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. Mantle cavity communicates with the exterior through an oval, closeable pneumostome. 1. Habitat: mostly marine; They burrow in mud and sand. 8. Members of class Monoplacophora (“bearing one plate”) posses a single, cap-like shell that encloses the body. 11. No distinct head; Examples: Mussels, Unio, Mytilus, Legal. The mantle cavity acting as the respiratory chamber. Content Guidelines 2. Their key characteristic is the torsion around the perpendicular axis on the center of the foot that is modified for crawling. Phylum Mollusca and Phylum Echinodermata belong to the Kingdom Animalia. This is a product of Mexus Education Pvt. 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. The Phylum Mollusca is considered one of the largest phyla in the animal kingdom.There are at least 100,000 known species. The shell is a curved tube open at both the ends, the opening being wider at the oral and narrower at the other end. The number of fossil species is estimated between 60,000 and 100,000 additional species. Examples: Lamellidens, Unio, Anodonta, Mytilus, Teredo, Oyster etc. Neometiia is hermaphroditic whereas Chaetoderma is dioecious. For more information contact us at info@libretexts.org or check out our status page at https://status.libretexts.org. Hermaphrodite; gonad single, mostly ovoviviparous; development direct or with suppressed larval stages. Sexual dimorphism is seen in this class of animals. Monoplacophora 2. The two subclasses of the class Amphineura sharing a similar primitive ner­vous system are very different in appear­ance, habitat and development. 2. Privacy Policy3. The body is elongated and worm-like and enveloped by the mantle. Class Aplacophora (“bearing no plates”) includes worm-like animals primarily found in benthic marine habitats. The rest of the foot forms a siphon on the ventral surface of the body. Body cylindrical or globose, often with fins. Body monomeric and highly variable in form, may possess a dorsal or lateral shells of protein and calcareous spicules. Watch the recordings here on Youtube! 6. They have a rudimentary mantle cavity and lack eyes, tentacles, and nephridia (excretory organs). 4. Respiratory structures—secondary gills, never ctenidia. Body without cavity. The respiratory organs consist either of one or two gills, or a gill and a pulmonary sac or a lung. Gastropoda This class of mollusca is the largest out of the major classes with over 37,500 species alive today. 4. The snail's cover for when it gets washed onto shore. Mollusks is one of the most fascinating and diverse animal groups on our planet.. Class # 1. Shell usually symmetrical, with dorsal hinge and ligament, and closed by 1 or 2 adductor muscles. 7. 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. The mouth and anus are situated at the anteromedian and posteromedian ends of the foot. The phylum Mollusca is commonly divided into seven classes: Gastropoda, Bivalvia, Cephalopoda, Scaphopoda, Monoplacophora, Polyplacophora and Aplacophora. Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda. Head with numerous retractile ten­tacular appendages which lack suckers. 3. Examples Achatina, Helix, Umax, Artalimax, Agriolimax (slug) Gonaxis, Lymnaea, Planorbis, Ferrisia,Physa, etc. Phylum Mollusca, contains 75-80% of all animals in Phylum. A flat creeping ventral foot is present. This group displays a broad range of morphological […] The typically elongated form, retain bilateral symmetry having terminal mouth and anus. Phylum Mollusca is a very diverse (85,000 species ) group of mostly marine species, with a dramatic variety of form. Pelecypoda or Bivalvia or Lamellibranchiata 6. Mnemonic Device: Some Grownups Can't See Magic Ponies But Children CAN Explanation: to remember the classes and sub-classes of Phylum Mollusca S - A pair of nephridia is present within the mantle cavity. Class Cephalopoda (“head foot” animals) includes octopi, squids, cuttlefish, and nautilus. 7. Marine molluscs with an elongated, bilaterally symmetrical flattened body and a broad foot ventrally. Mollusks may be primitively segmented, but all but the monoplacophorans characteristically lack segmentation and have bodies that are to some degree spirally twisted (e.g. 3.65. The foot is lobed and developed into tentacles and a funnel, which is used as the mode of locomotion. Largest is Gastropoda. 