Cnidaria and Protostomes: Lophotrochozoans and Nematodes

This lecture delves deeper into specific animal groups, following a general introduction in the previous session.
Warning: Some content may be graphic (e.g., relating to parasites), so snacking is not recommended during this lecture.

The phylogenetic tree serves as a touchstone for organizing animal groups and their defining characteristics.
Today's focus: Cnidaria (closer to the basal portion of the tree) and a large group called Protostomes.
Within Protostomes, we will cover Lophotrochozoans (Platyhelminthes, Annelida, Mollusca) and touch upon Ecdysozoans (specifically Nematoda). Rotifera will not be discussed.

Common Name: Nettle animals.
Characteristic Examples: Corals and jellyfish.
Diversity: Includes sea anemones, various jellyfish types (swimming, stalked, boxed), hydros, and siphonophores.

Two Basic Body Forms:
- Polyp: Sedentary (sessile, non-moving) form. E.g., Corals, sea anemones.
- Medusa: Mobile form. E.g., Jellyfish.
- Many cnidarians exhibit both forms in their life cycle, though one typically predominates (e.g., corals as polyps).
Tissue Layers: Cnidarians are diploblastic, meaning they possess two main germ layers: the ectoderm and the endoderm.
- Mesoglea: A gelatinous, non-cellular layer found between these two tissue layers, providing structural support.
Symmetry: Exhibit radial symmetry, meaning their body parts are arranged around a central axis.

Nematocyst: A primary adaptation for feeding and defense.
- Description: An organelle functioning like a mini-harpoon, unique to cnidarians.
- Mechanism: A trigger mechanism (e.g., foreign body, chemical profile) causes the harpoon to be released rapidly.
- Penetration: Harpoons are extremely sharp, capable of piercing tough exteriors like crab shells.
- Toxin Release: Releases toxins upon penetration; some species (like box jellyfish) have toxins fatal to humans, causing extreme pain or death.
- Uses: Capturing prey, defending against predators, and defending territory.
- Video Example: Anemones using nematocysts as a primary method for stunning and capturing small fish or crustaceans.

Key Characteristic: During embryonic development, the mouth typically develops from the first opening (blastopore), hence the name "protostome" (\text{proto} = \text{first}, \text{stoma} = \text{mouth}).
Major Clades: Divided into two main groups: Lophotrochozoa and Ecdysozoa, based on distinct developmental and morphological features.

Defining Features: This diverse clade is characterized either by the presence of a lophophore (a crown of ciliated tentacles for feeding) or a trochophore larva (a distinct larval stage with a band of cilia).
Groups Covered:
- Platyhelminthes (Flatworms):
- Examples: Tapeworms, flukes, and free-living planarians.
- Key Characteristics: Acoelomate (lacking a true body cavity), dorsoventrally flattened body, often parasitic (e.g., tapeworms in digestive tracts) but also includes free-living forms.
- Annelida (Segmented Worms):
- Examples: Earthworms, leeches, and marine polychaetes.
- Key Characteristics: Characterized by a segmented body, a true coelom (body cavity), and specialized organ systems for circulation, digestion, and excretion, which are often repeated in each segment.
- Mollusca (Mollusks):
- Examples: Snails, clams, oysters, squids, and octopuses.
- Key Characteristics: Soft-bodied invertebrates, typically protected by a calcareous shell (though some are shell-less or have internal shells), possess a muscular foot (for locomotion), a visceral mass (containing organs), and a mantle (a fold of tissue that secretes the shell).

Defining Feature: Members of this clade grow by ecdysis, the process of molting or shedding their tough external cuticle or exoskeleton as they grow.
Groups Covered:
- Nematoda (Roundworms):
- Examples: Various soil nematodes, parasitic worms like Ascaris, hookworms, and pinworms.
- **Key Characteristics