In these generally planktotrophic larvae, the girdle of ciliated cells widens to form a velum that entraps food and also propels the microscopic mollusk through the water. Molluscan systematics are still in flux. Like mollusks, annelids exhibit protostomic development. In the more aquatic gastropods, there are gills instead. Such feeding may have been selective or indiscriminate and will have encompassed algal, diatom, or cyanobacterial films and mats, or encrusting colonial animals. I. They have changed very little over geologic time, K.M.K. The Mollusca include some of the oldest metazoans known. 0000002491 00000 n As expected, both gastropods and bivalves show remarkably similar Ordovician diversity 7. Here we investigate richness patterns in the two largest classes of molluscs at both local and regional scales throughout the Southern Similarities between the veliger larvae of gastropods and lamellibranch bivalves have been long recognized. E^# d8T -!a7T_ah014G8D@ 1J#2&9r808ii{LaF# tq #Uh?NAAL%PXKp6=$-%]p",dN95(v!i1:{L- 1iM%? After a specific amount of time, which varies according to species and environmental conditions, the larva loses the velum and metamorphoses into an adult. Natl. Snails and slugs breathe through respiratory pores, the oxygen being absorbed directly into the abundant tiny blood vessels of the mantle. Biol. Bivalves, Cephalopods, and Gastropods. Mollusks, Available here. Cephalopods are generally carnivores, feeding on crustaceans and fishes, but some have adapted a microvorous diet of detritus and microscopic organisms and plants. Animals in the class Polyplacophora (bearing many plates) are commonly known as chitons and bear an armor-like, eight-plated shell (Figure 2). They do not have a foot like the bivalves, but do have a pedicle which aids attachment to rocks. shell is calcite. TABLE 3 Diet: Unlike many mollusks that feed using a rasping tongue, called a radula, bivalves feed by filtering food particles from the water. All species are obligate symbionts, meaning that they can only survive associated with their host, mainly with freshwater crayfish. You can unsubscribe at any time or request removal of street address, phone number, email address via Escoffier website. J. Molluscan Stud. Both are twin-valved and are filter-feeders. A.B.K., K.M.K. Gastropods are the largest group, comprising more than 80% of the molluscs. A single pair of nephridia for excretion is present. 0000005275 00000 n The few extant members of the class Monoplacophora inhabit secondary hard bottoms at depths of 175 to 6,500 metres and capture detritus by means of head appendages (velum) around the mouth. Get the most important science stories of the day, free in your inbox. Cephalopods are marine animals. Bioinformatics 24, 715716 (2008), Shimodaira, H. An approximately unbiased test of phylogenetic tree selection. 33, D476D480 (2005), Katoh, K., Kuma, K., Toh, H. & Miyata, T. MAFFT version 5: improvement in accuracy of multiple sequence alignment. 0000007279 00000 n They do this by siphoning water over the gills which traps it and propels it toward the mouth. Open Access WebThe species that characterized the similarities at the different scales of the model are listed in Supplementary Material Table S3; see also the results of the post-hoc tests (Supplementary Material Table S4). The change from grazing to other forms of food acquisition is one of the major features in the radiation of the group. of California Press, 2008), Scheltema, A. H. Aplacophora as progenetic aculiferans and the coelomate origin of mollusks as the sister taxon of Sipuncula. 6. Some gastropods are parasitic, while others are predatory. They breathe with ctenidia (gills) present ventrally. Cephalopods have a relatively complex brain and eyes with good, color vision. They are thought to be intelligent and can learn from experience. Caudofoveates (subclass Chaetodermomorpha, class Aplacophora) burrow in muddy sediments at depths of 10 to more than 7,000 metres (33 to 23,000 feet) and consume microorganisms and loose organic material (detritus). No # - Name some possible functions of the spines on this bivalve. 