11. The gills have evolved into ctenidia, specialised organs for feeding and breathing. 8. Classes and General characteristics of Vertebrates. They display vivid coloration, typically seen in squids and octopi, which is used for camouflage. Some species live quite high in the intertidal zone and are exposed to the air and light for long periods. Suckers are present on the tentacles in octopi and squid. Monoplacophora (Gk. 2. Between the edge of the foot and the girdle on each side is a mantle groove into which project a number of gills or ctenidia. Phylum Mollusca Classes: Gastropoda, Bivalvia, Cephalopoda 2 Who is in phylum mollusca? Phylum Mollusca In: Atkinson LJ and Sink KJ (eds) Field Guide to the Ofshore Marine Invertebrates of South Africa, Malachite Marketing and Media, Pretoria, pp. The nervous system consists of paired cerebral, pedal and visceral ganglia with their connectives. Class Scaphopoda consists of mollusks with a single conical shell through which the head protrudes, and a foot modified into tentacles known as captaculae that are used to catch and manipulate prey. The asymmetry of visceropallium is its funda­mental feature. 3. Monoplacophora: 1. This phylum contains about 100,000 described species. Bivalvia is a class of marine and freshwater molluscs with laterally compressed bodies enclosed by a shell in two hinged parts. Symmetry: bilateral and the body is laterally compressed. 7. 2. The odontophore is with a simple radula. Very few are terrestrial found in damp soil. Examples: Neometiia, Proneomenia, Chaetoderma, etc. Their majority live in salt water , some in fresh water and few on land , It is a soft mass , It has a calcareous shell which may be external , internal , absent or reduced , The majority are unisexual and few are hermaphrodites . The foot is ventral and plough-shaped. Their body has a cavity. Subclass i. Aplacophora or Solenogastres: 1. 6. Class Aplacophora (“bearing no plates”) includes worm-like animals primarily found in benthic marine habitats. 1. Mollusks can be segregated into seven classes: Aplacophora, Monoplacophora, Polyplacophora, Bivalvia, Gastropoda, Cephalopoda, and Scaphopoda. 4. The mouth lies centrally in a reduced head, the anal and excretory apertures are posterior. 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. This phylum can be segregated into seven classes: Aplacophora, Monoplacophora, Polyplacophora, Bivalvia, Gastropoda, Cephalopoda, and Scaphopoda. 6. 3. The classes are: 1. A complex radula is used by the digestive system and aids in the ingestion of food. The nervous system is primitive, with longitudinal pallial and pedal cords with cross anastomoses. 6. Animals in class Gastropoda (“stomach foot”) include well-known mollusks like snails, slugs, conchs, sea hares, and sea butterflies. 5. Members of class Monoplacophora (“bearing one plate”) posses a single, cap-like shell that encloses the body. Have questions or comments? Phylum Mollusca. 249-320. They appeared in the Lower Cambrian period. Untwisting and shortening of visceral loop and pleuroparietal nerve connectives due to de-torsion is pronounced. Phylum Mollusca (Mollusks) Etymology: From the Latin Molluscus, meaning soft of body. Chitons live worldwide, in cold water, warm water, and the tropics. Members of class Scaphopoda (“boat feet”) are known colloquially as “tusk shells” or “tooth shells,” as evident when examining Dentalium, one of the few remaining scaphopod genera. 9. 2. Marine molluscs with an elongated, bilaterally symmetrical flattened body and a broad foot ventrally. They exhibit organ system level of organization. 7. The body is divided into head, dorsal visceral mass, and ventral muscular foot and mantle. A looped digestive system, multiple pairs of excretory organs, many gills, and a pair of gonads are present in these animals. Animals, Biology, Diversity, Phylum Mollusca, Zoology. Most gastropods bear a head with tentacles, eyes, and a style. Class 1 Pelecypoda or Bivalvia. Hermaphrodite; gonad single, mostly ovoviviparous; development direct or with suppressed larval stages. Their key characteristic is the torsion around the perpendicular axis on the center of the foot that is modified for crawling. Captaculae serve to catch and manipulate prey. In Chaetoderma the ventral “foot- groove” is absent and cloaca is a discrete bell-like mantle cavity which also contains a pair or a series of ctenidia. Cephalopoda. Respiratory organ a lung, fused with the mantle in the adult. 