1. Many organisms belonging to this phylum have a calciferous shell. The metamerism is apparent and not represented externally. Mollusks are triploblastic (having 3 embryonic layers), closely related to flatworms/earthworms, and are the first animal phylum we have studied that have a true body cavity; a … The phylum Mollusca includes a wide variety of animals including the gastropods (“stomach foot”), the cephalopods (“head foot”), and the scaphopods (“boat foot”). Very few are terrestrial found in damp soil. Class Bivalvia consists of mollusks with two shells held together by a muscle; these include oysters, clams, and mussels. These animals continue to have an important role in the lives … Limpets. Phylum Mollusca is a very diverse (85,000 species ) group of mostly marine species, with a dramatic variety of form. Visceral complex usually incorporated into head-foot complex, leading to second­ary external bilateral symmetry. Marine, found in shallow (Neomenia) and deep sea (Chaetoderma), feeding upon hydroids, corals, etc. Question Set: Phylum Mollusca; Further Investigations: Phylum Mollusca; Phylum Arthropoda. Most gastropods bear a head with tentacles, eyes, and a style. The mouth of the shell can be closed by an aperculum borne on the foot. The mouth lies centrally in a reduced head, the anal and excretory apertures are posterior. 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. amphi- both + two neuron = nerve): The first two classes are thought to represent primitive molluscs, the last three classes include the great bulk of living species. The larva passes through trochophore and veliger stages. The buccal cavity contains an odontophore with a radula bearing rows of chitinoid teeth. 8. Example: Nautilus pompilius. Amphineura (Gk. 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. 10. Sexes separate, gonad solitary, opening in the right. Hermaphrodite; larva is a veliger, Car­boniferous to recent; 3,000 living and 300 fossil species. 3. 3. The gills one or two pairs (ctenidia or branchia), commonly plate-like. 2. Phylum Mollusca Classification Mollusca (mollusks) are classified into 6 classes according to their symmetry and the characters of food, shell, mantle, … The cephalopods are molluscs with large heads and tentacles. 3. 6. The monoplacophorans were believed extinct and only known via fossil records until the discovery of Neopilina galathaea in 1952. 3.65). 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. Phylum Mollusca is the second largest animal phylum. The head bears eight non-retractile ten­tacles which are provided with suckers in two rows. They can be found in terrestrial regions as well as in the depths of the seas. Head with one or two pairs of tentacles and one pair of eyes. The nervous system consists of paired cerebral, pedal and visceral ganglia with their connectives. The following diagram shows the major classes, which as can be seen are all based on variation in this same body plan. 3 Gastropods Sea snails (A) Sea slugs (B) Conchs (C) B C A 4 You will be amazed to know that the size of the molluscans can range from 20 metres to one millimetre, with a few microscopic animals too. Disclaimer Copyright, Zoology Notes | Exclusive Notes on Zoology for Students, Phylum Mollusca: Features and Classification, 3 Main Classes of Phylum Annelida | Zoology, 2 Main Classes of Phylum Hemichordata | Zoology, Essay on Earthworm: Feeding and Digestion. The foot is behind the head and is typically a flat, creeping organ. The giant squid, the largest invertebrate, reaches lengths of 15 m. Fig. The head bears a pair of large, simple eyes. 10. Large ventral foot. Weird Science: An Inordinate Fondness for Beetles; Activity: Aquatic Invertebrate Behavior; Question Set: Phylum Arthropoda; Further Investigations: Phylum Arthropoda; Phylum Echinodermata. A ventral longitudinal groove is present, which is connected anteriorly with a ciliated groove and posteriorly with the cavity of the cloaca. This phylum contains about 100,000 described species. Examples: Aplysia (sea hare), Actean, Gastropteron (sea slug), Cliona, Bertholimia, Polycera, Doris, Dendonotus, etc. 5. Sexes are separate or united; develop­ment with a veliger or glochidiam larva. They are bilaterally symmetrical, triploblastic and coelomate animal. Mollusks have a dramatic variety of form, ranging from large predatory squids and octopus, some of which show a high degree of intelligence, to grazing forms with elaborately sculpted and colored shells. We also acknowledge previous National Science Foundation support under grant numbers 1246120, 1525057, and 1413739. A free-swimming trochophore larva is succeeded by a veliger. Larval stages are trochophore and ve­liger. 