0000001816 00000 n Fresh seafood is a true culinary pleasure. Today, scientists have identified nearly two dozen living species. Recently, phylogenomic studies using dozens to hundreds of genes have greatly improved our understanding of deep animal relationships5. They can swim fast. Internet Explorer). Nature 471, 9598 (2011), Sigwart, J. D. & Sutton, M. D. Deep molluscan phylogeny: synthesis of palaeontological and neontological data. Mollusks have a wide range of locomotory patterns. HRmHQ=}}w8V[XIes}bI(2WH0"0B"ba)D7WX{=9p `2@yGs\d( k2.lCbUO The mantle cavity encloses the ctenidia as well as a pair of nephridia. Looking for some inspiration? They also use a lophophore to respire rather than gills. Habitat: They are found in both salt (marine) and freshwater habitats. What makes a bivalve a bivalve? The gastropods include snails, slugs, conchs, periwinkles and sea slugs. The visceral mass in the shelled species is characteristically twisted and the foot is modified for crawling. Members of the class first appear in the early Paleozoic and the taxon has maintained a slow and steady rate of increase in morphological diversification since then. The genomes of Capitella teleta, Helobdella robusta, Lottia gigantea and Nematostella vectensis were produced by the US Department of Energy Joint Genome Institute in collaboration with the user community. There are few features of the shell that are universal within each of these All rights reserved. 2. Phylum Annelida are segmented worms found in marine, terrestrial, and freshwater habitats, but the presence of water or humidity is a critical factor for their survival in terrestrial habitats. They have a variety of forms, ranging from large predatory squid and octopus, some of which show a high degree of intelligence, to small grazing forms with elaborately sculpted and colored shells. Earthworms are the most abundant members of the subclass Oligochaeta, distinguished by the presence of the clitellum, a ring structure in the skin that secretes mucus to bind mating individuals and forms a protective cocoon for the eggs. Burrowing occurs as an interaction between musculature and the hydrostatic skeleton (see below Internal features); it is performed in caudofoveates and several sea slugs by the whole anterior body but is restricted to the foot in scaphopods, bivalves, and some specialized gastropods. #26 - Notice the marks on this shell of some boring animal. Determining Evolutionary Relationships, 34. Bivalves can be eaten in a wide variety of ways, with techniques from raw consumption to deep frying and preparation in chowders all common. 2020, Available here. The sand irritates the soft tissue of the mantle. Fast evolving 18S rRNA sequences from Solenogastres (Mollusca) resist standard PCR amplification and give new insights into mollusk substitution rate heterogeneity. 2. USA 103, 77237728 (2006), Article In contrast, solenogasters (subclass Neomeniamorpha, class Aplacophora) prey on some members of the class Cnidaria (e.g., hydroids and corals) in five to 6,850 metres of water, on clay or muddy sand, or directly upon hydroid or coral colonies. This work represents contributions 82 and 4 to the Auburn University (AU) Marine Biology Program and Molette Biology Laboratory for Environmental and Climate Change Studies, respectively. This phylum includes gastropods, bivalves, scaphopods, cephalopods, and several other groups. Provided by the Springer Nature SharedIt content-sharing initiative. Chitons (class Polyplacophora) cling to hard bottoms of the intertidal zone, scraping algae from the rock surfaces by using their strong rasping teeth (radula); several members of the polyplacophoran family Lepidopleuridae consume detritus found at depths down to 7,000 metres, and Hanleyidae as well as Hopaliidae even depend on animal food. Webgastropods, bivalves, cephalopods The three classes of mollusks bright the ____ colors of these gastropods warn predators of the potential dangers bivalves which one of the three classes of mollusks does not have a distinct head or radula bivalves are filter feeders What is one of the main differences between gastropods and bivalves? II. Google Scholar, Giribet, G. et al. Watch this video for a close-up look at annelid anatomy. 0000009256 00000 n Within Conchifera, the previously most widely held hypothesis places Monoplacophora basal to two clades: Cyrtosoma (= Visceroconcha; Gastropoda and Cephalopoda) and Diasoma (= Loboconcha; Bivalvia, Scaphopoda, and the extinct class Rostroconchia) ( Runnegar and Pojeta 1974, Pojeta and Runnegar 1976, Salvini-Plawen Oysters (#1704, no #): some are attached, others rest in soft substrate. As the name suggests, bivalves are enclosed in a pair of shells (or valves) that are hinged at the dorsal side. They also have two pairs of tentacles, only one bearing eyes at the end.Class Cephalopoda. The subclass Hirudinoidea includes leeches. Nature (Nature) Cephalopods include squid, octopuses, cuttlefish, and chambered nautiluses while gastropods include snails, conchs, abalones, whelks, sea slugs, and garden slugs. Both shelled and naked gastropods have pelagic members that spend their entire lives swimming in the water; others penetrate marine hot vents or interstices between sand grains. Origins of Life Chemistries in an RNA World, 67. In order to catergorize bivalves, they are put into the Mollusca phylum. Based on our current understanding of relationships, the earliest molluscs grazed on encrusting animals and detritus. 