5. 8. Examples: Aplysia (sea hare), Actean, Gastropteron (sea slug), Cliona, Bertholimia, Polycera, Doris, Dendonotus, etc. The sexes are either separate or united. 5. 6. A pair of horny jaws and an odontophore with a radula are present. A pair of nephridia is present within the mantle cavity. Marine gastropods displaying reduc­tion or loss of shell (shell sometimes inter­nal). The following diagram shows the major classes, which as can be seen are all based on variation in this same body plan. They have a rudimentary mantle cavity and lack eyes, tentacles, and nephridia (excretory organs). Head with one or two pairs of tentacles and one pair of eyes. Before publishing your Notes on this site, please read the following pages: 1. Most chiton species inhabit intertidal or subtidal zones, and do not extend beyond the photic zone. Body without cavity. 9. The organisms belonging to phylum Mollusca exhibit the following characteristics: 1. Classes in Phylum Mollusca Phylum Mollusca is a very diverse (85,000 species ) group of mostly marine species, with a dramatic variety of form. 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. The head is surrounded by a series of arms bearing suckers, which are modified part of the foot. The class as a whole has become adapted for a free-swimming existence...... Click the link for more information. Many taxa remain poorly studied. The shell may be absent or rudimen­tary; when present may be internal or exter­nal, undivided or divided internally into a series of chambers. The number of fossil species is estimated between 60,000 and 100,000 additional species. Pelecypoda or Bivalvia or Lamellibranchiata 6. The following points highlight the six main classes of phylum mollusca. 2. Amphineura 3. The larva passes through trochophore and veliger stages. 4. The head is rudimentary and protrudes out of the posterior end of the shell. 3. But some species live in freshwater and terrestrial habitats. Locomotion in cephalopods is facilitated by ejecting a stream of water for propulsion (“jet” propulsion). The shell is absent and the body is covered with a cuticular mantle enclosing spicules of calcified material. Bilaterally symmetrical molluscs with a laterally compressed body enclosed in a bivalved calcareous shell. The shell is a curved tube open at both the ends, the opening being wider at the oral and narrower at the other end. Gastropoda 4. The proportion of undescribed species is very high. 2018. Primarily aquatic gastropods, retaining larval torsion and nervous asymmetry. The LibreTexts libraries are Powered by MindTouch® and are supported by the Department of Education Open Textbook Pilot Project, the UC Davis Office of the Provost, the UC Davis Library, the California State University Affordable Learning Solutions Program, and Merlot. 2. 3.65. Members of a species mate, then the female lays the eggs in a secluded and protected niche. They live in fresh water, salt water, and on land. 4. The heart consists of two auricles and a ventricle. (examples: snails, conch, slugs, nudibrancs) All Univalve (one-shelled or no shell) Operculum. torsion). 5. Phylum Mollusca, known as mollusks, are a group of invertebrate animals that include slugs, snails, mussels, octopus, bivalves, gastropods, etc.This phylum is attributed to 85,000 species with tens of thousands of extinct Mollusca species.. Class Bivalvia consists of mollusks with two shells held together by a muscle; these include oysters, clams, and mussels. Head with numerous retractile ten­tacular appendages which lack suckers. Bilaterally symmetrical, free-swimming, marine molluscs. This phylum can be segregated into seven classes: Aplacophora, Monoplacophora, Polyplacophora, Bivalvia, Gastropoda, Cephalopoda, and Scaphopoda. 1. Phylum Mollusca and Phylum Echinodermata belong to the Kingdom Animalia. 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. These animals are asymmetrical and usually present a coiled shell. 