Student Resources

Mollusca -Workbook Questions

How does a mollusc separate the organic food from the inorganic substrate that it ingests?  (p. 64)

Feeding with a radula creates a mixture of food and substrate particles that need to be separated from each other before digestion can proceed. Both types of particles enter the stomach embedded in a mucus string, and when the string reaches the stomach, digestive enzymes dissolve it to release the ingested particles. Ciliary sorting fields are located between the stomach and digestive gland where intracellular digestion of the food will occur. These sorting fields allow only particles with the proper size to enter the digestive gland. Those larger than this are either rejected and passed to the intestine or sent back into the stomach for further digestion. The longer a food particle is in the stomach, the smaller it becomes until it finds its way into the digestive gland where phagocytosis occurs. The digestive enzymes in the stomach can’t digest substrate particles; they never pass through the sorting fields but instead move directly to the intestine. Digestion may also occur along the length of the intestine, and phagocytosis takes up anything small enough.   

What are the three layers of the mollusc shell and of what is each made?  (p. 64)

The three layers include: the innermost nacreous layer, the middle prismatic layer, and the outer periostracum. The nacreous layer, or mother-of-meal, is formed from small, flattened calcareous tablets. This differs from the prismatic layer where the tall, parallel crystalline prisms are formed from the similar calcareous salts. In both these layers, cross-linked shell protein, conchin (or chonchiolin), is often used to cement the tablets and prisms together. The outmost periostracum is organic and consists of conchin.

How do metabolic wastes get from blood in the hemocoel to the fluid in the pericardial cavity that the metanephridia filters?  (p. 66)

Molluscs have an open-ended, funnel-shaped metanephridia, and like other animals with metanephridia it filters the coelomic fluid by pulling it into the funnel’s open end, the nephrostome. In molluscs, which have an open circulatory system, the largest body cavity is the hemocoel, filled with hemolymph (blood) that bathes the internal structures while collecting and circulating metabolic waste. The coelomic space and its fluids, which the metanephridia filter, are restricted to the small pericardial cavity surrounding the heart. There needs to be a way to get wastes from the hemolymph to the coelomic fluid so the metanephridia can do its job.

When the heart beats, it pulls oxygenated hemolymph inside the gills toward the heart. This part of the mollusc circulatory system is closed, and as the blood moves from the veins toward the heart, it passes through a large, thin-walled atrium suspended in the pericardial cavity and surrounded by pericardial fluid. Metabolic wastes diffuse across the wall, into the fluid, and are then filtered by the metanephridium that opens into the pericardial space. In marine molluscs, the gill surfaces are also an important site for diffusion of metabolic wastes, and the metanephridia are more important for osmoregulation compared to excretion. This is even more so in freshwater molluscs that are hypoosmotic to their surrounding environment. 

How does a clam burrow in the sediment?  (p. 66)

The clam uses its foot as a hydrostatic skeleton to dig into the substrate. Blood in the clam’s open circulatory system is a hydraulic medium. When the clam starts to burrow, contraction of the foot musculature forces the foot into the soft substrate. Once the foot has extended, the tip swells as blood is forced into the tip of the foot, anchoring it in the substrate. As the foot extended, the foot retractor muscles were stretched, and now they contract and pull the shell into the substrate. To help do this, the shell adductor muscles may contract and relax, opening and closing the shell, forcing water in and out of the mantle cavity. Finally the adductor muscles relax, and the shell opens and anchors the body so that the foot can once again dig farther into the substrate, pushing against the anchored shell.  

What causes the ridged rings on the surface of the clam’s shell?  (p. 67)

Molluscs are unable to regulate their body temperatures; when cold, their metabolism slows down and when warm, metabolism speeds up. The shell grows at its edges where the mantle deposits new shell, and they grow faster when it’s warm and slower when the temperature drops. The rate that new shell is added changes with temperature. The result is the growth rings that you see in the shell surface. 

Describe the route that water follows as it passes through a clam starting from the incurrent siphon and finishing with the excurrent siphon.  (p. 67) 

Water passes through these structures in the following order: incurrent (inhalant) siphon; inhalant space; through the interfilament spaces or ostia in the ctenidia; into the exhalant space inside the ctenidia; to the suprabrachial chamber; and out the excurrent (exhalant) siphon. 

