Introduction to Annelids and Nematodes
Course Logistics and Scheduling Updates
Bioassay Project Update:
The Daphnia planned for the project did not arrive as scheduled. They were expected by today, but the delivery has been delayed.
If the Daphnia arrive by Tuesday, they will be used for the lab session on that day. This will require rescheduling the previously planned activity, which was the grasshopper and crayfish dissection.
If the Daphnia have not arrived by the Tuesday class, the instructor will provide a backup project within the organ systems unit.
The goal is to conduct the experiment as soon as the live Daphnia are available, seizing the window of opportunity while they are alive.
Comparative Overview of Nematodes and Annelids
Taxonomic Context:
Annelids are segmented worms whose close ancestors include mollusks and flatworms.
Like mollusks and flatworms, annelids are classified as protostomes and coelomate protostomes.
They possess a true body cavity (coelom) surrounded by mesoderm.
They share a free-living larval stage known as a trochophore.
Both groups belong to the broader phylogenetic umbrella group called Lophotrochozoa.
Key Differences:
Nematodes: Unsegmented roundworms; many are parasites; characterized as pseudocoelomates (false coelom).
Annelids: Segmented worms (e.g., earthworms, polychaetes, leeches); true coelomates; bilaterally symmetrical with complete digestive tracts.
Characteristics and Examples of Phylum Nematoda
General Features:
Body Structure: Unsegmented, bilaterally symmetrical roundworms. They have a tubular digestive tract extending from the mouth to the anus.
Mobility/Anatomy: They lacks cilia.
Habitat: Naturally found in soil and water; many live within animal hosts.
Diversity: There are over identified species. Many are microscopic or tiny soil-dwellers that decompose organic matter, such as dead fruit or plant material.
Specific Parasitic Examples:
Pinworms: The most common nematode parasite in the United States, frequently affecting children.
Hookworms: Another common parasitic variety.
Ascaris: A human intestinal roundworm (pictured in the lecture materials).
Heartworms: Nematodes that commonly infect pets like cats and dogs.
Filarial Worms: The cause of filariasis. These worms enter the body and clog the lymphatic system, resulting in severe swelling of limbs (elephantiasis). While not common in the US, this is a significant health issue in parts of Africa.
General Anatomy and Physiology of Phylum Annelida
Segmentation (Metamerism):
External appearance shows obvious segments marked by lines or rings.
Septa: Internal partitions that divide one body segment from the next, similar to a nasal septum.
While some organs (like the digestive tract) pass through these partitions, other structures are repeated in each segment.
Repeated Structures: Each segment typically contains its own pair of blood vessels, a pair of nerves, and nerve endings for local muscle tissue.
Semi-repeated Structures: Some organs appear only in specific segments. For example, five specific body segments contain the five pairs of simple hearts.
Setae (Bristles): Most annelids have external bristles called setae (or chetae) that anchor their bodies during movement.
Taxonomy and Specialized Structures of Annelid Classes
Class Polychaeta:
Etymology: "Poly" (many) and "chaeta" (bristles/setae).
Characteristics: Marine worms that possess appendages called parapodia. These parapodia extend from the body and are covered in numerous bristles.
Example: Nereis (a marine worm specimen provided in the lab).
Class Oligochaeta (Earthworms and relatives):
Etymology: "Oligo" (few), referring to the fact they have much fewer bristles than polychaetes.
Locomotion: They use two layers of muscles: circular muscles (fibers running around the body) and longitudinal muscles (fibers running the length of the body). By alternatively contracting these, they move in an "inchworm" fashion.
Setae: Located on the sides of the worm; they feel like stubble and allow the worm to anchor itself in the soil to resist predators.
Class Hirudinea (Leeches):
Characteristics: Ectoparasites with smooth bodies. Unlike other annelids, they lack setae.
Feeding Mechanisms: They use suckers to attach to prey. They secrete an anesthetic to numb the bite site and an anticoagulant to prevent blood from clotting during the meal.
Medical Applications: Historically used for "bloodletting" to remove "bad blood." Modern medicine uses leeches after surgery to extract fluid from injuries, enhance healing, and prevent edema (fluid buildup).
Detailed Anatomy and Reproduction of Earthworms
External Anatomy:
Prostomium/Peristomium: The prostomium is the tip; the mouth is surrounded by the peristomium (technically the second segment).
Seminal Receptacles: Located between segments .
Gonopores: Segment contains both female and male gonopores (earthworms are hermaphrodites).
Clitellum: A swollen area consisting of fused segments located about one-third of the way back from the anterior (head) end.
Reproductive Process:
Two worms attach at the clitellum to exchange sperm.
Sperm mature in the seminal vesicles and exit through the male gonopore to the adjacent worm.
Sperm is stored temporarily in the partner's seminal receptacles.
Days later, the clitellum secretes a mucus band.
The band slides down, collecting eggs from the female gonopore and stored sperm from the receptacles.
Fertilization occurs within the mucus band, which then slides off to become a cocoon.
Internal Systems:
Digestive Tract: Mouth Pharynx (muscular band to move food) Esophagus Crop (temporary storage) Gizzard (physical breakdown/grinding) Intestine (nutrient absorption).
Typhlosole: An internal fold in the intestinal wall that increases the surface area for enzymes to contact and break down food particles.
Circulatory System: A closed system featuring five pairs of hearts (aortic arches) that clasp the esophagus, a dorsal blood vessel (visible as a dark line on the top), and a ventral blood vessel.
Nervous System: A rudimentary brain near the mouth branching into ventral and dorsal nerve cords.
Excretory System: Nephridia (primitive kidneys) found in segments. Waste enters through the nephrostome and exits through nephridiopores (excretory pores).
Lab Procedures and Dissection Techniques
Identification: Use the position of the clitellum and the dark dorsal blood vessel to identify the anterior end and the dorsal side.
Incision:
Always cut on the dorsal (top) side.
Make a shallow cut along the dark blood vessel line to avoid damaging internal organs.
"Cut a little, pry apart, and pin." This keeps the specimen stretched and accessible.
Pinning Technique:
Pins should be placed at an angle (almost sideways), not vertical. This secures the pin in the thin rubber of the dissection tray and keeps the pins from obstructing the view under the microscope.
Requirements:
Perform a sketch and label of the external anatomy.
Perform a sketch and label of the internal anatomy OR pin the structures and present them to the instructor for verification.
Specific labeling list: Prostomium, mouth, seminal receptacles, male/female gonopores, clitellum, pharynx, esophagus, crop, gizzard, intestine, hearts, nephridia.
Questions & Discussion
Student Question: "Earthworm guts on my face today. Should I wear goggles? Do you think my glasses are okay?"
Instructor Response: "I think your glasses will be okay as long as you keep your glasses on."
Student Interaction during Dissection: A student noted that their specimen was "falling apart" and appeared "segmented." The instructor remarked that the specimen might be old but confirmed that the segmentation is a natural feature of the annelid.
Identification Inquiry: Students discussed the location of the prostomium and peristomium. The instructor provided a model/poster for reference to help distinguish the tiny openings and segments at the anterior end.
Microscope Usage: Students were instructed to use dissecting microscopes to identify small structures like the setae and specific pores (gonopores and nephridiopores) which appear as "tiny little dots" on the surface.