1. Examples: Haliotis, Pila, Viviparus, Pa­tella, Acmaea, Fissurella, Cypraea, Littorina, Murex, Buccinum, Conus, etc. 2. The two subclasses of the class Amphineura sharing a similar primitive ner­vous system are very different in appear­ance, habitat and development. Class 1. Two pairs of ctenidia, osphradia, renal organs, and auricles. Scaphopoda 5. This class contains the cephalopodscephalopod, member of the class Cephalopoda, the most highly organized group of mollusks (phylum Mollusca), and including the squids, octopuses, cuttlefish, and nautiluses. The mantle encloses a large mantle cavity in which are situated the gills, the renal, reproductive and the anal apertures. Siphonal funnel of two separate folds. The buccal cavity contains an odontophore with a radula bearing rows of chitinoid teeth. The head bears a pair of large, simple eyes. Largest is Gastropoda. The head comprises of tentacles and compound eyes. Figure from D. R. Prothero, 1998, Bringing Fossils to Life , McGraw-Hill, p. 277; from Clarkson, 1993 - this image from LAB 3 Chapter 8: Phylum Mollusca and Biological Diversity and Evolution through Time The Phylum Mollusca is familiar to us as invertebrate animals, as it includes snails, clams, squid, oysters, sea-mice and tusk shells. Gastropods (Phylum Mollusca, Class Gastropoda) Including forms commonly known as snails and slugs, the gastropods are the most diverse taxonomic class within the phylum Mollusca, with an estimated 60,000 – 80,000 known living species, accounting for over 80% of all known living mollusks. Weird Science: An Inordinate Fondness for Beetles; Activity: Aquatic Invertebrate Behavior; Question Set: Phylum Arthropoda; Further Investigations: Phylum Arthropoda; Phylum Echinodermata. Members of class Monoplacophora have a single shell that encloses the body. 249 2. Sexes separate, gonad solitary, opening in the right. Phylum Mollusca is the second largest animal phylum. The nervous system consists of cere­bral, pleural, buccal, pedal, visceral and ab­dominal ganglia with their connectives and commissures. Mollusca makes the second largest phylum of non-chordate animals including snails, octopuses, sea slugs, squid, and bivalves such as clams, oysters, and mussels. 3. The foot is lobed and developed into tentacles and a funnel, which is used as the mode of locomotion. Many organisms belonging to this phylum have a calciferous shell. The body is elongated and worm-like and enveloped by the mantle. 3. They have die characters of both the phylum Annelida and phylum Mollusca. Phylum Mollusca is the Large phylum of invertebrates having 100,000 species distributed in 8 classes. Phylum Mollusca is a very diverse (85,000 species ) group of mostly marine species, with a dramatic variety of form. 3 Gastropods Sea snails (A) Sea slugs (B) Conchs (C) B C A 4 Neometiia is hermaphroditic whereas Chaetoderma is dioecious. 6. Phylum Mollusca. Hermaphrodite; larva is a veliger, Car­boniferous to recent; 3,000 living and 300 fossil species. The body is divided into segments. Question Set: Phylum Mollusca; Further Investigations: Phylum Mollusca; Phylum Arthropoda. Mnemonic Device: Some Grownups Can't See Magic Ponies But Children CAN Explanation: to remember the classes and sub-classes of Phylum Mollusca S - The nervous system is highly devel­oped; the principal nerve ganglia are aggre­gated around the oesophagus and a carti­laginous skeleton supports and protects the nerve centre. They are bilaterally symmetrical, triploblastic and coelomate animal. The snail's cover for when it gets washed onto shore. This video explains the characteristics and Classes of the Phylum Mollusca. The head bears eight non-retractile ten­tacles which are provided with suckers in two rows. All animals in this class are carnivorous predators and have beak-like jaws at the anterior end. Disclaimer: The Animal Diversity Web is an educational resource written largely by and for college students.ADW doesn't cover all species in the world, nor does it include all the latest scientific information about organisms we describe. The mantle has siphonophores that facilitate exchange of water. The bodies are generally very soft and are covered by the hard exoskeleton. The rectum pierces the pericardium and the ventricle, and opens in the exhalant siphon. The mouth of the shell can be closed by an aperculum borne on the foot. TOS4. The cephalopods are molluscs with large heads and tentacles. Head is not distinct and without eyes and tentacles. Animals in the class Polyplacophora (“bearing many plates”) are commonly known as “chitons” and bear an armor-like, eight-plated dorsal shell. 