1. Gastropods are a group of molluscs that include snails, conchs, abalones, whelks, sea slugs, and garden slugs. to rock in intertidal or subtidal, smooth rounded shapes (#458)- deep burrowing in sand and mud, high unornamented spires (brown unnumbered)- shallow burrowing in sand, thick ornamentation (#20, 1555)- defense against crushing predators A new study answers some questions about the base of the molluscan tree, showing that, contrary to the traditional view, bivalves and gastropods are members of sister taxa. snail's life habit? Biology & Philosophy The organism lives head-down Understanding the Naturalistic Fallacy, 58. A single large muscle closes the shell A substantial change in shell morphology usually marks the transition to adult form. The greatest ecological diversity is shown by the gastropods. The key difference between cephalopods and gastropods is that cephalopods are exclusively marine animals, while the majority of gastropods are terrestrial, and some are from marine and freshwater. 2. 1. including humans. Mollusks are eucoelomates, but the coelomic cavity is restricted to a cavity around the heart in adult animals. volume477,pages 452456 (2011)Cite this article. 1-833-443-0618, 637 South Broadway, Suite H & Madan, A. CAP3: a DNA sequence assembly program. Copulation in solenogasters, often by means of a special genital cone, may be supported by copulatory stylets. Grapevinesnail 01a By Grapevinesnail_01.jpg: Jrgen Schonerderivative work: Tim Ross (talk) Grapevinesnail_01.jpg (CC BY-SA 3.0) via Commons Wikimedia. The Importance of Biodiversity to Human Life. Her research interests include Bio-fertilizers, Plant-Microbe Interactions, Molecular Microbiology, Soil Fungi, and Fungal Ecology. Molluscs occur in almost every habitat found on Earth, where they are often the most conspicuous The three groups of molluscs are gastropods,cephalopods, and bivalves. They have a rough, tongue-like structure, called a radula, with which they scrape away at food. While most are found in the marine environment, extending from the intertidal to the deepest oceans, several major gastropod clades live predominantly in freshwater or terrestrial habitats. more heavily sculptured shells? ISSN 1476-4687 (online) Reproduction via an unfertilized gamete (parthenogenesis) is also found among gastropods of the subclass Prosobranchia. Title: Subtitle: Section of Page if appropriate. Scallops burrow or swim freely, using their shell to clap water propelling them as needed. 10, 323 (2010), Caron, J. What are the shaped burrowers. To help students learn the anatomy of the Mollusks, print out a black and white version of the color following diagrams: Assess content comprehension about Mollusks (Octopus, Squid, Snails, Clams, etc.) The Mollusca Classification: The traditional classification of bivalves 6. Clicking the "Get the Workbook Now" button constitutes your express written consent to be contacted via phone, text, and/or emails by Auguste Escoffier School of Culinary Arts at the number(s)/email you provided, regarding furthering your education. Webdetermine the similarities between the defense mechanisms of two venerid clams, T. semidecussatus and T. decussatus, when they are infected by Perkinsus sp. -between digestive tube & nerve cord 2. While gastropods have many different shell shapes (or no shells at all) and lifestyles, they are united by undergoing a process called torsion during their development (see below). Your email address will not be published. Cephalopod diversity is much more variable through the Phanerozoic, whereas the remaining groups (monoplacophorans, rostroconchs, polyplacophorans, and scaphopods) maintain low diversity over the entire Phanerozoic or became extinct. Most notably, both possess larval retractor muscles and a velum muscle ring 18. While diners enjoy bivalves, crustaceans and gastropods at all times of the year, summer is traditionally associated with eating many of these mostly hard-shelled creatures. 0000005523 00000 n How do you explain this? Other Acellular Entities: Prions and Viroids, 111. Annelids have a well-developed nervous system with two ventral nerve cords and a nerve ring of fused ganglia present around the pharynx. BMC Bioinformatics 9, 5 (2008), Ebersberger, I., Strauss, S. & Von Haeseler, A. HaMStR: Profile hidden markov model based search for orthologs in ESTs. When citing a WEBSITE the general format is as follows. Typically, at least in the more primitive members of each group, there are one or more pairs of gills (called ctenidia) which lie in a posterior cavity (the pallial cavity) or in a posterolateral groove surrounding the foot. If you find something abusive or that does not comply with our terms or guidelines please flag it as inappropriate. Mussels use their foot to anchor to rocks and other objects. 