Invertebrate Zoology: Nemerteans and Molluscs

Nemerteans

• Rhynchocoel: A fluid-filled cavity unique to this group, often containing a proboscis used for prey capture. The proboscis may have a stilet
• Prey capture: Nemerteans use their proboscis, often extending it to stab or encircle prey. Some secrete sticky substances for capture.
• Distinguishing Nemerteans from Platyhelminthes: Nemerteans have a more three-dimensional body shape, varying eye spot counts, and closed circulatory systems (though a hemal system is more accurate).

Molluscs

• Radula: A rasping tongue-like structure in most mollusks (except bivalves). It has a cartilaginous support and rows of tiny teeth constantly produced.

• Shell layers:

  • Periostracum: The outermost, rough, and thin but tough proteinaceous layer.
  • Prismatic layer: The middle layer, hard due to calcium carbonate, and relatively smooth.
  • Nacreous layer: Innermost layer (mother-of-pearl), composed of calcium carbonate and proteins

• Torsion: A 180-degree rotation of the visceral mass during development, placing the mantle cavity and gills above the head. This has advantages (e.g., protection of the foot and gills) and disadvantages (e.g., digestive and respiratory tract).

• Feeding in Bivalves: Water is drawn into the mantle cavity via the incurrent siphon. Cilia move food particles to the labial palps, and then to the mouth.

• Bivalve meaning: This refers to the two-part shell characteristic of this phylum.

Cephalopods

• Nautilus buoyancy: Gas exchange, adjusting gas and fluid content in interconnected chambers of the shell.
• Cephalopod shells: Variable among different cephalopod groups. Octopuses typically lack an external shell. Squid have a pen. Cuttlefish have a cuttlebone.
• Cephalopod feeding: Use a chitinous beak and venom to capture prey (many have powerful bite).
• Cephalopod circulatory system: A closed system with separate branchial and systemic hearts pumping blood through the body.
• Cephalopod subclasses: Nautiloidea, Coleoidea (Ammonoidea - extinct; Sepia, Teuthidia, Octopoda).

Annelids

• Metamerism: Segmentation, repeating body units, allowing for greater specialization and regeneration capability.
• Polychaetes: Paraphyletic group, meaning it does not include all the descendants of their most recent common ancestor.
• Polychaete head parts: Prostomium (pre-oral), peristomium (first body segment), trunk segments, pygidium (posterior).
• Atoke/Epitoke: Atokes are the non-reproductive part of the body; epitokes are the specialized reproductive part, often separated and swimming.
• Siboglinids: Deep-sea tube worms lacking a digestive tract, utilizing symbiotic bacteria for nutrition.
• Clitellum: A thickened band of segments in some annelids, involved in reproduction.

Other Phyla

• Trochophore larvae: A planktonic larval stage in certain mollusks and annelids, characterized by bands of cilia.

• Echiurans: Also known as spoon worms. They have an extensible proboscis used to capture food in U-shaped burrows.

• Sipunculans: Also known as peanut worms. They are benthic organisms with a well-defined body composed of two major body parts; the introvert and the trunk. They feed in U shaped burrows.

• Tardigrades: Microscopic animals that can survive extreme conditions (cryptobiosis). They're also known as water bears.

• Tardigrade feeding: They usually feed on fluids, but some will chew or scrape food.

• Cycliophorans: Small marine organisms, found on crustacean appendages and displaying both asexual and sexual reproduction.

• Lophophores: Ciliated feeding structures in certain phyla.

• Brachiopods: Bivalve-like, but with the shell parts situated dorsally and ventrally, not laterally. This is a key difference between brachiopods and bivalve mollusks.

Explore the unique characteristics of Nemerteans and Molluscs in this quiz. Learn about the rhynchocoel, proboscis, radula, and shell layers, distinguishing features, and their adaptations. Perfect for students studying invertebrate zoology.