Chapter 11 Evolujonary Perspecjve Molluscan Characterisjcs Body

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Evoluonary Perspecve

• Triploblasc • Coelomate Chapter 11 • Very successful – 100,000 living species Molluscan Success • Relaonships to other animals – Lophotrochozoans

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Figure 11.1 Evoluonary Relaonships of Molluscs to Other Animals. Table 11.1.

Molluscan Characteriscs Body Organizaon • Head-foot – Elongate 1. Body of two parts: head-foot and visceral mass – Mouth 2. Mantle that secretes a calcareous shell and covers the visceral mass – Foot for aachment and locomoon 3. Mantle cavity funcons in excreon, gas exchange, eliminaon of digesve wastes, and release of reproducve products. • Visceral mass 4. Bilateral symmetry – Dorsal to head-foot 5. Trochophore larvae, spiral cleavage, and schizocoelous coelom formaon – Organs of digeson, circulaon, reproducon, excreon 6. Coelom reduced • Mantle 7. Open circulatory system (except Cephalopoda) – Enfolds body 8. Radula usually present – Secretes shell • Mantle cavity – Gas exchange, excreon, eliminaon of digesve wastes and reproducve products • Radula – Supported by odontophore – Rasping food Copyright © 2016 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written Copyright © 2016 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. consent of McGraw-Hill Education.

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Figure 11.2 Molluscan body organizaon. Figure 11.3 Molluscan shell and mantle.

Figure 11.4 Radular structure. Class Gastropoda • Snails, limpets and slugs • 35,000 living species • Torsion – 180o counterclockwise twisng of visceral mass, mantle, and mantle cavity during development – Possible adapve signiﬁcance • Head enters shell ﬁrst. • Clean water enters anteriorly oriented mantle cavity opening. • Mantle sensory organs move to head region.

Figure 11.5 Torsion in Gastropods. (a) A pretorsion gastropod larva. (b) Aer torsion the digesve tract is looped and mantle opens near head. (c) Hypothecal adult ancestor prior to torsion. (d) Modern adult gastropod aer torsion. Class Gastropoda • Shell coiling – Earliest fossils, one plane – Modern, asymmetrical • More compact • Internal organs asymmetrical and somemes no longer paired • Locomoon – Flaened foot – Cilia propel over mucous trail – Muscular waves

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Class Gastropoda Figure 11.6 Gastropod structure.

• Feeding and digeson – Most scrape algae and aached organisms – Herbivores, predators, scavengers – Digesve tract • Ciliated • Food incorporated into mucous mass called protostyle. • Gas exchange – One or two gills in mantle cavity – Land snails (pulmonates) • Mantle cavity richly vascular for gas exchange with air

Other Maintenance Funcons Other Maintenance Funcons • Open circulatory system – Blood bathes ssues in sinuses. • Excreon – Heart • Single ventricle and single auricle – Single nephridium – Funcons • Result of shell coiling • Transports nutrients, wastes and gases – Discharges into mantle cavity or adjacent to • Hydraulic skeleton mantle cavity (pulmonates) • Nervous system – Six ganglia plus nerve cords – Ammonia (aquac species) • Sensory structures – Uric acid (pulmonates) – Eyes at base or end of tentacles – Statocysts in foot – Osphradia in mantle cavity

Reproducon • Dioecious Gastropod Diversity – External ferlizaon • Eggs shed singly or in masses • Subclasses – Prosobranchia • Some dioecious marine species • 20,000 species • Monoecious • Mostly marine – Opisthobranchia – Copulaon for internal ferlizaon • 2,000 species • Sperm transfer may be mutual or one-way. • Mostly marine • Eggs shed in strings or in masses • Sea hares, sea slugs • Shell, mantle cavity, and gills reduced or lost – Larval stages – Pulmonata • Trochophore • 17,000 species • Veliger – foot, eyes, tentacles, shell • Freshwater or terrestrial • Vascular mantle cavity serves as lung

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Figure 11.7 Variaons in the Gastropod Body Form. Class Bivalvia

• Clams, oysters, mussels, scallops • 30,000 species • Shell and associated structures – Single shell consisng of two hinged valves (ﬁgure 11.8) – Mantle sheetlike and covers laterally compressed body.