0000006439 00000 n K.M.K. Unlike cephalopods, gastropods have an open circulatory system. For that reason, we will dip a tiny bit into the traditional of the animals. supporting your hypothesis: #1074, 22, 25, 592. Sutton. Prevention and Treatment of Viral Infections, 105. Author Last Name, First Name(s). F. W. Goetz, A. Gracey and M. L. Blaxter kindly provided sequence quality data for Dreissena rostriformis, Mytilus californianus and Lumbricus rubellus, respectively. The resolved relationships shown (such as cephalopods, scaphopods, and gastropods) are recent discoveries. WebThe scaphopod mollusc (Figure 9) combines gastropod-like shell morphology with bivalve-like development with cephalopod-like anatomy. Pharyngeal slits-gills-jaw support, hearing 4. Evol. Brachiopods - Have one valve (pedicle valve) larger than the other and tend to grow to 2-10cm in length; smaller than the average bivalve. and abalone, several groups with low translation rates and simple aperatures, uncoiled or flat-bottomed shells (#1189, abalone) - often live attached 150 0 obj << /Linearized 1 /O 153 /H [ 1816 675 ] /L 792692 /E 89318 /N 32 /T 789573 >> endobj xref 150 62 0000000016 00000 n Google Scholar, Wilson, N. G., Rouse, G. W. & Giribet, G. Assessing the molluscan hypothesis Serialia (Monoplacophora+ Polyplacophora) using novel molecular data. The larva in primitive bivalves is a pericalymma (test cell) larva in which the embryo is protected below a covering (test) of cells provided with one to four girdles of cilia, at the apex of which is a sensory plate of ciliated cells. Some having up to a 100 tentacles. WebGastropods include shell-bearing species as well as species with a reduced shell. 9, 868877 (1999), Lottaz, C., Iseli, C., Jongeneel, C. V. & Bucher, P. Modeling sequencing errors by combining Hidden Markov models. Mussels (987): live in the rocky intertidal, attached to rocks with Websimilarities between gastropods and bivalves Non Pathogenic Gram-positive Bacteria , Pachgaon Kolhapur Pin Code , World Youth Lacrosse Championships , Function Of We thank W. Jones and K. T. Fielman for help with cDNA library preparation, R. M. Jennings, N. Mikkelsen, and the crews of the RV Hkon Mosby, RV Hans Brattstrom and RV Laurence M. Gould for assistance collecting aplacophorans, and J. C. Havird, P. J. Krug, S. C. Kempf, D. R. Lindberg, M. V. Matz, L. R. Page and T. H. Struck for discussions. However, limited genomic resources spanning molluscan diversity has prevented use of a phylogenomic approach. and K.M.K. Visit the mollusca pages on the Tree of Life for more on molluscan systematics. D. Speiser kindly shared the photo of Argopecten. Bivalves include clams, oysters, mussels and scallops. Boulder, CO 80305 33, 511518 (2005), Misof, B. Phylogenet. 7, (2007), Okusu, A. these orders just fall apart. 10 September 2022, Scientific Reports Cephalopods have a series of tentacles that circle the head. Cephalopods are a group of molluscs comprising of marine animals such as octopi, cuttlefish, and nautili, etc. 0000023604 00000 n Understanding Altruism: Self and Other Concerns, 62. Iberus 21, 6797 (2003), Meyer, A., Todt, C., Mikkelson, N. & Lieb, B. One-to-one online tuition can be a great way to brush up on your Geology knowledge. anchored in the sediment. Suckers are present on the tentacles in octopuses and squid. This pattern of movement is supported or replaced in larger mollusks by the propulsive waves that run along the surface of the foot and are controlled by the actions of the dorsoventral musculature (Figure 1). 74, 177190 (2009), Simone, L. R. L. Filogenia das superfamlias de Caenogastropoda (Mollusca) com base em morfologia comparativa. Moreover, Cephalopods have a closed circulatory system, while gastropods have an open circulatory system. Introduction to Origins of Life of Earth, 63. Nucleic Acids Res. Buoyancy floating and jet propulsion are found in cephalopods; floating is also known in gastropods, and swimming of a different kind is practiced by some opisthobranch and prosobranch gastropods as well as in scallops and related bivalves. Nature 442, 159163 (2006), Scheltema, A. H., Kerth, K. & Kuzirian, A. M. Original molluscan radula: comparisons among Aplacophora, Polyplacophora, Gastropoda, and the Cambrian fossil Wiwaxia corrugata. L.L.M., A.B.K., K.M.K., J.T.C. Some gastropods are aquatic: salt and freshwater animals. Citing for websites is different from citing from books, magazines and periodicals. Assess anatomical vocabulary comprehension with the Snail Anatomy Labeling Page. They are strictly marine molluscs. In some groups, like slugs and octopuses, the mantle is secondarily lost, while in others, it is used for other activities, such as respiration. WebPhylum - Mollusca (Gastropods, Bivalves, Cephalopods) There are more than 50,000 species in this phylum. 