Phylum Mollusca is the predominant phylum in marine environments. The is estimated that 23 percent the all known marine types are mollusks; there room over 75,000 explained species, making them the 2nd most varied phylum the animals. The name “mollusca” signifies a soft body, due to the fact that the earliest descriptions of mollusks come from monitorings of unshelled cuttlefish. Mollusks are mainly a marine team of animals; however, lock are well-known to inhabit freshwater and terrestrial habitats. Mollusks screen a wide range of morphologies in every class and also subclass, yet share a couple of key characteristics, consisting of a muscular foot, a visceral mass containing internal organs, and a mantle that may or might not secrete a covering of calcium lead carbonate (Figure 1).

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Figure 1. There room many types and variations of mollusks; this illustration shows the anatomy of one aquatic gastropod.


Practice Questions

Which the the complying with statements around the anatomy that a mollusk is false?

Mollusks have actually a radula for grinding food.A digestive gland is associated to the stomach.The tissue beneath the covering is dubbed the mantle.The digestive system contains a gizzard, a stomach, a digestive gland, and the intestine.

Mollusks have actually a muscular foot, i m sorry is provided for locomotion and also anchorage, and varies in shape and also function, relying on the kind of mollusk under study. In shelled mollusks, this foot is typically the very same size as the opening of the shell. The foot is a retractable and also an extendable organ. The foot is the ventral-most organ, whereas the mantle is the limiting dorsal organ. Mollusks space eucoelomate, yet the coelomic cavity is minimal to a cavity roughly the love in adult animals. The mantle cavity develops independently that the coelomic cavity.

The visceral fixed is present above the foot, in the visceral hump. This consists of digestive, nervous, excretory, reproductive, and respiratory systems. Mollusk types that are exclusively aquatic have gills because that respiration, whereas some terrestrial varieties have lungs because that respiration. Additionally, a tongue-like organ called a radula, which bears chitinous tooth-like ornamentation, is current in plenty of species, and also serves to shred or scrape food. The mantle (also well-known as the pallium) is the dorsal epidermis in mollusks; shelled mollusks are committed to secrete a chitinous and also hard calcareous shell.

Most mollusks are dioecious animals and fertilization occurs externally, back this is no the situation in terrestrial mollusks, such together snails and also slugs, or in cephalopods. In part mollusks, the zygote hatches and undergoes two larval stages—trochophore and veliger—before becoming a young adult; bivalves may exhibit a third larval stage, glochidia.

Classification the Phylum Mollusca

Phylum Mollusca is a an extremely diverse (85,000 species) team of mostly marine species. Mollusks have actually a dramatic range of form, varying from big predatory squids and octopus, some of which display a high degree of intelligence, come grazing develops with elaborately sculpted and also colored shells. This phylum can be segregated right into seven classes: Aplacophora, Monoplacophora, Polyplacophora, Bivalvia, Gastropoda, Cephalopoda, and Scaphopoda.


Figure 2. This chiton from the class Polyplacaphora has the eight-plated shell that is indicative of its class. (credit: Jerry Kirkhart)


Class Aplacophora (“bearing no plates”) consists of worm-like animals primarily uncovered in benthic marine habitats. These animals lack a calcareous shell yet possess aragonite spicules on your epidermis. They have a rudimentary mantle cavity and also lack eyes, tentacles, and nephridia (excretory organs). Members of class Monoplacophora (“bearing one plate”) posses a single, cap-like shell that encloses the body. The morphology of the shell and the underlying pet can differ from circular to ovate. A looped digestive system, multiple bag of excretory organs, numerous gills, and a pair of gonads are current in this animals. The monoplacophorans were believed extinct and only well-known via fossil records until the exploration of Neopilina galathaea in 1952. Today, scientists have identified practically two dozen extant species.

