Which molluscan group has a radula

The mantle itself cannot be seen because it is on the inner surface of the shell. In some gastropods, such as the cowries, the mantle extends over the shell, keeping the shell shiny and new in appearance. In other gastropods, like the sea hares, and in some cephalopods, like the squid and the octopus, the shell is very small and the mantle covers the shell completely Fig. The chambered nautilus is one cephalopod that secretes an external shell.

Squid and cuttlefish produce internal shells that are contained within the mantle, and octopus do not produce shells at all. The mouth structures of many molluscs include a specially adapted rasp-like tongue called a radula. The radula is a hard ribbon-shaped structure covered in rows of teeth.

Herbivorous snails have a mouth with a radula of usually five to seven complex teeth. There is a great diversity of radula forms in the mollusca Fig. The snail uses its radula like a file, rasping it back and forth over the substrate to scrape off small bits of food Fig.

As radular teeth wear down or break off, new teeth are formed to replace them. The tooth patterns of snail radulas are distinctive to species, and scientists can identify snails by looking at their radulas. Some radulas are highly specialized. A group of gastropods called cone snails are carnivorous meat-eating hunters that produce venom in glands near the mouth.

Their radulas are shaped into long, hollow teeth, which they thrust one at a time into their prey like harpoons Fig. A barbed radular tooth fires through the proboscis, which is an extension of the mouth. It pierces the prey, paralyzing it with venom and preventing its escape.

In this way cones stalk and capture worms, molluscs, and even fish. Some cones produce a poison strong enough to kill humans who handle them carelessly. Their poison is a neurotoxin that attacks and destroys nerves. Molluscs breathe with gills called ctenidia that sit in a cavity between the mantle and body mass Fig.

In some molluscs, most notably bivalves like oysters and mussels, the ctenidia are also used as filter feeding apparatus to strain particulate food from the water. Molluscs have a complete digestive tract surrounded by a small coelom. The molluscan circulatory system is composed of a series of blood sinuses or cavities, rather than closed, discrete vessels. This is referred to as an open circulatory system.

Molluscs display a large diversity of nervous systems, from the rudimentary nervous system of the brainless bivalves to the complex systems of the cephalopods, who have well-developed brains and are considered the most intelligent of invertebrates. Chitons Polyplacophora are basal relative to other extant molluscs Fig. Their soft bodies are covered with a series of eight shell plates.

The joints between these shell plates enable to chitons to roll up for protection. Chitons are mobile and contract their muscular foot in waves to move about. The primarily herbivorous chitons have a well-developed radula. Their nervous system is a series of ladder-like nerves and only a few species have poorly developed ganglia.

Chitons are found only marine environments. They are most commonly found in tide pools and rocky intertidal zones. Chitons can tolerate the harsh conditions of these habitats where ocean and land meet.

Gastropods are the most diverse group of molluscs Fig. The ones we usually think of are snails and slugs. Most gastropods have a calcareous shell protecting the soft-bodied animal inside. Some gastropods, such as sea slugs, sea hares, and garden slugs, lack a shell or have a reduced shell buried in the folds of their mantle. Most creep about on a flattened foot, but some swim, using extended folds of their mantle as fins. Most snails and terrestrial slugs are herbivorous.

They use their radula to scrape algae from surfaces Fig. For this reason, gardeners consider snails and slugs to be pests. Some gastropods are carnivores, stalking other snails, worms, and fish for food Fig. The colorful and striking nudibranchs contain many carnivorous specialists. Many nudibranchs feed on only one type of sponge; their body coloration and their eggs are patterned to blend in with their prey.

Other gastropods use their radula and acidic secretions to bore holes in shells and prey on other molluscs. In the Hawaiian Islands, the terrestrial cannibal snail Euglandina rosea ; Fig. The giant East African land snail is considered to be an agricultural pest and is also known to be a carrier of the parasitic nematode rat lungworm Angiostrongylus cantonensis ; Fig. Unfortunately, the cannibal snails also predated native land snails, nearly driving them to extinction.

Marine and freshwater gastropods breathe using ctenidia or gills. In many of these gastropods the ctenidia are protected within the mantle cavity. This distinctive trait makes nudibranchs an easily identifiable group of molluscs. Terrestrial slugs and snails, by contrast are primarily in a subgroup known as the pulmonates that actually have a mantle cavity that has become connected to the circulatory system vascularized to function as a lung.