0000042082 00000 n These snails are predators, catching bivalves and drilling holes through the Molluscs are a clade of organisms that all have soft bodies which typically have a "head" and a "foot" region. Tooth shells have a radula and a foot modified into tentacles, each with a bulbous end that catches and manipulates prey (Figure 5). Phylum Mollusca- Characteristics, Classification, Examples. Microbe Notes, 23 Mar. The octopuses and squid are an exception to this and have a closed circulatory system with two hearts that move blood through the gills and a third, systemic heart that pumps blood through the rest of the body. Look for the muscle scar. Biol. 2, 3442 (2011), Wanninger, A. 11. PubMedGoogle Scholar. (like crabs), so living out in the open either on rocky or sandy substrates. 226270). published in 2005. Evolutionary relationships among the eight major lineages of Mollusca have remained unresolved despite their diversity and importance. & Misof, K. A Monte Carlo approach successfully identifies randomness in multiple sequence alignments: a more objective means of data exclusion. 0000001591 00000 n Phylum Mollusca includes a group of soft-bodied invertebrates with bilateral symmetry. About 1380 individuals of gastropods and bivalves were found in 8 transects. Their well-developed camera-type eyes help them catch prey. The Gastropod Mollusks: Nudibranchs, Sea Slugs, Sea Hares, Snails, Limpets and More The gastropods make up the largest class of mollusks with In more advanced mollusks (such as in marine gastropods and bivalves), the trochophore larva develops into a veliger larva. They feed on the algae that grows on the carapace of the crayfish. In order to close the valves, a pair of adductor muscles contract to keep valves closed (leave scars at anterior and posterior). Biol. The adaptive features of Watch animations of clams and mussels feeding to understand more about bivalves. Recognize a few groups 5. #1329 - How is this gastropod preserved? For example, on the Pacific coast of California, Native Americans consumed large quantities of abalone and especially owl limpets. Waterford's Energy Flow Through Ecosystems, 118. Most gastropods have a coiled or conical shell, which may be extremely reduced in some species or lost entirely as in slugs. 0000004331 00000 n Have a Free Meeting with one of our hand picked tutors from the UK's top universities. Biol. organisms. Overview and Key Difference Fertilization by transfer of capsules containing sperm (spermatophores) typically occurs in cephalopods and some gastropods. Another metabolic 0000004905 00000 n Here we investigate richness patterns in the two largest classes of molluscs at both local and regional scales throughout the Southern WebSize of foot in bivalves The foot is used primarily for attachment in gastropods, and partly locomotion. It is unclear whether similarities or differences between sites or regions persist over time because large differences in Pismo clam population density can Beyond congruence: evidential integration and inferring the best evolutionary scenario, Mito-nuclear coevolution and phylogenetic artifacts: the case of bivalve mollusks, Elemental analyses reveal distinct mineralization patterns in radular teeth of various molluscan taxa, http://www.ncbi.nlm.nih.gov/projects/dbEST, Diversification of the aquaporin family in geographical isolated oyster species promote the adaptability to dynamic environments, The rhodopsin-retinochrome system for retinal re-isomerization predates the origin of cephalopod eyes. Google Scholar, Haszprunar, G. Is the Aplacophora monophyletic? muscle. The evolutionary benefit of such a body plan is thought to be the capacity it allows for the evolution of independent modifications in different segments that perform different functions. Large concentrations of gastropods and bivalves are found at hydrothermal vents in the deep sea. The phylum includes earthworms, polychaete worms, and leeches. The skin of annelids is protected by a cuticle that is thinner than the cuticle of the ecdysozoans and does not need to be molted for growth. External shells present in some species, 6. into an order given the descriptions of the orders. Polychaete worms have paired, unjointed limbs called parapodia on each segment used for locomotion and breathing. Gas exchange occurs across the moist body surface. Escoffier is still open for enrollment & student support. Eggs are deposited singly or in groups, generally on some hard surface and often within jelly masses or leathery capsules. The fossil record of mollusks is relatively complete, and they are well represented in most fossil-bearing marine strata. The fossil record Wide