4. Examples of cephalopod molluscs include squid, octopus, cuttlefish, and nautilus (Fig. 1. Class Aplacophora includes worm-like animals with no shell and a rudimentary body structure. The sexes are separate and develop­ment is direct. 4. Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda. The mantle cavity encloses the ctenidia (singluar: ctenidium) as well as a pair of nephridia (singular: nephridium). Privacy Policy3. The classes are: 1. Body possesses a through gut with mouth and anus. 7. A ventral longitudinal groove is present, which is connected anteriorly with a ciliated groove and posteriorly with the cavity of the cloaca. 5. Phylum Mollusca Classification Mollusca (mollusks) are classified into 6 classes according to their symmetry and the characters of food, shell, mantle, … Unless otherwise noted, LibreTexts content is licensed by CC BY-NC-SA 3.0. Most bivalves bury themselves in sediment on the seabed, while others lie on the sea floor or attach themselves to rocks or other hard surfaces. The members are known as molluscs or mollusks . The monoplacophorans were believed extinct and only known via fossil records until the discovery of Neopilina galathaea in 1952. These animals bear a single conical shell, which has both ends open. These are joined together along one edge by a flexible ligament that, in conjunction with interlocking “teeth” on each of the valves, forms the hinge. But the giant octopus (Enteroctopussp. 10. Body is covered by a calcareous shell and is unsegmented with a distinct head, muscular foot and visceral hump. Around 85,000 extant species of molluscs are recognized. The phylum Mollusca is divided into the following seven classes: Monoplacophora, Polyplacophora, Aplacophora, Scaphopoda, Gastropoda, Bivalvia, Cephalopoda. 2. 3. Today, scientists have identified nearly two dozen extant species. Pelecypoda or Bivalvia or Lamellibranchiata 6. Sexual dimorphism is seen in this class of animals. Marine molluscs with an elongated worm-like body enclosed in a bilaterally cy­lindrical shell. The mouth opens at the end of a short buccal tube, at the base of which is a circlet of tentacles. 1. The first two classes are thought to represent primitive molluscs, the last three classes include the great bulk of living species. The gills one or two pairs (ctenidia or branchia), commonly plate-like. 8. The key difference between Mollusca and Echinodermata relies on the habitat of the organisms belonging to these two classes.Molluscs live in both terrestrial and aquatic environments while echinoderms live strictly in marine environments. Phylum Mollusca is the predominant phylum in marine environments. Gastropoda includes shell-bearing species as well as species with a reduced shell. 3.65. Watch the recordings here on Youtube! Mollusks are soft-bodied invertebrates of the phylum Mollusca, usually wholly or partly enclosed in a calcium carbonate shell secreted by a soft mantle covering the body. 1. Eyes may be absent in some gastropods species. Legal. Only one living genus with three species of Nautilus in eastern Pacific and Indian Oceans at depths to 560 metres; 2,500 fossil species. Visceral complex usually incorporated into head-foot complex, leading to second­ary external bilateral symmetry. 8. The mollusca phylum is split up into eight classes of mollusks, the three major ones being Gastropoda, Bivalvia, and Cephalopoda. 3. One pair of ctenidia, osphradia, au­ricles and renal organs present. 5. 7. The rest of the foot forms a siphon on the ventral surface of the body. Our mission is to provide an online platform to help students to discuss anything and everything about Zoology. Welcome to NotesOnZoology.com! 1. The single dorsal shell is thin and sub circular.