Ch. 11 Flashcards
(27 cards)
Key Features of Phylum Nemertea:
Defining Characteristic:
• Muscular eversible proboscis housed in a fluid-filled, schizocoelous cavity called the rhynchocoel. This proboscis is a unique feature of nemerteans, used for capturing prey.
Key Features of Phylum Nemertea:
Body Structure:
• Elongated, unsegmented, soft-bodied worms, most of which are marine, though some freshwater and terrestrial species exist.
• Nemerteans are often flattened dorsoventrally and have circular, longitudinal, and dorsoventral muscles that assist with movement.
Key Features of Phylum Nemertea:
Habitat:
• The majority of the 1,300 described species are marine and found in shallow-water environments, often crawling on solid substrates, burrowing in sediment, or lurking under rocks and algae.
• A few species are found in freshwater and terrestrial environments, most likely having evolved from marine ancestors.
• Some nemerteans live commensally with other invertebrates, particularly arthropods and mollusks, while a few species are parasitic.
Key Features of Phylum Nemertea:
Movement:
• Small species move using ciliary beating, while larger nemerteans generate muscle contractions for movement, particularly when navigating hard substrates or soft
sediments.
• Some species can swim by creating muscle contractions.
Key Features of Phylum Nemertea:
Nervous System:
• The nervous system is similar to that of turbellarian flatworms with cerebral
ganglia forming a ring anteriorly, giving rise to longitudinal nerves with lateral connectives.
• Chemoreceptors, mechanoreceptors, pigmented photoreceptors (eyes), and balance organs (statocysts) are present in most species. The number of eyes varies, ranging from 0 to over 80 per individual.
Key Features of Phylum Nemertea:
Excretory and Digestive Systems:
• The excretory system typically consists of protonephridia, like in flatworms.
• Protonephridia: Nemerteans have protonephridia for excretion, with each unit consisting of collecting tubules leading to a bladder. The bladder expels fluid into the cloaca, which also handles digestive and reproductive system products.
• Digestion is primarily intracellular, similar to that of Platyhelminthes (flatworms).
Key Features of Phylum Nemertea:
Larval Development:
• The larval stages of nemerteans share key characteristics with those of flatworms, such as having similar developmental patterns and features.
• Most species have microscopic larvae resembling the juvenile worm.
This larva features a functional rhynchocoel, proboscis, and digestive tract.
• Heteronemertea species have pilidium larvae, which are ciliated, long-lived, and feed. Only part of the pilidium develops into the juvenile worm, and the remaining larval body either starves to death or is eaten by the juvenile.
• Trochophore Larvae: Studies suggest that some primitive nemerteans have a trochophore larva, suggesting that the pilidium might have evolved from this type of larvae.
Key Features of Phylum Nemertea:
Size and Structure:
• Most nemerteans are several to 20 cm long, though some species can reach lengths of up to 30 meters. This makes the term “ribbon worm” especially fitting for these elongated animals.
• They have a flat body that is permeable, allowing for gas exchange through diffusion across the body surface.
Key Features of Phylum Nemertea:
Classification:
• Despite their superficial similarities to flatworms, nemerteans differ significantly in their systems for gas exchange, digestion, and food capture, placing them in a separate phylum.
• Molecular and developmental studies suggest that nemerteans are more closely related to coelomates (like annelids and mollusks) than to flatworms. They belong to the Lophotrochozoa group, which includes other coelomate animals.
• Two Major Classes: Nemerteans are divided into two classes based on features of the proboscis and mouth placement:
• Anopla: Members of this class lack stylets on their proboscis, and the mouth is located posterior to the brain.
• Enopla: These species possess an armed proboscis (with a stylet) and the mouth is anterior to the brain. All armed species belong to the Hoplonemertea order.
Key Features of Phylum Nemertea:
Circulatory System:
• Unlike flatworms, nemerteans have a true circulatory system with contractile blood vessels. The blood circulates throughout the body, though the flow is erratic due to the lack of a heart and one-way valves.
• Some species have hemoglobin, though most lack oxygen-carrying pigments in their blood.
Key Features of Phylum Nemertea:
Digestive System:
• Nemerteans possess a one-way digestive tract with a mouth at the front and a separate anus at the posterior end, unlike flatworms, which lack an anus. This unidirectional flow of food allows for more efficient digestion, with digestive enzymes secreted in sequence as food moves through the gut.
• Nutrient absorption occurs in the posterior part of the gut, and the circulatory system helps distribute absorbed nutrients throughout the body.
Key Features of Phylum Nemertea:
Proboscis:
• A unique feature of nemerteans is the presence of a muscular, hollow proboscis used for capturing prey. This proboscis is not homologous with the flatworm’s pharynx, as it is a distinct structure from the digestive tract.
• The proboscis is housed in a separate, fluid-filled cavity called the rhynchocoel.
This cavity forms as a split in the mesodermal tissue, and although it has similarities to the coelomic cavities of
other coelomates, its exact relationship to those cavities is still uncertain.
