Billman, Bonin, & Olson Per. 5 Mollusca Billman, Bonin, & Olson Per. 5

General Characteristics 1)Bilaterally symmetrical. 2)Body has more than two cell layers, tissues and organs. 3)Body without cavity. 4)Body possesses a through gut with mouth and anus. 5)Body monomeric and highly variable in form, may possess a dorsal or lateral shells of protein and calcareous spicules. 6)Has a nervous system with a circum-oesophagal ring, ganglia and paired nerve chords. 7)Has an open circulatory system with a heart and an aorta. 8)Has gaseous exchange organs called ctenidial gills. 9)Has a pair of kidneys. 10)Reproduction normally sexual and gonochoristic. 11)Feed a wide range of material. 12)Live in most environments.

General Characteristics After the Arthropods the Mollusks are the most successful of the animal phyla in terms of numbers of species. There are about 110,000 species known to science most of which are marine. They also exhibit an enormous range in size, from species which are almost microscopic to the largest of all invertebrates the giant squid which can weighs 270 kg and measures up to 12 meters long in the body, with tentacles as much as another 50 meters in length.

Classes of Mollusca

Amphineura http://animaldiversity.ummz.umich.edu/site/resources/jack_burch/002.rjb1.jpg/medium.jpg

Monoplacophora http://biology.fullerton.edu/biol317/im/s02/bc/lined_chiton.jpg

Gastropoda http://www.pdphoto.org/jons/pictures2/snail_2_bg_112302.jpg

Scaphopoda http://www.sanibel-international.com/SSpincushionl.jpg

Bivalvia http://www.diggerschoice-seafood.com/images/littlenecksweb.jpg

Cephalopoda http://chemistry.csudh.edu/faculty/jim/cozaugo4-600/octopus.jpg

Body Plan

Body Plan Head-foot Visceral Lump The body is divided into two functional regions, the head-foot and the visceral lump. Head-foot It is mostly a muscular organ covered in cilia and rich in mucous cells includes a mouth, eyes and tentacles, the last two may be much reduced or even absent. species with shells the head-foot can be drawn into the shell Visceral Lump entirely nonmuscular and contains the organs of digestion and reproduction it includes the gonads, the kidney, the heart and the digestive diverticulum.

Feeding Digestion can occur in a ciliated tract or intracellularly Pelecypods are mainly filter feeders Cephalopods are active predators Gastropods have a sharp radula for drilling through shells some species have a single radula tooth while others may have several hundred thousand. In some the teeth are hollow and poison containing and are used as weapons

Respiration Use external gills for respiration Diffusion also occurs through the moist skin Respiration is through gills called ctenidia.

Circulation Have an open circulatory system, meaning blood does not circulate entirely within vessels but is collected from gills, pumped through heart, and released directly into spaces in tissues from which it returns to gills and then to heart. Such a blood-filled space is known as a hemocoel ("blood cavity"). hemocoel has largely replaced coelom, which is reduced to a small area around the heart and to the cavities of the organs of reproduction and excretion

Excretion Carried out by a pair of nephridia Tubular structures that collect fluids from coelom and exchange salts and other substances with body tissues as the fluid passes along tubules for excretion. Nephridia empty into the mantle cavity Excretion of wastes is through structures called metanephridia and through the body and gill surfaces.  

Response Cephalization is present complexity of the nervous system varies with each species Neurons are arranged in a ganglial pattern with two longitudinal nerve chords and a circum-oesophagal ring cephalopods have eyes capable of image formation Bivalves don’t have eyes, but they have cells that are tactile and photosensitive

Movement the head-foot is the muscular organ with cilia and many mucous cells Herbivorous forms glide by way of waves from muscular contraction Carnivorous forms such as cephalopods use jet propulsion (water sprayed from the mantle cavity by a siphon) Sea Hares and Cuttlefish use lateral fins Bivalves use the foot to burrow into the sand

Reproduction visceral mass contains the organs of reproduction (gonads) usually sexual and gonochoristic Fertilization is external Mostly dioecious but Gastropods are monoecious Egg becomes juvenile (cephalopods, snails, and some bivalves) or the egg becomes a trochophora larva (Chitons, monoplacophorans, scaphopods)

Bibliography "The Molluscs (Phylum Mollusca) ." The Earth Life Web. N.p., n.d. Web. 15 Apr. 2010. <http://www.earthlife.net/inverts/mollusca.html>. "The Mollusca." UCMP - University of California Museum of Paleontology. N.p., n.d. Web. 15 Apr. 2010. <http://www.ucmp.berkeley.edu/taxa/inverts/mollusca/mollusca. php>. "Phylum Mollusca." Infusion. N.p., n.d. Web. 15 Apr. 2010. <http://infusion.allconet.org/webquest/PhylumMollusca.html>. “Phylum Mollusca Lecture Outline.” N.p., n.d. Web. 15 Apr. 2010. <http://www.d.umn.edu/biology/courses/bio3701/Mollusca.htm>.