Q.Differentiate between peristomium and prostomium in earthworm.

The earthworm (Pheretima posthuma) has a long, cylindrical, segmented (metameric) body without a distinct head, bearing a reproductive clitellum, genital pores, and chaetae used for burrowing and movement.

Morphological features of the earthworm:

1. Shape and segmentation: The body is long, cylindrical and bilaterally symmetrical, divided into about 100–120 ring-like segments (metameres) — metameric segmentation.
2. No distinct head: There is no clearly differentiated head; the mouth lies in the first segment (peristomium) and is overhung by a small fleshy lobe, the prostomium, used to sense food and help in burrowing.
3. Clitellum: Segments 14, 15 and 16 are swollen, glandular and darker, forming a collar-like clitellum ('saddle') that secretes mucus for cocoon formation during reproduction.
4. Genital openings:
   • A single male genital pore lies mid-ventrally on segment 18.
   • A single female genital pore lies mid-ventrally on segment 14.
   • Four pairs of spermathecal apertures lie on the ventro-lateral sides in the grooves between segments 5/6, 6/7, 7/8 and 8/9.
5. Dorsal pores: Minute pores in the mid-dorsal line from about the 12th segment onward release coelomic fluid that keeps the body surface moist. …