Animals in the course Polyplacophora (“bearing countless plates”) are generally known as “chitons” and also bear an armor-like eight-plated covering (Figure 2). These pets have a broad, ventral foot that is adapted for suction to rocks and also other substrates, and a mantle the extends beyond the covering in the kind of a girdle. Calcareous spines might be present on the girdle to offer security from predators. Respiration is helped with by ctenidia (gills) that are existing ventrally. These animals possess a radula that is modified for scraping. The nervous device is rudimentary with only buccal or “cheek” ganglia current at the anterior end. Eyespots are absent in this animals. A solitary pair the nephridia for excretion is present.


Figure 3. This mussels, found in the intertidal ar in Cornwall, England, room bivalves. (credit: note A. Wilson)


Class Bivalvia (“two shells”) contains clams, oysters, mussels, scallops, and also geoducks. Members that this course are found in marine as well as freshwater habitats. Together the surname suggests, bivalves space enclosed in a pair of shells (valves are commonly called “shells”) that space hinged at the dorsal end by shell ligaments and also shell teeth (Figure 3). The in its entirety morphology is laterally flattened, and the head an ar is poorly developed. Eyespots and statocysts may be lacking in some species. These pets are suspension feeders—they eat material, such together plankton, that is rely in the water roughly them. Because of their diet, this class of mollusks lacks a radula. Respiration is facilitated by a pair that ctenidia, vice versa, excretion and also osmoregulation room brought about by a pair of nephridia. Bivalves often possess a huge mantle cavity. In some species, the posterior edges of the mantle may fuse to kind two siphons that offer to take it in and exude water.

One that the features of the mantle is to secrete the shell. Part bivalves like oysters and mussels own the unique capacity to secrete and also deposit a calcareous nacre or “mother that pearl” around foreign particles that may enter the mantle cavity. This property has been commercially exploited to create pearls.


Animals in course Gastropoda (“stomach foot”) include well-known mollusks like snails, slugs, conchs, sea hares, and sea butterflies. Gastropoda consists of shell-bearing species as fine as types with a lessened shell. These animals are asymmetrical and usually present a coiled covering (Figure 4). Shells may be planospiral (like a garden hose wound up), frequently seen in garden snails, or conispiral, (like a spiral staircase), generally seen in naval conches.


Figure 4. (a) Snails and also (b) slugs are both gastropods, yet slugs absence a shell. (credit a: change of occupational by Murray Stevenson; credit transaction b: change of work by Rosendahl)


The visceral fixed in the shelled types displays torsion around the perpendicular axis top top the facility of the foot, which is the an essential characteristic that this group, along with a foot that is modified because that crawling (Figure 5). Many gastropods bear a head v tentacles, eyes, and also a style. A complicated radula is used by the cradle system and aids in the sloop down of food. Eyes might be absent in part gastropods species. The mantle cavity encloses the ctenidia and a pair of nephridia.


Figure 5. During embryonic development of gastropods, the visceral mass undergoes torsion, or counterclockwise rotation that anatomical features. Together a result, the anus that the adult animal is located over the head. Torsion is one independent procedure from coiling of the shell.


Figure 6. Members that the genus Conus produce neurotoxins that may one day have actually medical uses. (credit: David Burdick, NOAA)


Marine snails that the genus Conus (Figure 6) attack prey with a awful sting. The toxin released, known as conotoxin, is a peptide with internal disulfide linkages. Conotoxins can bring about paralysis in humans, indicating that this toxin attacks neurological targets. Part conotoxins have actually been displayed to block neuronal ion channels. This findings have actually led researchers to examine conotoxins for feasible medical applications.

Conotoxins room an interesting area of potential pharmacological development, due to the fact that these peptides might be possibly modified and also used in particular medical problems to inhibit the task of certain neurons. For example, this toxins may be supplied to induce paralysis in muscles in specific health applications, comparable to the usage of botulinum toxin. Since the whole spectrum that conotoxins, and their instrument of action, room not completely known, the study of their potential applications is quiet in that is infancy. Most research to day has concentrated on their usage to treat neurological diseases. Lock have also shown some efficacy in relieving chronic pain, and also the pain associated with conditions like sciatica and shingles. The study and also use that biotoxins—toxins derived from living organisms—are an excellent example of the application of organic science to modern medicine.