Gastropods move by contracting their muscular foot in a series of waves to creep forward. These trails also provide chemical communication among gastropods. The cannibal snail, for example, tracks its prey by following the mucus trail left behind. The gastropod nervous system includes bodily nerves and anterior ganglia with relatively sophisticated sensory systems, including light receptors and well developed chemosensory abilities.

The bivalve molluscs get their name from the two door-like valves or shells that make up their exoskeleton Fig. Foot size varies among marine bivalves. Clams have a muscular hatchet-shaped foot for moving about and for burrowing in mud or sand Fig. They swim in short bursts by jet propulsion, clapping their shells together and forcing water out the rim.

Bivalves are more enclosed by their shells than other molluscs. Water enters and leaves a bivalve by way of two tubes called siphons. One siphon takes in water while the other expels water and waste. The water taken in contains oxygen and food particles. Most bivalve species acquire energy and nutrients through filter feeding.

Filter feeding or suspension feeding is the process of ingesting water and filtering out food particles. The diversity of molluscs impressively demonstrates how a basic body plan can evolve into a variety of different forms adapted to specific environments.

The ancestors of Nautilus evolved buoyant shells, a trait that allowed them to launch off the seafloor and become swimming predators known as cephalopods squid, octopus and cuttlefish. As each generation struggled against increasingly clever predators like vertebrates, the cephalopods accumulated more and more sophisticated innovations through evolution.

One adaptation lay in speed. In creatures such as squid, the shell became small, moved inside the body and all but vanished. Another adaptation lay in brainpower.

This chiton from the class Polyplacaphora has the eight-plated shell that is indicative of its class. These animals lack a calcareous shell but possess aragonite spicules on their epidermis.

They have a rudimentary mantle cavity and lack eyes, tentacles, and nephridia excretory organs. The morphology of the shell and the underlying animal can vary from circular to ovate. A looped digestive system, multiple pairs of excretory organs, many gills, and a pair of gonads are present in these animals. The monoplacophorans were believed extinct and only known via fossil records until the discovery of Neopilina galathaea in Today, scientists have identified nearly two dozen extant species.

These animals have a broad, ventral foot that is adapted for suction to rocks and other substrates, and a mantle that extends beyond the shell in the form of a girdle. Calcareous spines may be present on the girdle to offer protection from predators. Respiration is facilitated by ctenidia gills that are present ventrally. These animals possess a radula that is modified for scraping. Eyespots are absent in these animals. A single pair of nephridia for excretion is present.

Figure 3. These mussels, found in the intertidal zone in Cornwall, England, are bivalves. Members of this class are found in marine as well as freshwater habitats. The overall morphology is laterally flattened, and the head region is poorly developed. Eyespots and statocysts may be absent in some species. These animals are suspension feeders—they eat material, such as plankton, that is suspended in the water around them. Due to their diet, this class of mollusks lacks a radula.

Respiration is facilitated by a pair of ctenidia, whereas excretion and osmoregulation are brought about by a pair of nephridia. Bivalves often possess a large mantle cavity. In some species, the posterior edges of the mantle may fuse to form two siphons that serve to take in and exude water. One of the functions of the mantle is to secrete the shell.

This property has been commercially exploited to produce pearls. Gastropoda includes shell-bearing species as well as species with a reduced shell. These animals are asymmetrical and usually present a coiled shell Figure 4. Shells may be planospiral like a garden hose wound up , commonly seen in garden snails, or conispiral , like a spiral staircase , commonly seen in marine conches.

Figure 4. The visceral mass in the shelled species displays torsion around the perpendicular axis on the center of the foot, which is the key characteristic of this group, along with a foot that is modified for crawling Figure 5.

Most gastropods bear a head with tentacles, eyes, and a style. A complex radula is used by the digestive system and aids in the ingestion of food. Eyes may be absent in some gastropods species. The mantle cavity encloses the ctenidia as well as a pair of nephridia. Figure 5. During embryonic development of gastropods, the visceral mass undergoes torsion, or counterclockwise rotation of anatomical features. As a result, the anus of the adult animal is located over the head.

Torsion is an independent process from coiling of the shell. Figure 6. Members of the genus Conus produce neurotoxins that may one day have medical uses. Marine snails of the genus Conus Figure 6 attack prey with a venomous 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.