5. 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. […] But some species live in freshwater and terrestrial habitats. The following points highlight the six main classes of phylum mollusca. Phylum : Mollusca . Bivalves include clams, oysters, mussels, scallops, and numerous other families of shells. 3. Among all known marine species, 23% are mollusks. 2. The single dorsal shell is thin and sub […] Examples: Neometiia, Proneomenia, Chaetoderma, etc. The shell may be absent or rudimen­tary; when present may be internal or exter­nal, undivided or divided internally into a series of chambers. Mollusca makes the second largest phylum of non-chordate animals including snails, octopuses, sea slugs, squid, and bivalves such as clams, oysters, and mussels. Amphineura 3. This video explains the characteristics and Classes of the Phylum Mollusca. 4. 9. Devonian to recent; 300 fossil and 100 living species. Marine gastropods displaying reduc­tion or loss of shell (shell sometimes inter­nal). Phylum Mollusca. Examples: Haliotis, Pila, Viviparus, Pa­tella, Acmaea, Fissurella, Cypraea, Littorina, Murex, Buccinum, Conus, etc. Body possesses a through gut with mouth and anus. Digestive glands are not distinct; sali­vary gland is usually present. (B) The co… 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. Gastropoda 4. Shell single piece with a simple spiral or none. Monoplacophora 2. Cephalopods are a class of shell-bearing animals as well as mollusks with a reduced shell. Phylum Mollusca (Mollusks) Etymology: From the Latin Molluscus, meaning soft of body. 8. 249-320. Most cephalopods are relatively small. 4. 3. Classes and General characteristics of Vertebrates. 6. The shell is absent and the body is covered with a cuticular mantle enclosing spicules of calcified material. It is estimated that 23 percent of all known marine species are mollusks; there are around 85,000 described species, making them the second most diverse phylum of animals. The class as a whole has become adapted for a free-swimming existence...... Click the link for more information. The shell consists of eight separate pieces on dorsal surface and studded later­ally with scales or spines to form a girdle. Ctenidia are enclosed in a large mantle cavity serviced by blood vessels, each with its own associated heart. Symmetry: bilateral and the body is laterally compressed. 6. 4. 3. Members of a species mate, then the female lays the eggs in a secluded and protected niche. Gastropoda includes shell-bearing species as well as species with a reduced shell. Unless otherwise noted, LibreTexts content is licensed by CC BY-NC-SA 3.0. The phylum Mollusca is divided into the following seven classes: Monoplacophora, Polyplacophora, Aplacophora, Scaphopoda, Gastropoda, Bivalvia, Cephalopoda. The head comprises of tentacles and compound eyes. These animals do not possess eyes, but they have a radula, as well as a foot modified into tentacles with a bulbous end, known as captaculae. The organisms belonging to phylum Mollusca exhibit the following characteristics: 1. Chitons live worldwide, in cold water, warm water, and the tropics. Class 1. Most chiton species inhabit intertidal or subtidal zones, and do not extend beyond the photic zone. Ctenidia are absent in these animals. Phylum Mollusca is a very diverse (85,000 species) group of mostly marine species. Members of class Monoplacophora have a single shell that encloses the body. 5. The mollusca phylum is split up into eight classes of mollusks, the three major ones being Gastropoda, Bivalvia, and Cephalopoda. Scaphopoda 5. They display vivid coloration, typically seen in squids and octopi, which is used for camouflage. 8. 4. Scaphopoda 5. 1. 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. Eyes may be absent in some gastropods species. 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 Phylum Mollusca is familiar to us as invertebrate animals, as it includes snails, clams, squid, oysters, sea-mice and tusk shells. 4. 7. TOS4. Welcome to NotesOnZoology.com! Marine molluscs with an elongated worm-like body enclosed in a bilaterally cy­lindrical shell. The classes are: 1. 4. ), which lives along the west coast of the United States, can grow to 1.5 m or more. The bodies are generally very soft and are covered by the hard exoskeleton.