Systematic Adaptations of Phylum Mollusca

1. Systematic Adaptations of Phylum Mollusca The Chitons, Gastropods, Bivalves and Cephalopods

2. General Characteristics • The molluscs are a diverse group of soft bodied invertebrates • Some varieties of molluscs produce an external shell (“test” or “valve”)made of calcium carbonate • All molluscs have a fold of skin associated with the dorsal body wall called the Mantle which encloses the Mantle Cavity. The mantle cavity houses the gills, if present.

3. Body Form & Development • Triploblastic (well developed mesoderm) • Organ System level of body organization • True body cavity (eucoelomate) – the coelom is completely bound by mesodermal tissue and lined with peritoneum. • Complete gastrulation – one way digestive tract with a mouth and an anus • Protostomes – the first opening of the gastrula forms the mouth • Bilateral symmetry

4. Body Plan • Molluscs have a unique body plan with 2 general body regions – the Head-Foot and the Visceral Mass • The Head-Foot, as the name implies, contains the primary sense organs characteristic of cephalization, the mouth and feeding apparatus, and locomotor organs • The Visceral Mass contains the “guts” – the organs associated with digestion, respiration, reproduction, and circulation

5. Mollusc Body Plan

6. Cephalization - Photoreception • Varying degrees of cephalization occur in the molluscs. Some (particularly the cephalopods, which include squid and octopus) have very well developed eyes.

7. Mollusc Eyes – Scallop and Snail

8. Cephalization – Chemoreception and tactile pressoreception • Many molluscs (notably the gastropods and cephalopods) have tentacles associated with the Head-Foot region • The tentacles are used for “touch” and “taste”

9. Octopus Senses and Behavior • https://www.youtube.com/watch?feature=player_detailpage&v=yE0QqxwyL_8

10. Feeding apparatus • Most molluscs (all gastropods, chitons and cephalopods) have a specialized rasplike feeding organ called a Radula. The radula is a muscular tongue covered with teeth for scraping off food

11. Radula Teeth from a Slug

12. Snail feeding behavior • Video of a snail eating • Note the antennae, eyes, and mouth • Note the texture and movement of the radula • http://www.youtube.com/watch?v=cEMK3VN8pW8 • http://www.youtube.com/watch?v=mLVDwlrSq5U

13. The Head-Foot & Locomotion • The foot portion of the head-foot region is used for locomotion and varies greatly within the phylum • Gastropods (“stomach foot”) and Chitons have a broad, laterally flattened ventral foot • Pelecypods (“hatchet foot”) have a muscular foot elongated along the dorso-ventral midline • Cephalopods (“head foot”) have modified the foot to form a siphon to eject water for propulsion

14. Gastropod Locomotion Snails • https://www.youtube.com/watch?v=pRmChG3Jf_Q • https://www.youtube.com/watch?v=utxH9MH4tOI Sea Slugs • https://www.youtube.com/watch?NR=1&v=EwpexfYnlDE • https://www.youtube.com/watch?v=9VL_p2A7DPg

15. Cephalopod Locomotion Octopus • https://www.youtube.com/watch?v=rOvhs5icTfg • https://www.youtube.com/watch?v=t-LTWFnGmeg&feature=player_detailpage Squid • https://www.youtube.com/watch?v=Nf2iRRWH5OI • https://www.youtube.com/watch?v=EeaMOOnmHAA • https://www.youtube.com/watch?v=5o-OMa53F1s

16. Visceral Mass - Respiration • The Visceral Mass contains what we normally think of as the “internal organs” • Aquatic forms have Gills for respiration • Gills are enclosed in the Mantle Cavity, and covered by the Mantle. All molluscs have a mantle and mantle cavity • Some terrestrial forms have diminished gills, and perform gas exchange across the moist membranes of the mantle cavity

