Platyhelminthes, Nemertea, Nematoda, and Tardigrada

Phylum Platyhelminthes: The Flatworms

Overview

• Belongs to Superphylum Lophotrochozoa.

Characteristics

• Feeding:

  • Have either a lophophore feeding apparatus or cilia around the mouth.

  • Exhibit trochophore larvae with two bands of cilia.

• Body Plan:

  • Bilateral symmetry.

  • Triploblastic, acoelomate.

  • Epidermis: One layer of epithelial cells from ectoderm.

  • Mesoderm: Differentiates into longitudinal and circular muscle layers, and mesenchymal cells that are excretory.

  • Endoderm: Forms the digestive system.

• Development: Proto-stome pattern of development.

Nervous System

• Cephalization: Simple.

• Nerve tissue and sensory organs concentrated in a defined head region.

• Presence of eye spots (ocelli) to sense light intensity.

Feeding Habits

• Free-living species act as predators or scavengers.

• Parasitic species absorb nutrients from a host's gut.

• Digestive System: Incomplete:

  • Mouth also serves as anus.

  • Contains branching gastrovascular cavity (GVC).

  • Food is digested in GVC and absorbed by cells lining the cavity; transport occurs through diffusion.

Excretory and Circulatory Systems

• Excretory System: Network of tubules connected to flame cells; expels waste through pores.

• Nervous System: Two lateral nerve cords connected by transverse nerves; two cerebral ganglia surround the ocelli.

• Respiratory System: Absent; O2 and CO2 exchange occurs through the epidermis.

• Circulatory System: Absent; substances move by diffusion.

Reproduction

• Most species are monoecious and highly regenerative.

• Asexual Reproduction: Through fragmentation.

• Sexual Reproduction: Involves internal fertilization through "penis fencing".

  • Sperm released into the body of a stabbed individual; fertilization occurs in oviducts.

  • Fertilized eggs released; larvae develop through multiple stages.

Phylum Nemertea: The Ribbon Worms

Overview

• Also in Superphylum Lophotrochozoa.

Characteristics

• Body Plan:

  • Bilateral symmetry, triploblastic, acoelomate.

  • Epidermis: One layer of epithelial cells from ectoderm.

  • Mesoderm: Differentiates into muscle tissue.

  • Endoderm: Forms the digestive system.

  • Proto-stome development with defining characteristic: eversible proboscis located in the rhynchocoel (used to harpoon prey).

  • Size varies widely.

Nervous System

• Cephalization: Simple; cerebral ganglia form a rudimentary brain.

• Presence of sensory papillae and ocelli for touch and light detection.

Feeding Habits

• Mostly carnivorous with a complete digestive system.

  • Food travels from mouth to foregut to intestines.

Circulatory and Excretory Systems

• Circulatory System: Closed, formed by a pair of connected blood vessels with muscle contractions to maintain blood flow.

• Excretory System: Composed of protonephridia to remove metabolic waste and maintain osmoregulation.

• Nervous System: Comprised of four ganglia around the head.

• Respiratory System: Absent; respiratory gases diffuse through the epidermis.

Reproduction

• Highly regenerative, usually dioecious with gonads along the digestive tract.

• Asexual reproduction via fragmentation; external fertilization with trochophore larvae developing through several stages.

Phylum Nematoda: The Round Worms

Overview

• In Superphylum Ecdysozoa.

Characteristics

• Features a tough, flexible cuticle made of collagen and chitin; undergoes molting to grow.

• Body Plan: Bilateral, triploblastic, pseudocoelomate with cylindrical tubular morphology.

Nervous System

• Cephalization: Has a simple "brain" with sensory papillae and bristles for touch.

Feeding Habits

• Complete digestive system with mouth and anus; food travels unidirectionally through mouth, pharynx, and intestines.

Circulatory and Excretory Systems

• No circulatory system; nutrients and gases diffuse throughout the body.

• Excretory System: Simplified; metabolic waste is ammonia removed via diffusion.

Reproduction

• Varies by species; can be monoecious, dioecious, or parthenogenic with internal fertilization in dioecious species.

• Eggs develop for about two weeks before larvae molt four times.

Phylum Tardigrada: The Water Bears

Overview

• Also part of Superphylum Ecdysozoa.

Characteristics

• Tardigrade cuticle made of chitin divided into plates, bilaterally symmetrical, triploblastic, eucoelomate with hemocoel.

• Unique capability for cryptobiosis; present in diverse habitats.

Morphology

• Cylindrical body with four pairs of legs (first three for locomotion, last for clinging).

• Simple cephalization; has eye spots for light detection and sensory papillae for touch.

Feeding Habits

• Possesses a complete digestive system; food travels from mouth to anus.

Circulatory and Excretory Systems

• No circulatory system; bodily fluids are contained within hemocoel.

• Simple excretory system via malpighian tubules transports metabolic waste to the digestive system.

Reproduction

• Dioecious with sexual reproduction and external fertilization.

• Eggs attached to the molted cuticle of the parent; young undergo multiple molts during growth into adulthood.