Brian McCauley 12/12/07 Page 1 of 8 Animals 2 : Mollusc a , Ec hi no dermata , Arthropod a B i o 6 A / S a n h i t a D a t t a W i n t e r 2 0 0 8 This lab will cover three animal phyla: • Mollusca: snails, clams, octopus, etc. • Echinodermata: Sea urchins, starfish, sea cucumbers, etc. • Arthropoda: Crabs, shrimp, pillbugs, spiders, insects, etc. There is a lot of terminology in this lab; try to remember the terms in bold type. Refer to chapter 33 in Campbell for more information on the phyla covered in this lab. P h y l u m M o l l u s c a : S n a i l s , C l a m s , O c t o p u s , e t c . TISSUES: • Three well-defined tissue layers in embryo. SYMMETRY: • Bilateral. BODY CAVITY: • True coelom. PROTO/DEUTEROSTOME: • Protostome. DIGESTIVE TRACT: • Complete digestive tract. OTHER FEATURES • Gills contained in mantle cavity. • Many have a radula, which is a hard, tonguelike structure used for feeding. • Most have external shell. Mollusk means soft, and the members of this phylum have soft bodies – but generally inside hard shells. Mollusks include snails, octopus, clams, and some others. Mollusks have a true coelom, which seems to open the way to more complex development. One key effect of the coelom is the circulatory system. Mollusks have circulatory system with a heart, which develops in the coelom. Octopus and squid, the most active mollusks, have a closed circulatory system similar to that found in vertebrates. Most other mollusks have an open circulatory system, in which hemolymph is pumped by a heart but is not confined to blood vessels. In adult mollusks, the coelom is small, typically just a space that contains the heart. The mantle cavity of mollusks provides an excellent example of the way a structure that functions a particular way can change over evolutionary time to become something quite different. In the earliest mollusks, the mantle cavity was a space beneath the shell containing the gills. The mantle cavity serves different functions in various classes of mollusks, as described below. Four classes of Mollusks: • Chitons (class Polyplacophora): Marine. Shell consists of 8 plates. Chitons are grazers; they use a radula (hard, tonguelike scraper) to scrape algae off rocks. The mantle cavity contains gills, used for gas exchange. • Snails (class Gastropoda): Includes freshwater and marine, and terrestrial snails and slugs. Single shell, often curled into a tight spiral. Many snails are grazers, like chitons, and scrapeBrian McCauley 12/12/07 Page 2 of 8 algae or plants with their radula. They have gills, used for gas exchange, in the mantle cavity. Terrestrial gastropods don’t have gills; the mantle cavity functions like a lung. Some gastropods, such as cone snails are predators. • Bivalves (Class Bivalvia or Pelecypoda): Includes clams, mussels, oysters, scallops, etc. Two shells that can be closed together tightly. Bivalves are filter feeders. They use siphons to pump water into and out of their mantle cavity. In the mantle cavity, the water passes over the gills. The gills not only do gas exchange, they also capture small particles of food from the water. • Cephalopods (Class cephalopda): Includes squid, octopus, cuttlefish, and nautilus. Single shell, which becomes very small or disappears in some cephalopods (such as octopus). Cephalopods are active predators, with large focusable eyes, large brains, a closed circulatory system, and the ability to swim rapidly. Cephalopods can pump water through the mantle cavity for gas exchange and also use this pumping action for jet propulsion when swimming. Anatomy of a Bivalve: Specimens:  Live specimens of various mollusks. Take a look at the various species. How would you recognize them as mollusks? How would you know what class they belong to?  Model of clam anatomy. Note the mantle cavity, gills, siphons, foot, and gonad. These same basic features are also found in most other kinds of mollusks, though they may look very different. We also have a number of preserved specimens in jars:  Class Gastropoda. (specimens in jars). Gastropods (“stomach foot”) are snails, including marine and terrestrial forms, slugs, and various other animals that don’t look much like snails.  Class Bivalvia. (specimens in jars). Bivalves (named for the two “valves” of the shell) like clams, oysters, and mussels use their gills for filtering food particles from the water.Brian McCauley 12/12/07 Page 3 of 8 Most have a siphon for sucking water in and another for expelling water. The one-way flow of water across the gill is similar to that of a fish.  Class Cephalopoda. (specimens in jars). Cephalopods (“head foot”) are the high-performance mollusks. With big eyes, closed circulatory systems, and the ability to swim fast, they are active predators.  Class Polyplacophora. (specimens in jars). The Polyplacophora, commonly called chitons, are named for the multiple plates of their shells. The gills are along the side, under the edge of the mantle. They don’t have a big, highly developed mantle cavity like the others. And a single microscope slide:  Snail radula. (prepared slide). Snails use this hard, tonguelike organ for scraping off bits of food to eat. You may also be able to see the radula on live snails, if they are feeding – especially if they are scraping algae from the inside of a glass aquarium. P h y l u m E c h i n o d e r m a t a : S t a r f i s h , S e a U r c h i n s TISSUES: • Three well-defined tissue layers in embryo. SYMMETRY: • Bilateral in the larva, but adults are more or less radially symmetrical (pentaradial, since they have 5 arms). BODY CAVITY: • Coelom PROTO/DEUTEROSTOME: • Deuterostome DIGESTIVE TRACT: • Complete OTHER FEATURES • Calcareous endoskeleton • Hydrostatic skeleton (water vascular system) • Planktonic larva You might not guess it from looking, but the echinoderms are probably our closest relatives outside our own phylum. Key echinoderm features include: • Endoskeleton. Much like our own skeleton, the echinoderm skeleton consists of numerous calcified structures joined by more