Phylum Mollusca is the primary phylum in naval environments. It is estimated that 23 percent the all recognized marine types are mollusks; there are over 75,000 defined species, making castle the second most varied phylum of animals. The name “mollusca” signifies a soft body, because the earliest explanation of mollusks come from observations of unshelled cuttlefish. Mollusks are mainly a marine group of animals; however, they are well-known to inhabit freshwater as well as terrestrial habitats. Mollusks display screen a wide variety of morphologies in every class and also subclass, yet share a couple of key characteristics, including a muscular foot, a visceral massive containing internal organs, and also a mantle that might or may not secrete a shell of calcium lead carbonate (Figure 1).

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Figure 1. There room many varieties and sport of mollusks; this illustration reflects the anatomy of an aquatic gastropod.


Practice Questions

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

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

Mollusks have a muscular foot, which is provided for locomotion and also anchorage, and also varies in shape and function, depending upon the type of mollusk under study. In shelled mollusks, this foot is typically the same size as the opened of the shell. The foot is a retractable and an extendable organ. The foot is the ventral-most organ, vice versa, the mantle is the limiting dorsal organ. Mollusks space eucoelomate, yet the coelomic cavity is restricted to a cavity around the heart in adult animals. The mantle cavity creates independently of the coelomic cavity.

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

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

Classification the Phylum Mollusca

Phylum Mollusca is a an extremely diverse (85,000 species) group of greatly marine species. Mollusks have a dramatic range of form, ranging from big predatory squids and octopus, several of which display a high level of intelligence, to grazing forms with elaborately sculpted and colored shells. This phylum deserve to be segregated right into seven classes: Aplacophora, Monoplacophora, Polyplacophora, Bivalvia, Gastropoda, Cephalopoda, and Scaphopoda.


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


Class Aplacophora (“bearing no plates”) contains worm-like pets primarily uncovered in benthic maritime habitats. These pets lack a calcareous shell but possess aragonite spicules on your epidermis. They have actually a rudimentary mantle cavity and also lack eyes, tentacles, and also 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 vary from circular come ovate. A looped digestive system, multiple pairs of excretory organs, plenty of gills, and also a pair of gonads are existing in these animals. The monoplacophorans were believed extinct and also only known via fossil documents until the exploration of Neopilina galathaea in 1952. Today, scientists have actually identified practically two dozen extant species.

Animals in the class Polyplacophora (“bearing countless plates”) are commonly known together “chitons” and also bear one armor-like eight-plated shell (Figure 2). These animals have a broad, ventral foot that is adapted for suction to rocks and other substrates, and also a mantle that extends past the shell in the kind of a girdle. Calcareous spines may be present on the girdle to offer defense from predators. Respiration is assisted in by ctenidia (gills) the are existing ventrally. These animals possess a radula the is modified for scraping. The nervous mechanism is rudimentary with only buccal or “cheek” ganglia current at the anterior end. Eyespots are absent in this animals. A single pair of nephridia for excretion is present.


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


Class Bivalvia (“two shells”) consists of clams, oysters, mussels, scallops, and also geoducks. Members the this class are uncovered in marine and freshwater habitats. As the surname suggests, bivalves space enclosed in a pair that shells (valves are generally called “shells”) that space hinged at the dorsal end by shell ligaments and also shell this (Figure 3). The overall morphology is laterally flattened, and also the head region is poorly developed. Eyespots and statocysts may be absent in some species. These animals are suspension feeders—they eat material, such together plankton, the is suspended in the water approximately them. As result of their diet, this course of mollusks lacks a radula. Respiration is promoted by a pair of ctenidia, vice versa, excretion and osmoregulation room brought about by a pair of nephridia. Bivalves often possess a large mantle cavity. In part species, the posterior edge of the mantle may fuse to type two siphons that offer to take it in and exude water.

One of the features of the mantle is come secrete the shell. Part bivalves prefer oysters and mussels possess the unique ability to secrete and deposit a calcareous nacre or “mother that pearl” approximately foreign particles the may get in the mantle cavity. This property has been commercially exploited to create pearls.


Animals in course Gastropoda (“stomach foot”) include well-known mollusks choose snails, slugs, conchs, sea hares, and sea butterflies. Gastropoda includes shell-bearing types as well as varieties with a diminished shell. These pets are asymmetrical and also usually present a coiled shell (Figure 4). Shells might be planospiral (like a garden hose wound up), typically seen in garden snails, or conispiral, (like a spiral staircase), frequently seen in naval conches.


