Invertebrates-st-2011-2

Invertebrates-st-2011-2 - Invertebrates-Chap 33 Visceral...

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Unformatted text preview: Invertebrates-Chap 33 Visceral mass Mantle Foot Coelom Intestine Gonads Mantle cavity Anus Gill Nerve cords Esophagus Stomach Shell Radula Mouth Mouth Nephridium. Excretory organs called nephridia remove metabolic wastes from the hemolymph. Heart. Most molluscs have an open circulatory system . The dorsally located heart pumps circulatory fluid called hemolymph through arteries into sinuses (body spaces). The organs of the mollusc are thus continually bathed in hemolymph. The long digestive tract is coiled in the visceral mass. Radula. The mouth region in many mollusc species contains a rasp-like feeding organ called a radula. This belt of backward- curved teeth slides back and forth, scraping and scooping like a backhoe. The nervous system consists of a nerve ring around the esophagus, from which nerve cords extend. Figure 33.16 Phylum Mollusca 2 nd most abundant Mantle-leathery, secretes shell, needs calcium Coelom around the heart There are four major classes of molluscs Table 33.3 Most are marine, some have adapted to land [snails] Free swimming larval stage Evolved from platyhelminthes Chitons Class Polyplacophora is composed of the CHITONS Oval-shaped marine animals encased in an armor of eight dorsal plates Radula is present, no internal segmentation Figure 33.17 Gastropods About three-quarters of all living species of molluscs Snails belong to class Gastropoda (largest class in phylum mollusca) Ancestor was bilaterally symmetrical [larva] A land snail (a) A sea slug. Nudibranchs, or sea slugs, lost their shell during their evolution. (b) Figure 33.18a, b Torsion One side of the visceral mass grows at a much faster rate than the other. Causes the visceral mass to rotate 180 degrees relative to the head-foot. Disadvantage: causes fouling [anus comes near mouth] Bivalves Molluscs of class Bivalvia Include many species of clams, oysters, mussels, and scallops Have a shell divided into two halves No distinct head, no radula [filter feeders] Paired gills, mantle forms siphons Figure 33.20 The Shell The mantle is responsible for secreting the shell. The shell is comprised of three layers: The outside of the shell is covered by an organic layer - periostracum The middle prismatic layer is characterized by densely packed prisms of calcium carbonate laid down in a protein matrix The inner nacreous layer is composed of calcium carbonate sheets laid down over a thin layer of protein [secreted around an irritant to create pearls] Cephalopods Class Cephalopoda includes squids and octopuses Carnivores with beak-like jaws surrounded by tentacles of their modified foot Nautilus- only cephalopod that has a shell [in stasis] Only molluscs with a closed circulatory system Class Cephalopoda squids, octopus, cuttlefish, nautilus...
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This note was uploaded on 11/06/2011 for the course BIL 160 taught by Professor Krempels during the Spring '08 term at University of Miami.

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Invertebrates-st-2011-2 - Invertebrates-Chap 33 Visceral...

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