Figure 11.8 Inside View of a Bivalve Shell. Gas Exchange, Filter Feeding, and Digeson • Sedentary filter feeders – Loss of head and radula – Expansion of cilia-covered gills into folded sheets (lamellae) • Cilia create water currents into and through mantle cavity. – Gas exchange in water tubes (figure 11.9) – Food trapped along gill surface and transported to food grooves and labial palps (figure 11.10). – Digeson (figure 11.11) • Crystaline style and gastric shield

Figure 11.10 Bivalve feeding. Solid purple arrows show path of food parcles. Dashed purple arrows show path of parcles being rejected. Figure 11.9 Lamellibranch gill of a bivalve. Blue arrows show water movement. Red arrows show blood movement.

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Figure 11.11 Bivalve structure. Other Maintenance Funcons • Open circulatory system – Mantle and gills oxygenate blood • Nephridia – Below pericardial cavity – Open to suprabranchial chamber • Nervous system – Three pairs of interconnected ganglia – Sensory receptors at mantle margin

Figure 11.12 Reproducon and Development Bivalve Circulaon.

• Mostly dioecious • Gonads within visceral mass • Mostly external ferlizaon • Trochophore and veliger larval stages (ﬁgure 11.13a, b) • Freshwater in family Unionidae – Parasic larval stage – Glochidium (ﬁgures 11.13c and 11.14)

Figure 11.13 Trochophore (a) and veliger (b) larval stages. Glochidia of an unionid bivalve. Figure 11.14 Mantle lure of a freshwater bivalve (Lampsilis reeviana).

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Figure 11.15b Bivalve diversity. Rock scallop (Hinnites giganteus). Figure 11.15a Bivalve Diversity. Giant clam (Tridacna dersa).

Figure 11.15c Bivalve diversity. The goeduck (Panopea generosa). Class Cephalopoda

• Squid, octopuses, culefish, and nauluses • Foot modified into circle of tentacles or arms and incorporated into siphon • Head in line with visceral mass • Muscular mantle (figure 11.17)

Figure 11.17 Internal structure of the squid, Loligo. Class Cephalopoda • Shell – Reduced or absent except in naulus • Locomoon – Jet propulsion using muscles of mantle compressing water within mantle cavity and siphon • Feeding and Digeson – Predators • Tentacles, jaws, radula – Digesve tract muscular with large digesve glands

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Figure 11.18 Cephalopod arms and tentacles. Other Maintenance Funcons

• Closed circulatory system • Nervous system – Large brains – Complex sensory structures • Eyes • Statocysts • Chromatophores – Color changes involved with courtship and other displays • Ink glands

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Figure 11.19 The cephalopod eye. Learning

• Unparalleled in comparison to any other invertebrate and many vertebrates • Evolved in response to predatory lifestyles

Reproducon and Development Class Polyplacophora • Chitons • Dioecious • Reduced head, flaened foot, shell consisng • Male produces spermatophores of eight dorsal valves, muscular mantle – Transfers to female’s mantle cavity using modified extends beyond margin of shell (figure 11.20) tentacle (hectocotylus) • Feed on aached algae • Eggs deposited singly or in masses aached to • Ladderlike nervous system substrate. • Dioecious with external ferlizaon • Eggs tended by parents.

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Figure 11.20 Class Polyplacophora. (a) Tonicella lineata. Class Scaphopoda

• Tooth shells or tusk shells • Marine, burrowing • Conical shell open at both ends • Dioecious with trochophore and veliger larvae

Figure 11.21 Class Scaphopoda. Class Monoplacophora

• Marine • Undivided, arched shell • Broad, ﬂat foot • Serially repeated pairs of gills and foot retractor muscles • Dioecious • Embryology unknown

Figure 11.22 Class Monoplacophora. Class Solenogastres

• Marine substrates Figure 11.23 Class Solenogastres • Lack shell • Crawl on ventral foot • Minute calcareous spicules • Carnivores

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Class Caudofoveata Further Phylogenec Consideraons • More than 500 million years old • Deep sea • Lophotrochozoa • Wormlike • Shell and muscular foot not ancestral • Feed on foraminifera – Solenogaster spicules may be similar to ancestral • Lack shell, foot, and nephridia “shell”. • 120 species – Muscular foot first seen in Polyplacophora. • Quickly diversified into modern classes (figure 11.24)

Figure 11.24 Molluscan Phylogeny.