Class Cephalopoda (“head foot” animals), incorporate octopi, squids, cuttlefish, and nautilus. Cephalopods space a course of shell-bearing animals and also mollusks with a diminished shell. They display screen vivid coloration, typically seen in squids and octopi, i m sorry is supplied for camouflage. All animals in this course are carnivorous predators and also have beak-like jaws at the anterior end. Every cephalopods present the existence of a an extremely well-developed worried system in addition to eyes, as well as a closed circulatory system. The foot is lobed and also developed into tentacles, and also a funnel, which is used as their mode of locomotion. Suckers are existing on the tentacles in octopi and squid. Ctenidia are enclosed in a big mantle cavity and also are serviced by large blood vessels, each with its very own heart linked with it; the mantle has siphonophores the facilitate exchange that water.

Locomotion in cephalopods is assisted in by ejecting a present of water for propulsion. This is referred to as “jet” propulsion. A pair that nephridia is current within the mantle cavity. Sex-related dimorphism is viewed in this course of animals. Members the a types mate, and the female then lays the egg in a secluded and also protected niche. Females the some varieties care for the eggs for an extended period of time and may end up dying during that time period. Cephalopods such together squids and also octopi also produce sepia or a dark ink, which is squirted upon a predator to help in a fast getaway.

Reproduction in cephalopods is different from other mollusks in that the egg hatches to develop a youth adult without undergoing the trochophore and also veliger larval stages.

In the shell-bearing Nautilus spp., the spiral covering is multi-chambered. These chambers space filled with gas or water to regulate buoyancy. The shell structure in squids and cuttlefish is reduced and also is present internally in the kind of a squid pen and also cuttlefish bone, respectively. Examples are shown in Figure 7.


Figure 7. The (a) nautilus, (b) gigantic cuttlefish, (c) reef squid, and (d) blue-ring octopus space all members that the course Cephalopoda. (credit a: modification of work by J. Baecker; credit b: change of work by Adrian Mohedano; credit transaction c: change of job-related by Silke Baron; credit d: alteration of work by Angell Williams)


Figure 8. Antalis vulgaris shows the standard Dentaliidae form that provides these animals their common name of “tusk shell.” (credit: Georges Jansoone)


Members of class Scaphopoda (“boat feet”) are known colloquially as “tusk shells” or “tooth shells,” as apparent when examining Dentalium, one of the couple of remaining scaphopod genera (Figure 8).

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Scaphopods are usually buried in sand v the anterior opened exposed to water. These animals bear a single conical shell, which has both ends open. The head is rudimentary and protrudes out of the posterior finish of the shell. These animals do not possess eyes, yet they have a radula, as well as a foot modified right into tentacles with a bulbous end, well-known as captaculae. Captaculae serve to catch and also manipulate prey. Ctenidia are absent in these animals.


In Summary: Phylum Mollusca

Phylum Mollusca is a large, marine team of invertebrates. Mollusks show a selection of morphological variations within the phylum. This phylum is also distinct in that part members exhibition a calcareous covering as an external means of protection. Some mollusks have developed a diminished shell. Mollusks room protostomes. The dorsal epidermis in mollusks is modified to type the mantle, which encloses the mantle cavity and also visceral organs. This cavity is quite distinctive from the coelomic cavity, which in the adult pet surrounds the heart. Respiration is assisted in by gills recognized as ctenidia. A chitinous-toothed tongue referred to as the radula is current in many mollusks. Early breakthrough in some varieties occurs via two larval stages: trochophore and also veliger. Sex-related dimorphism is the predominant sexual strategy in this phylum. Mollusks deserve to be split into 7 classes, each with distinctive morphological characteristics