Experiment 04Annelid AnatomyHistological T.S.
Earthworm (Pheretima posthuma) Transverse Section through Typhlosolar Region & Chitinous Setae Mount
Lab Duration: 3 Hours • Post-Clitellar Intestine • KOH Maceration for Setae
🎯 Aim: To examine stained transverse histological sections (T.S.) of the earthworm (*Pheretima posthuma*) passing through the post-clitellar typhlosolar intestinal region, identify body wall musculature, coelomic cavities, blood vessels, ventral nerve cord, and the internal typhlosolar fold, and prepare a permanent whole mount of isolated chitinous setae.
🔬 Principle & Annelid Coelomic Architecture
The common Indian earthworm (*Pheretima posthuma*, Phylum Annelida, Class Oligochaeta) exemplifies the true eucoelomate metamerically segmented body plan:
- Body Wall: Consists of a thin, non-cellular iridescent cuticle perforated by minute pores, a single-layered cellular epidermis containing supporting cells, gland cells (mucous and albumen), and sensory cells, an outer layer of circular muscles, and an inner layer of thick longitudinal muscles arranged in feather-like (penniform) blocks separated by connective tissue.
- Perichaetine Setae: S-shaped (sigmoid) chitinous rods ($sim 0.25,text{mm}$ long) embedded in epidermal setal sacs in a continuous ring around the middle of every segment (except segment 1, the clitellum, and the anal segment). Each seta possesses a nodulus (swollen middle), a root, and a curved neck. Actuated by protractor and retractor muscles to anchor segments into burrows during hydrostatic peristalsis.
- Schizocoelomic Cavity: A fluid-filled true coelom partitioned internally by transverse intersegmental septa, functioning as a hydrostatic skeleton.
- Intestinal Typhlosole (Segment 26 to pre-anal): The dorsal wall of the intestine projects downward into the intestinal lumen as a prominent inverted U-shaped or T-shaped longitudinal fold termed the Typhlosole. The typhlosole increases the internal absorptive and secretory surface area of the simple tubular intestine by $> 250%$, an evolutionary innovation that compensates for the lack of intestinal coiling.
- Chloragogen Tissue: The outer peritoneal lining of the intestine is modified into thick, yellow, club-shaped chloragogen cells. Functionally analogous to the vertebrate liver, chloragogen cells store glycogen and lipids, synthesize urea and uric acid from deaminated amino acids, and detach as eleocytes into the coelomic fluid.
- Ventral Nerve Cord & Giant Axons: Situated in the mid-ventral coelom, containing two fused lateral nerve cords and a dorsal neural sheath housing 3 large medial and lateral giant axons that mediate ultra-fast escape withdrawal reflexes.
Figure 4.1: Transverse section through the post-clitellar intestinal region of Pheretima posthuma displaying the inverted typhlosole fold, chloragogen tissue, and ventral nerve cord.
📋 Step-by-Step Laboratory Protocol
- Setae Isolation via KOH Maceration: Cut a $1,text{cm}$ segment from the post-clitellar region of an earthworm. Place the body wall piece in a test tube containing $5,text{mL}$ of $10%,text{KOH}$ solution. Boil gently over a bunsen burner for 3 to 5 minutes until all soft tissues and muscles are completely dissolved, leaving only chitinous setae.
- Setae Mounting: Allow the tube to stand until setae settle as a white sediment. Decant KOH, wash with distilled water twice, and transfer a drop of sediment to a slide. Stain with $1%$ eosin or safranin for 1 minute, dehydrate with absolute ethanol, clear in xylene, and mount in DPX. Observe under $400times$: identify the curved nodulus, root, and apex of the sigmoid setae.
- Typhlosolar T.S. Slide Examination: Place a permanent slide of earthworm intestinal T.S. under low power ($100times$). Identify the outermost body wall, the coelomic cavity, the central intestine, and the prominent dorsal typhlosole hanging into the lumen.
- Dorsal-Ventral Orientation: Orient the section: the dorsal surface is identified by the presence of the large thin-walled Dorsal Blood Vessel and the downward-projecting typhlosole. The ventral surface is identified by the solid double Ventral Nerve Cord, subneural vessel, and ventral blood vessel.
- High-Power Cellular Inspection: Under $400times$, inspect:
– Epidermis and circular/longitudinal muscle fibers.
– S-shaped setae seated in their setal sacs in the body wall.
– Chloragogen cells covering the intestine.
– Microvillar columnar intestinal lining epithelium.
– Giant fibers within the ventral nerve cord. Sketch and label all layers.