17. Mantle and Mantle CavityChiton and Mussel

18. Mantle and Mantle CavityOctopus and Squid

19. The Mollusc Shell • Many molluscs secrete protein and calcium carbonate from the mantle to create a shell • The shell typically has 3 layers: • The periostacum is the outer layer, made of protein secreted by the edge of the mantle • The prismatic layer is the middle layer, made of densely packed prisms of calcium carbonate in a protein matrix • The nacreous layer is secreted continuously by the surface of the mantle. This layer is what we call Mother of Pearl

20. Circulation • The mechanism of blood circulation varies • Bivalves have an Open Circulatory System. Blood pumped by the heart circulates through blood vessels in the gills, but empties into the coelom where the visceral organs are bathed by blood • Cephalopods have a Closed Circulatory System. Blood is circulated through blood vessels, including capillaries, into all body regions

21. Excretion & Osmoregulation • Metabolic wastes and excess water are removed by the kidneys (metanephridia) • The kidneys open on the inside to the coelom, communicating with the blood, and discharge wastes into the mantle cavity • The ducts of the excretory system are shared by the Reproductive system, providing passageways for sperm and egg to the mantle cavity

22. Reproduction: Trochophore Larvae The larval form of a mollusc is called a Trochophore See diagrams and descriptions p. 330 in your textbook

23. Mollusc Diversity • Mollusca is one of the most diverse and widespread phyla in the animal kingdom, second only to the arthropods • We will focus on only a few of the major classes of molluscs • Polyplacophora (Chitons) • Gastropods (snails and slugs) • Pelecypods (Bivalves – Clams, Oysters, Mussels & Scallops) • Cephalopods (Squid, Nautilus and Octopus)

24. Polyplacophora - Chitons

25. Gastropoda – Slugs and Snails • Foot is flattened and tight to the belly (thus “gastro” “pod” – belly foot) • Definite cephalization with both eyes and tentacles (for tactile sense and chemoreception) • Slugs – no shell. Snails – single shell • Asymmetrical deposition of calcium carbonate by the mantle causes the shell of snails to coil (see page 335 in text)

26. Torsion in gastropods • See page 334 in the textbook • During embryonic development, the mantle cavity of gastropods undergoes torsion – the mantle twists so that the posterior of the mantle is relocated over the anterior of the head-foot • Torsion allows the snail to more readily retract the head into the shell for protection • Torsion also positions the excretory openings over the gills. This problem is resolved by bringing water into the mantle cavity from the left and expelling waste water from the right

27. Gill variations in gastropods • Due to shell coiling, most snails have the right gill diminished or absent. The whorls of the coiled shell compress the right side of the mantle cavity and visceral mass • Terrestrial forms of both snails and slugs are lacking gills entirely, using the moist skin of the mantle cavity for gas exchange. These forms are called Pulmonates • Nudibranchs are slugs with exposed gills (nudi = naked, branch = gill)

28. Pelecypoda – The Bivalves • Pelecy = hatchet. The pelecypods are molluscs with a hatched shaped foot • Pelecypods are bivalves (2 shells). The shell has 2 distinct parts, attached by a hinge ligament • Clams, oysters, and mussels are all pelecypods • All are filter feeders and lack a radula • All have little or no cephalization (although some have eyes at the rim of the mantle)

29. Filter Feeding in the Bivalves • Many bivalves use their hatchet shaped foot to burrow into the soil • The mantle is organized to form an incurrent siphon and an excurrent siphon – which may extend significantly out of the shell • Water circulates through the mantle cavity, and ciliary action of the gill surface channels particles of food trapped in the gill mucus to the mouth • See textbook pages 340-342

30. Bivalve shells • Bivalve shells have a distinct Umbo, the oldest part – representing the position of the embryonic shell • The two valves of the shell are connected by a protein hinge ligament. The elasticity of the hinge ligament causes the shell to open • A pair of adductor muscles (one anterior and one posterior) contract to close the shell • Sand particles trapped between the shell and the mantle may be covered with secretions of calcium carbonate, forming a pearl

31. Bivalve shells

32. Cephalopods – Squid, and Octopus

33. Cephalopods – Nautilus and Ammonites