Question

Figure 15.24 Which of the following statements about the anatomy of a mollusk is false? a. Mollusks have a radula for scraping food. b. Mollusks have ventral nerve cords. c. The tissue beneath the shell is called the mantle. d. The mantle cavity contains hemolymph.

Short answer

The false statement about the anatomy of a mollusk is 'The mantle cavity contains hemolymph.'

Step-by-step solution

Analyze statement a

Mollusks have a radula for scraping food. This is true. The radula is a characteristic feature of mollusks. It's a scraping organ used to bring food into the mouth.

Analyze statement b

Mollusks have ventral nerve cords. This is also true. Mollusks have two main nerve cords, one of which is the ventral cord. The nervous system of mollusks runs along the belly side, hence it's called the 'ventral nerve cord'.

Analyze statement c

The tissue beneath the shell is called the mantle. This is correct. The mantle is a significant part of a mollusk's anatomy. It's a protective layer that, in some species, can secrete a shell.

Analyze statement d

The mantle cavity contains hemolymph. This is false. The mantle cavity of a mollusk usually contains air, water, or sometimes even a lung, but it does not contain hemolymph. Hemolymph is the equivalent of blood in invertebrates like mollusks, but it is found within vessels and sinuses in the circulatory system, and not within the mantle cavity.

Key concepts

Radula in Mollusks

The radula is an adaptable anatomical feature exclusive to mollusks, often likened to a microscopic conveyor belt lined with teeth. Its primary function is to acquire nutrients by scraping or cutting food before ingestion. This organ is incredibly diverse and reflects the varied diets of mollusks.

Composed of chitin, the same material found in arthropod exoskeletons, the radula's tiny teeth engage in a continual cycle of wear and renewal, ensuring the mollusk’s feeding efficiency. Snails, for instance, use their radulae to graze on algae, while predatory species like the cone snail have evolved a modified radula to harpoon and envenomate prey. Although most mollusks possess a radula, bivalves such as clams and oysters are an exception, having lost this structure during their evolution towards filter feeding.

Ventral Nerve Cords

The ventral nerve cords in mollusks play an essential role in coordinating movements and sensory processes. These cords run along the underside or 'ventral' side of the organism and form part of a nervous system that can range from simple to complex, depending on the species.

In many mollusks, the ventral nerve cords are connected to ganglia, which are clusters of nerve cells that act as local control centers processing sensory information, managing reflexes, and conveying motor commands to the creature’s muscles. The existence of these cords highlights the bilateral symmetry typical of mollusks and underscores the evolutionary advances that allow these animals to interact with their environment in intricate ways, including locomotion, predation, and responding to sensory cues.

Mantle in Mollusks

The mantle is the tissue in mollusks that lies beneath the shell and in many ways, serves as the interface between the mollusk and its external environment. Acting as a multifunctional organ, the mantle is involved in respiration, excretion, shell formation, and, in some species, even sensory perception.

The mantle can secrete calcium carbonate, giving rise to the hard, protective shell that many associate with these creatures. This ability to create and fortify a shell is critical for mollusks, offering them protection from predators and harsh environmental conditions. Beyond shell production, the mantle also contributes to forming the structure of the siphon in species like squid and octopuses, allowing them to draw in water for respiration and propulsion.

Mantle Cavity Function

The mantle cavity is a central feature in the biology of mollusks, serving various functions that are vital for survival. Positioned between the mantle and the body proper, this cavity can function as a respiratory chamber in aquatic mollusks where gills are located, facilitating gas exchange with the surrounding water. Additionally, the mantle cavity houses the excretory and reproductive organs in many mollusks.

In terrestrial mollusks, such as land snails and slugs, the mantle cavity often evolves into a lung, with a richly vascularized surface area for breathing air. Contrary to the misconception that the mantle cavity contains hemolymph, it is typically filled with either air or water, and its composition and functionality can dramatically affect the mollusk's ability to breathe, expel waste, and reproduce, underscoring its significance in molluscan physiology.