classes are distinguished by the presence and types of shells which they possess, among other criteria.

• Class Aplacophora includes worm-like, non-shell animals with a rudimentary body structure.

• Monoplacophora class members have a single shell with the body enclosed.

• Class Polyplacophora members are better known as "chitons;" on the ventral side these molluscs have a large foot, and on the dorsal side a shell consisting of eight hard plates.

• Class Bivalvia is made up of mollusks with two muscle-holding shells; these include oysters, clams, and molds.

• Class Gastropoda members have an asymmetric body plan, and typically have a shell that can be either planospiral or conispiral. Their key feature is the torsion on the center of the foot around the perpendicular axis, which is modified for crawling.

• Class Scaphopoda comprises mollusks with a specific conical shell via which the head projects, and a foot altered to tentacles known as captaculae used to catch and manipulate prey.