What are the presumed advantages of torsion, and for what stage in the gastropod life cycle was it an advantage?  (p. 68)

Biologists don’t agree on what was, or even if there was, an advantage to torsion. One thought is that torsion was advantageous for the larval stages of the animal. Rotation of the visceral muscles caused the pedal retractor muscle to cross each other; when the muscles started to contract, they pulled the head and its ciliated velum inside the shell first. Without torsion, muscles would have pulled the middle of the foot inside the shell first, and this would have prolonged the time the head and velum were exposed for attack by predators. The other school of thought presumes that the advantage of torsion was for the adult, rather than the larval, stage of the gastropod. As the animal creeps across the substrate, it detects water quality using special sensory osphradia located inside the mantle cavity. If the mantle is behind the animal, it could become clogged with particles of sediments swept into the mantle cavity as a result of locomotion. Having the osphradia in front, one of the consequences of torsion, prevented this from happening. It had the added advantage of placing the main sensory structures of the animal in an anterior position, like other animals. 

In snails, describe the events from insemination to oviposition.  (p. 68)

Being monoecious organisms, snails have separated the events of sperm transfer and fertilization to prevent self-fertilization. What complicates the organization of their reproductive system is that torsion has resulted in lost structures and other structures are shared by the female and male reproductive systems. Although the events differ between species, the main events are similar to those of Helix where the single ovary and testis are combined to form the ovotestis located deep in the body whorl, surrounded by the digestive gland. Sperm forms in the ovotestis; passes down the hermaphrodite duct and the sperm duct (vas deferens); and is stored prior to mating in the distal end of the sperm duct, which may be modified into a seminal vesicle. When snails mate, the penis is everted and inserted in the female. Sperm is transferred to the spermatheca (or copulatory bursa) and stored there until it moves up the oviduct to fertilize the egg. Eggs formed in the ovotestis pass down the hermaphrodite duct and are fertilized at the junction of the hermaphrodite duct, albumen gland, and the oviduct. Once the eggs are fertilized they are provisioned with nutrients and a protective covering, or egg case, as they pass down the length of the oviduct before being oviposited. 

How does the nautalid shell differ from that of a gastropod?  (p. 69) 

It differs in the shape of the spiral, a flat planospiral, and the absence of a periostracum. 

What route does blood follow as it moves through the squid? Start at the systemic heart.  (p. 69)

Unlike other molluscs, the circulatory system in the cephalopods is closed. Blood flows from the systemic heart to either the anterior part of the body through the anterior (or cephalic) aorta or toward the posterior through the posterior aorta that divides to form the left, right, and median lateral mantle arteries. Anterior arteries supply internal organs and structures in the head, and as their names imply, the branches of the posterior arteries supply the mantle. Blood returns from the cephalic region through the anterior vein (cephalic vein or anterior vena cava). The anterior vein branches into left and right anterior veins and each combines with corresponding left and right posterior veins (also referred to as the posterior vena cava) before emptying into the corresponding branchial heart on the left or right sides. Each branchial heart pumps blood to the afferent branchial vein, through the gills, into the afferent branchial vein, and back to the systemic heart.  

How do cephalopods solve the problem of getting sperm from the male to the genital pore of the female, hidden inside the mantle cavity?  (p. 70)

The streamlining and modifications of the cephalopod body plan left the genital opening of the male and female deep inside the mantle cavity and the males with a problem of getting the sperm to the female. The fourth arm in squids and the third in octopods is the hectocotylus and is modified for sperm transfer. During mating the male reaches inside its mantle cavity and picks up spermatophores, packages of sperm, which he places inside the mantle cavity of the female near the opening to the oviductal gland. The spermatophores open, and sperm is transferred to the seminal receptacle of the female where it is stored until it’s time to fertilize the eggs.

Protozoa || Porifera || Cnidaria || Platyhelminthes || Nematoda || Annelida || Mollusca || Arthropoda
Echinodermata || Chordate Origins || Jawed Fishes || Amphibia || Mammalia