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

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Figure 1. There space many species and sports of mollusks; this illustration reflects the anatomy of an aquatic gastropod.


Practice Questions

Which the the following statements about the anatomy the a mollusk is false?

Mollusks have actually a radula for grind food.A digestive gland is linked to the stomach.The tissue beneath the covering is called the mantle.The cradle system has a gizzard, a stomach, a cradle gland, and the intestine.

Mollusks have actually a muscular foot, which is offered for locomotion and anchorage, and varies in shape and also function, depending upon the type of mollusk under study. In shelled mollusks, this foot is usually the exact same size together the opening of the shell. The foot is a retractable and also an extendable organ. The foot is the ventral-most organ, conversely, the mantle is the limiting dorsal organ. Mollusks are eucoelomate, however 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 mass is present above the foot, in the visceral hump. This contains digestive, nervous, excretory, reproductive, and also respiratory systems. Mollusk varieties that are specifically aquatic have gills because that respiration, whereas part terrestrial varieties have lungs because that respiration. Additionally, a tongue-like organ called a radula, which bear chitinous tooth-like ornamentation, is current in numerous species, and also serves to piece or scrape food. The mantle (also well-known as the pallium) is the dorsal epidermis in mollusks; shelled mollusks are devoted to secrete a chitinous and also hard calcareous shell.

Most mollusks space dioecious animals and fertilization wake up externally, return this is not the case in terrestrial mollusks, such together snails and slugs, or in cephalopods. In some mollusks, the zygote hatches and undergoes 2 larval stages—trochophore and veliger—before coming to be a young adult; bivalves may exhibit a 3rd larval stage, glochidia.

Classification that Phylum Mollusca

Phylum Mollusca is a very diverse (85,000 species) group of mostly marine species. Mollusks have a dramatic selection of form, ranging from large predatory squids and also octopus, some of which present a high level of intelligence, come grazing develops with elaborately sculpted and also colored shells. This phylum deserve to be segregated into seven classes: Aplacophora, Monoplacophora, Polyplacophora, Bivalvia, Gastropoda, Cephalopoda, and Scaphopoda.


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


Class Aplacophora (“bearing no plates”) includes worm-like animals primarily discovered in benthic naval habitats. These pets lack a calcareous shell yet possess aragonite spicules on their 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 covering that encloses the body. The morphology the the shell and also the underlying pet can vary from circular come ovate. A looped digestive system, multiple pairs of excretory organs, numerous gills, and also a pair that gonads are present in this animals. The monoplacophorans were thought extinct and also only well-known via fossil documents until the exploration of Neopilina galathaea in 1952. Today, scientists have identified practically two dozen extant species.

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


Figure 3. This mussels, discovered in the intertidal zone in Cornwall, England, are bivalves. (credit: note A. Wilson)


Class Bivalvia (“two shells”) contains clams, oysters, mussels, scallops, and also geoducks. Members the this course are found in marine and also freshwater habitats. Together the surname suggests, bivalves space enclosed in a pair that shells (valves are commonly called “shells”) that room hinged at the dorsal end by shell ligaments and shell this (Figure 3). The as whole morphology is laterally flattened, and also the head region is poorly developed. Eyespots and statocysts may be lacking in part species. These animals are suspension feeders—they eat material, such together plankton, the is suspended in the water around them. Due to their diet, this course of mollusks lacks a radula. Respiration is helped with by a pair the ctenidia, whereas excretion and also osmoregulation room brought around by a pair the nephridia. Bivalves regularly possess a large mantle cavity. In some species, the posterior edge of the mantle may fuse to kind two siphons that serve to take it in and also exude water.

One of the features of the mantle is come secrete the shell. Some bivalves choose oysters and mussels possess the unique ability to secrete and also deposit a calcareous nacre or “mother of pearl” approximately foreign particles that may enter the mantle cavity. This property has actually been commercially exploited to develop pearls.


