Describe the unique anatomical features of mollusksDescribe the functions of an animal classified in phylum Annelida

The mollusks room a varied group (85,000 described species) of mainly marine species. They have actually a variety of forms, ranging from big predatory squid and octopus, few of which display a high level of intelligence, to little grazing creates with elaborately sculpted and also colored shells. The annelids traditionally include the oligochaetes, which include the earthworms and also leeches, the polychaetes, which are a maritime group, and two other smaller classes.

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The phyla Mollusca and also Annelida belong come a clade referred to as the Lophotrochozoa, which also includes the phylum Nemertea, or ribbon worms (). They are distinct from the Ecdysozoa (nematodes and also arthropods) based on evidence from evaluation of your DNA, i beg your pardon has changed our see of the relationships amongst invertebrates.

Phylum Mollusca

Mollusca is the predominant phylum in maritime environments, wherein it is estimated that 23 percent of all well-known marine varieties belong to this phylum. It is the 2nd most varied phylum of pets with over 75,000 explained species. The surname “mollusca” signifies a soft body, as the earliest descriptions of mollusks come from observations of unshelled, soft-bodied cuttlefish (squid relatives). Return mollusk body forms vary, they share key characteristics, such as a ventral, muscular foot the is commonly used because that locomotion; the visceral mass, which includes most that the interior organs that the animal; and a dorsal mantle, i beg your pardon is a flap of tissue over the visceral mass that creates a space called the mantle cavity. The mantle may or might not secrete a covering of calcium carbonate. In addition, plenty of mollusks have a scraping framework at the mouth, dubbed a radula (

).

The muscular foot varies in shape and also function, depending on the kind of mollusk (described below in the ar on mollusk diversity). The is a retractable as well as extendable organ, offered for locomotion and anchorage. Mollusks room eucoelomates, but the coelomic cavity is limited to a cavity about the love in adult animals. The mantle cavity, developed inside the mantle, develops independently the the coelomic cavity. The is a multi-purpose space, housing the gills, the anus, organs for sensing food corpuscle in the water, and also an outlet because that gametes. Many mollusks have actually an open circulatory device with a heart the circulates the hemolymph in open up spaces approximately the organs. The octopuses and squid space an exemption to this and also have a close up door circulatory system with 2 hearts that relocate blood through the gills and also a third, systemic heart that pumps blood v the rest of the body.


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

Class Bivalvia (“two shells”) consists of clams, oysters, mussels, scallops, and geoducks. They are uncovered in marine and also freshwater habitats. As the surname suggests, bivalves space enclosed in a pair the shells (or valves) that space hinged in ~ the dorsal side. The human body is planarization on the sides. They feed by filtering particles from water and a radula is absent. Castle exchange gases using a pair of ctenidia, and also excretion and osmoregulation are lugged out through a pair the nephridia. In some species, the posterior edges of the mantle may fuse to form two siphons that inhale and exhale water. Part bivalves choose oysters and mussels have the unique capacity to secrete and also deposit a calcareous nacre or “mother the pearl” roughly foreign particles that go into the mantle cavity. This building is commercially exploited to produce pearls.


Gastropods (“stomach foot”) incorporate well-known mollusks prefer snails, slugs, conchs, sea hares, and sea butterflies. Gastropods incorporate shell-bearing varieties as fine as species with a diminished shell. These animals are asymmetrical and also usually present a coiled shell (

).

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Figure 4: The (a) nautilus, (b) giant cuttlefish, (c) reef squid, and also (d) blue-ring octopus room all members the the course Cephalopoda. (credit a: modification of work-related by J. Baecker; credit transaction b: alteration of work by Adrian Mohedano; credit c: alteration of occupational by Silke Baron; credit d: modification of work by Angell Williams)

Members that the class Scaphopoda (“boat feet”) are recognized colloquially as “tusk shells” or “tooth shells.” this shells are open at both ends and also usually lie buried in sand through the front opening exposed come water and the decreased head end projecting from the earlier of the shell. Tooth shells have actually a radula and also a foot modified right into tentacles, each through a bulbous finish that catches and also manipulates food (

).

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Figure 6: In this schematic showing the basic anatomy that annelids, the digestive device is indicated in green, the nervous system is shown in yellow, and the circulatory system is indicated in red.

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Annelids might be one of two people monoecious with irreversible gonads (as in earthworms and also leeches) or dioecious with short-lived or seasonal gonads (as in polychaetes).