Laboratory Notes for BIO 1003
© 30 August 1999, John H. Wahlert & Mary Jean Holland
The Mollusca is the second largest animal phylum after the Arthropoda. Mollusks, which are familiar shallow marine and fresh water creatures, include clams, mussels, oysters, snails, slugs, octopus, and squid. They are an important source of food for people and for many other animals. All mollusks possess two characteristics that are special to the phylum: a ventral, muscular foot, and the mantle, a dorsal tissue layer that deposits the shell and may also be a respiratory organ. Mollusks are acoelomate (contrary to what the text and many other sources say), and the internal chamber is simply a pericardial sac; their bodies are derived from the three embryonic tissue layers. Externally they have three body regions: head, mantle, and foot. They are bilaterally symmetrical. Respiration is with gills (water breathing) or lungs (air breathing). The circulatory and digestive systems are separate. Some have a larval stage in development.
Class Pelecypoda or Bivalvia
These are bivalves, which means that they have two shells; these are hinged together at the dorsal apex. Examples include clams, mussels, oysters, and scallops. The body is flat and the foot is hatchet shaped (pelecy-pod). Examine a preserved clam. The tiny dorsal hump at the apex of each valve is called an umbo; the number of growth rings indicates the number of seasons or years of the clam's age.
Clam dissection: Slip your scalpel between the valves into one end of the clam and cut dorsally. You will cut through one of the adductor muscles, which hold the two valves closed. At rest the clam is open. Repeat at the other end of the clam. Then, holding the tissues in place, remove the valve that is facing up. A thin tissue layer, the mantle, lines the inside of each valve; the cells of the mantle actually extracted the calcium carbonate from the water and deposited it as shell material. The mantle of the two sides comes together at the posterior end of the clam to form the incurrent and excurrent siphons. Clams filter water for feeding and breathing. The leaf-like gills are just internal to the mantle on both sides of the body. Remove a gill, cut across it, and examine it with the dissecting microscope; what evidence do you see of increased surface area for molecular exchange? Water passes over them and gas exchange is accomplished; cilia move trapped food particles to the mouth. Labial palps, which flank the mouth, sort the particles by taste and permit certain ones to enter the mouth. The body of the clam contains the digestive system and gonad. The intestine curves dorsally and empties at the posterior end near the excurrent siphon. The tube of the intestine can be seen dorsal to the body mass. The heart is wrapped around it. The circulatory system is open, and blood travels in open sinuses through the body. Clams use the foot to burrow into the sand or mud, and only the siphons at the rear of the clam project up into the water; a living clam may thus be mostly hidden.
Fertilization in clams is internal; sperm are swept in through the incurrent siphon. Larvae called glochidia mature in the gill area and then parasitize fish for from two to three months by clamping on to their gills or fins with their valves. This parasitic stage serves to disperse the individuals far from their parents, as clams take over the world.
Gastro-pod means "stomach foot." These creatures include the snails, which are marine, fresh water and terrestrial, the terrestrial slugs, and the marine nudibranchs. The shell of a snail coils either to the animals left or to its right and provides a home into which the soft animal can withdraw. A slime gland in the front of the foot secretes mucus through which the snail glides by muscle contractions of the foot. The digestive system, excretory organ, mantle cavity and gills are housed within the shell. The mouth of a gastropod contains a rasp-like structure called a radula that can be used to scrape algae off rocks or make holes in the shells of other mollusks. Slugs and nudibranchs have lost the torsion or twisting of a snail's body. The mantle cavity in a slug functions as a lung, and its entrance, a hole into the side of the animal, can be seen to open and close.
Put a land snail on a microscope slide; observe it move under the dissecting microscope. The gills have been lost and the mantle cavity functions as a lung for gas exchange. There are two pairs of tentacles and the second has eyes are at the tip. This example is a herbivore; you may be able to see its radula working agains the glass, and you can feed it a lettuce leaf.
Cephalopods (head foot) include squid, octopus, chambered nautilus, and cuttlefish and are marine animals. The mantle forms a cavity with a funnel-shaped siphon. When the mantle relaxes, the cavity fills with water; contraction of muscle in the mantle squirts out a jet of water. By manipulating the direction of the siphon, the cephalopod can orient its swimming. A threatened octopus can add a dark secretion, ink, to the outflow so that it can swim away behind a concealing cloud.
Cephalopods are predators that use tentacles with suction discs to capture their prey. A radula moves the prey to the beaklike jaws of the mouth that bite and crush it. Some cephalopods also secrete venom into the prey. The high levels of activity incur a great demand for oxygen. The circulatory system is closed and highly specialized to pump blood through the gills. At rest a cephalopod rhythmically relaxes and contracts the mantle cavity to facilitate gas exchange in the gills and to expel waste that is discharged into the mantle cavity. The nervous system is highly developed, and cephalopods are capable of learning. They also have excellent vision with eyes that are in some ways similar to mammalian eyes.
Return to Index
Last updated 13 November 2016 (JHW)