environmental tolerances. This handbook will help you plan your study time, beat procrastination, memorise the info and get your notes in order. Creative Commons Attribution-NonCommercial 3.0 Unported License. Diet: They are predators, using their arms (or tentacles) to trap prey, and a sharp beak to bite prey. The species is prey for a variety of predators such as fishes, gastropods, crustaceans, birds, and mammals, particularly the sea otter (Shaw and Hassler 1989; McLachlan et al. Crustaceans also benefit from a number of tried and true preparations and presentations. Molluscs can also be nuisances, such as the common garden snail; and molluscs make up a major component of fouling communities both on docks and on the hulls of ships. Most gastropods bear a head with tentacles that support eyes. google mountain view charge cash app; wect news bladen county; similarities between cephalopods and bivalves; similarities between cephalopods and bivalves. There are more than 50,000 species in this phylum. The body is flattened on the sides. Most mollusks have an open circulatory system with a heart that circulates the hemolymph in open spaces around the organs. III. Austin, TX 78752 (adsbygoogle = window.adsbygoogle || []).push({}); Copyright 2010-2018 Difference Between. Phylum Annelida includes the classes Polychaeta and Clitellata (Figure 7); the latter contains subclasses Oligochaeta, Hirudinoidea, and Branchiobdellida. Recognize a few groups by their common names (below). modified the morphological character matrix. They have a highly developednervous systemwith a complex brain. They also have a mantle, which is a thin layer of tissue covering their body organs. As the larva continues to develop, the shell, mantle cavity, tentacles, and foot appear. They feed by filtering particles from water and a radula is absent. Some gastropods live in marine water, while some other types live in freshwater. 50. There are many bivalves regularly eaten around the world, from clams and scallops to mussels and oysters. What is your evidence? WebBrachiopods and Bivalves A theory on the morphology ERTH2312A Paleontology Presented to Professor T. Patterson Joachim de Fourestier Student Number: 101022736 27/03/2017 Brachiopods and Bivalves are superficially similar, but are rather distantly related. 54. morphology of the mussel. Intelligent, well developed nervous system, 4. Sea slugs, squid, snails, and scallops. After the developing juvenile has grown out apically of the test (which then is lost), the animal settles and develops into an adult. with "wings" (auricles), single large adductor muscle, flat swimmers: one valve flatter than the other, vertical swimmers: valves equally rounded, byssal (attched with threads) (no #): adductor muscles of unequal size, The polytomies shown indicate that the question of which molluscs are the most closely related is still a matter of debate. Major Histocompatibility Complex (MHC) Proteins, 43. and K.M.K. They are called Bivalves because they have two shells. Cnidarians such as Jelly Fish do not have shells which define a bivalve (mollusks). What is the nervous system of a Cephalopod? Cephalopods are widely regarded as the most intelligent of the invertebrates, and have well developed senses and large brains (larger than those of gastropods). Cephalopods are the most intelligent invertebrates. In some snails, fertilization occurs internally through copulation, but in others it involves external spawning. 0000010295 00000 n To work the valves, adductor muscles contract to close valves (pair of scars in pedicle valve, 2 in brachial) and then the didcutor muscles contract to open valves (run from cardinal process to floor of pedicle valve). 0000001752 00000 n Most bivalves are either suspension or deposit feeders that indiscriminately take in particles, but then elaborately sort them based on size and weight, typically assimilating bacteria, protists, and diatoms. 0000004561 00000 n A cladistic point of view. %PDF-1.4 % So, this is the key difference between cephalopods and gastropods. 184, 5778 (1993), Salvini-Plawen, L. On the phylogenetic significance of the aplacophoran Mollusca. where the soft tissue of the snail was attached to the shell. They have no head, but a diffuse nerve network made up of three nerve centers (or ganglia) distributed around their visceral mass. We thank A. Degree in Plant Science, M.Sc. Despite their amazing diversity, all molluscs share some unique characteristics that define their body plan. Proc. WebSome littoral bivalves, such as Tridacna, as well as some sea slugs, such as Aeolidia, share an obligatory symbiosis with zooxanthellae (a group of algae). Archeogastropods (#579, 1305, 578): Paleozoic to Recent, herbivores, Mesogastropods (#1604, 474): Late Paleozoic to Recent, herbivores and