Mollusca

Billman, Bonin, & Olson

Per. 5

General Characteristics1)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– 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.

The body is divided into two functional regions, the head-foot and the visceral lump.

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.

CirculationHave 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 nephridiaTubular 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 cavityExcretion 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>.