• The rhynchocoel is a defining feature of nemerteans, leading to the alternative name for the phylum, Rhynchocoela (meaning “snout cavity”).
Key Features of Phylum Nemertea:
Feeding and Diet:
• Nemerteans are carnivorous, often preying on small annelids, crustaceans, and specific prey species. Their proboscis helps them capture and immobilize prey before ingestion.
Key Features of Nemertean Proboscis
Functioning:
Proboscis Extension:
• The proboscis of nemerteans can be shot out with explosive force, a remarkable feature not shared by flatworms.
• This rapid extension is powered by the fluid-filled rhynchocoel, which surrounds the proboscis and plays a crucial role in its operation.
Key Features of Nemertean Proboscis
Functioning:
Mechanism of Extension:
• The rhynchocoel acts like a constant-volume cavity, and when the surrounding muscles contract, the pressure inside it increases.
• Hydrostatic pressure builds up because the fluid inside the rhynchocoel is essentially incompressible, creating a high-pressure environment that forces the proboscis out quickly once the sphincter muscles at the anterior end
relax.
Key Features of Nemertean Proboscis
Functioning:
Analogy to a Rubber Glove:
• To understand the mechanism, imagine a rubber glove with a finger pushed inside.
When the glove is squeezed (simulating contraction), the finger (proboscis) is forced out due to the increased internal pressure.
If a hole is poked in the glove, the pressure would not increase in the same way, demonstrating the importance of maintaining a constant-volume cavity (like the rhynchocoel) for this process.
Key Features of Nemertean Proboscis
Functioning:
Proboscis Opening:
• In most species, the proboscis is everted through a proboscis pore distinct from the mouth. However, in some species, the proboscis and digestive tract share a common opening due to evolutionary adaptations like the loss of the mouth or proboscis pore.
Key Features of Nemertean Proboscis
Functioning:
Feeding Mechanism:
• Nemerteans are carnivores, and many species use their proboscis for prey capture:
• Some species have a piercing stylet (a sharp, barbed structure) on their proboscis that injects a paralytic toxin into the prey after it is pierced.
• The proboscis can also coil around the prey, holding it tightly and then stab it repeatedly with the stylet.
• For species without a stylet, the proboscis wraps around the prey and retracts back into the rhynchocoel, transferring the captured prey to the mouth for digestion.
Key Features of Nemertean Proboscis
Functioning:
Proboscis Retraction:
• After the prey is captured, the proboscis retractor muscle (running from the proboscis tip to the inner wall of the rhynchocoel) contracts, pulling the proboscis back inside the rhynchocoel.
• In species with stylets, the prey is paralyzed by the toxin, and the proboscis is fully retracted before the nemertean moves on to consume the immobilized prey.
• In species without a stylet, the prey is ingested whole, or peristaltic waves in the nemertean’s foregut extract juices from the prey, which are then pumped into the digestive tract.
Key Features of Burrowing and Locomotion in Nemerteans:
Burrowing Mechanism:
• Body Wall Musculature and Proboscis:
Many nemerteans burrow into soft substrates, such as sediment. Some species use their body wall muscles to create internal pressure, while others use their proboscis for burrowing.
Key Features of Burrowing and Locomotion in Nemerteans:
Proboscis Extension:
• Internal Pressure: The contraction of the body wall musculature elevates the internal pressure of the rhynchocoel, which houses the proboscis.
• Eversion of the Proboscis: The proboscis is everted (turned inside out)
into the sediment. At the tip of the proboscis, relaxation of the circular muscles causes the tip to widen and anchor itself in the substrate.
Key Features of Burrowing and Locomotion in Nemerteans:
Anchoring and Movement:
• Once the tip of the proboscis is anchored in the substrate, the longitudinal muscles of the body wall and proboscis retractor muscles contract. This pulls the body downward, into the sediment.
• Fluid Pressurization: The contraction of the body wall muscles forces additional fluid into the proboscis, increasing its pressure. This causes further dilation at the tip of the proboscis, ensuring better anchorage in the sediment.
Key Features of Burrowing and Locomotion in Nemerteans:
Hydrostatic Skeleton:
• The rhynchocoel acts like a hydrostatic skeleton, a constant-volume, fluid-filled cavity that plays a crucial role in locomotion and burrowing. This concept is shared with other animals that have
evolved similar structures for support and movement.
• The hydrostatic skeleton is key to nemerteans’ ability to burrow effectively.
By manipulating the internal pressure of the rhynchocoel, they gain mechanical advantages that allow them to perform tasks like burrowing and food capture.
Key Features of Burrowing and Locomotion in Nemerteans:
Evolution of Fluid-filled Cavities:
• The hydrostatic skeleton in nemerteans is part of a broader evolutionary trend where fluid-filled cavities (like the rhynchocoel) have evolved in various ways across
different phyla. These cavities may form through enterocoely, schizocoely, blastocoel persistence, or, as in nemerteans, schizocoelous formation of the rhynchocoel.
• The evolution of these cavities has provided mechanical advantages, contributing to new ways of locomotion and food capture, enhancing the survival and adaptability of species that possess them.