Figure 4. (a) Snails and (b) slugs room both gastropods, but slugs lack a shell. (credit a: alteration of work by Murray Stevenson; credit transaction b: change of work by Rosendahl)


The visceral massive in the shelled types displays torsion approximately the perpendicular axis top top the facility of the foot, which is the an essential characteristic of this group, in addition to a foot that is modified because that crawling (Figure 5). Many gastropods be affected by each other a head with tentacles, eyes, and a style. A complex radula is supplied by the digestive system and aids in the ingestion of food. Eyes may be missing in some gastropods species. The mantle cavity encloses the ctenidia and also a pair that nephridia.


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


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


Marine snails that the genus Conus (Figure 6) assault prey with a poisonous sting. The toxin released, recognized as conotoxin, is a peptide with interior disulfide linkages. Conotoxins can bring about paralysis in humans, indicating that this toxin strikes neurological targets. Some conotoxins have actually been displayed to block neuronal ion channels. These findings have led researchers to research conotoxins for possible medical applications.

Conotoxins space an amazing area of potential pharmacological development, since these peptides might be probably modified and used in specific medical problems to inhibit the activity of particular neurons. For example, these toxins may be offered to induce paralysis in muscles in specific health applications, similar to the usage of botulinum toxin. Due to the fact that the whole spectrum that conotoxins, and also their mechanisms of action, room not fully known, the examine of your potential applications is tho in the infancy. Most research to day has focused on their usage to law neurological diseases. Lock have also shown part efficacy in relieving chronic pain, and the pain connected with conditions like sciatica and also shingles. The study and also use of biotoxins—toxins derived from life organisms—are wonderful example of the applications of organic science to contemporary medicine.


Class Cephalopoda (“head foot” animals), incorporate octopi, squids, cuttlefish, and nautilus. Cephalopods room a course of shell-bearing animals and mollusks through a decreased shell. They display vivid coloration, typically seen in squids and octopi, i m sorry is offered for camouflage. All pets in this class are carnivorous predators and have beak-like jaws at the anterior end. Every cephalopods show the visibility of a really well-developed worried system together with eyes, and a close up door circulatory system. The foot is lobed and developed into tentacles, and also a funnel, which is provided as their setting of locomotion. Suckers are existing on the tentacles in octopi and also squid. Ctenidia room enclosed in a huge mantle cavity and are serviced by huge blood vessels, each v its very own heart associated 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 dubbed “jet” propulsion. A pair the nephridia is current within the mantle cavity. Sex-related dimorphism is watched in this course of animals. Members the a varieties mate, and also the female climate lays the egg in a secluded and also protected niche. Females of some species care for the eggs because that an extended duration of time and also may finish up dying throughout that time period. Cephalopods such together squids and octopi additionally produce sepia or a dark ink, i beg your pardon is squirted upon a predator to help in a fast getaway.

Reproduction in cephalopods is various from various other mollusks in that the egg hatches to create a juvenile adult there is no undergoing the trochophore and also veliger larval stages.

In the shell-bearing Nautilus spp., the spiral covering is multi-chambered. This chambers room filled through gas or water to control buoyancy. The shell structure in squids and also cuttlefish is reduced and also is current internally in the kind of a octopus pen and cuttlefish bone, respectively. Examples are presented in Figure 7.


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


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


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

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Scaphopods room usually hidden in sand with the anterior opened exposed come water. These animals bear a solitary conical shell, which has both ends open. The head is rudimentary and also protrudes the end of the posterior finish of the shell. These pets do not possess eyes, but they have a radula, as well as a foot modified into tentacles with a bulbous end, 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 group of invertebrates. Mollusks display a selection of morphological variations within the phylum. This phylum is also distinct in that some members exhibition a calcareous shell as an external way of protection. Some mollusks have developed a reduced shell. Mollusks space protostomes. The dorsal epidermis in mollusks is amendment to form the mantle, i beg your pardon encloses the mantle cavity and also visceral organs. This cavity is quite distinct from the coelomic cavity, i m sorry in the adult animal surrounds the heart. Respiration is facilitated by gills known as ctenidia. A chitinous-toothed tongue referred to as the radula is existing in most mollusks. Early development in some types occurs via two larval stages: trochophore and also veliger. Sexual dimorphism is the predominant sexual strategy in this phylum. Mollusks deserve to be divided into seven classes, every with distinctive morphological characteristics