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Molluscs Snails, Slugs, Clams, Oysters, Octopi, Squids, Cuttlefish, etc. Mollusc Characteristics • 1. Coelomates • 2. Bilateral symmetry. • 3. First appearance of a one way digestive tract, • • • • with both mouth and anus. 4. Have a visceral mass, which houses the organs of digestion, circulation, reproduction, and excretion. 5. Have a mantle which secretes the shell. 6. Have a mantle cavity which is open to the outside for gas exchange and elimination of waste. 7. Most also have a beak which is similar to a bird’s beak, and a tongue-like radula used to scrape food. Trochophore Larva • Remember that a larva is an immature form that does not resemble the adult, but that eventually develops into an adult. 3 Class Gastropoda – Snails & Slugs • The soft body is protected by a • • • • hard shell, which the snail retreats into when alarmed. Found worldwide in the seas, in fresh water, and in moist areas on land. Locomotion: Snails move by crawling, swimming, or floating with currents. A special gland in the foot secretes mucus that helps the snail move. Snails have two pairs of tentacles on the head. Land snails have a light-sensitive eyespot located on each of the larger tentacles. The smaller pair of tentacles is used for the sense of smell and the sense of touch. Diet: Most snails eat living and decaying plants. They eat using a radula, a rough tongue-like organ that has thousands of tiny denticles (tooth-like protrusions). Snail Biology 5 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Radular Structure Source: From A Life of Invertebrates, Copyright © 1979 W. D. Russell-Hunter. 6 Class Bivalvia Class Bivalvia: Clams, Oysters, Scallops, & Mussels • Bivalves (pelecypoda) are • • • • soft-bodied animals that are protected by two hard shells, hinged together. Oyster – stationary Scallop – moves by “jet propulsion” Clams – burrowing foot Open circulatory system – no veins or arteries, just free flowing blood 9 Clam anatomy Pericardial cavity Metanephridium Heart Intestine Mouth Excurrent Stomach Incurrent Intestine Gonad 10 Pearl formation Shell Developing pearl Epithelium Irritant lodged between shell and mantle Layers of nacre secreted around foreign material 11 Class Cephalopoda: Octopi, Squid, & Cuttlefish • Cephalopod means "head-foot." The foot of the • • • • • cephalopod is a cluster of tentacles that connects directly to the head. The brain, the eyes, and the other sensory organs are well-developed, more than any other mollusc. Cephalopods breathe using gills. Closed circulatory system They swim by jet-propulsion; they rapidly expel water from the mantle cavity through a specialized, tube-like organ called the hyponome. Many can also eject a cloud of ink to confuse enemies. Many can change color for camouflage or to attract mates. Squid Tentacle Arm Funnel (siphon) Collar Eye Fin 13 Squid Shell (Pen) Systemic heart Brachial heart Ctenidium (gills) Funnel 14 Squid Male Testis Penis Hectocotylous arm – transfers sperm 15 Squid Female Ovary with eggs Oviducal gland Nidamental glands – (of or pertaining to eggs or a nest) Oviduct opening 16 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Cephalopod Eye (The most well-developed eye of all the invertebrates! They can see just as well as we can, and in perfect color!) 17