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


Figure 4. (a) Snails and (b) slugs room both gastropods, but slugs absence a shell. (credit a: alteration of work-related by Murray Stevenson; credit transaction b: modification of job-related by Rosendahl)


The visceral massive in the shelled species displays torsion about the perpendicular axis top top the facility of the foot, i m sorry is the crucial characteristic that this group, in addition to a foot the is modified for crawling (Figure 5). Many gastropods bear a head with tentacles, eyes, and a style. A complex radula is supplied by the cradle system and also aids in the sloop down of food. Eyes may be lacking in part gastropods species. The mantle cavity encloses the ctenidia as well as a pair of nephridia.


Figure 5. Throughout embryonic advancement of gastropods, the visceral mass experience torsion, or counterclockwise rotation that anatomical features. Together a result, the anus of the adult pet is situated over the head. Torsion is an independent procedure from coiling that 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) assault prey through a venomous sting. The toxin released, known as conotoxin, is a peptide with internal disulfide linkages. Conotoxins can bring about paralysis in humans, indicating the this toxin attacks neurological targets. Some conotoxins have been presented to block neuronal ion channels. These findings have led researcher to research conotoxins for feasible medical applications.

Conotoxins are an exciting area of potential pharmacological development, due to the fact that these peptides might be probably modified and used in specific medical problems to inhibit the activity of particular neurons. Because that example, these toxins might be provided to induce paralysis in muscles in specific health applications, comparable to the usage of botulinum toxin. Due to the fact that the entire spectrum of conotoxins, and also their instrument of action, space not completely known, the study of their potential applications is still in that infancy. Most research to date has concentrated on their usage to law neurological diseases. They have also shown part efficacy in relieving chronic pain, and the pain connected with conditions like sciatica and shingles. The study and also use the biotoxins—toxins acquired from life organisms—are an excellent example that the applications of biological science to modern-day medicine.


Class Cephalopoda (“head foot” animals), incorporate octopi, squids, cuttlefish, and also nautilus. Cephalopods are a course of shell-bearing animals as well as mollusks through a lessened shell. They screen vivid coloration, frequently seen in squids and octopi, which is offered for camouflage. All animals in this course are carnivorous predators and also have beak-like jaws in ~ the anterior end. All cephalopods present the existence of a an extremely well-developed nervous system together with eyes, as well as a close up door circulatory system. The foot is lobed and also developed right into tentacles, and a funnel, i beg your pardon is offered as their setting of locomotion. Suckers are present on the tentacles in octopi and squid. Ctenidia room enclosed in a big mantle cavity and are serviced by large blood vessels, each with its own heart connected with it; the mantle has actually siphonophores the facilitate exchange of water.

Locomotion in cephalopods is assisted in by ejecting a present of water because that propulsion. This is called “jet” propulsion. A pair that nephridia is existing within the mantle cavity. Sexual dimorphism is viewed in this course of animals. Members of a varieties mate, and the female climate lays the egg in a secluded and protected niche. Females that some species care because that the eggs for an extended period of time and also may finish up dying throughout that time period. Cephalopods such as squids and octopi likewise produce sepia or a dark ink, i beg your pardon is squirted ~ above a predator to help in a quick getaway.

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

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


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


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


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

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Scaphopods space usually buried in sand through the anterior opened exposed to water. These pets bear a solitary conical shell, which has both ends open. The head is rudimentary and protrudes out of the posterior end of the shell. These pets do no possess eyes, however they have a radula, as well as a foot modified right into tentacles through a bulbous end, well-known as captaculae. Captaculae offer to catch and also manipulate prey. Ctenidia are missing in these animals.


In Summary: Phylum Mollusca

Phylum Mollusca is a large, marine group of invertebrates. Mollusks show a selection of morphological variations in ~ the phylum. This phylum is additionally distinct in that part members exhibition a calcareous shell as an external way of protection. Some mollusks have advanced a diminished shell. Mollusks room protostomes. The dorsal epidermis in mollusks is modification to kind the mantle, which encloses the mantle cavity and also visceral organs. This cavity is quite unique from the coelomic cavity, which in the adult animal surrounds the heart. Respiration is facilitated by gills recognized as ctenidia. A chitinous-toothed tongue called the radula is present in many mollusks. Early breakthrough in some types occurs via two larval stages: trochophore and also veliger. Sex-related dimorphism is the primary sexual strategy in this phylum. Mollusks can be split into 7 classes, every with unique morphological characteristics