Earthworm : Morphology and Anatomy MCQ - Practice Questions with Answers

Morphology of Earthworm - Prostomium, Peristomium and Clitellum

• The earthworm is a reddish brown in colour due to presence of porphyrin pigment
• This pigment protects the body of earthworms from UV Rays
• It belongs to the Phylum Annelida of Kingdom Animalia
• Therefore, it shows true segmentation or metamerism wherein, the segments appear on the external surface as annuli and internally as septation
• Each of these segments has a repetition of organs to a certain extent
• Earthworms inhabit the upper layer of the moist soil
• Pheretima and Lumbricus are the common Indian earthworms
• They have a long cylindrical body with 100-120 segments or metameres
• The dorsal surface of the body is marked by a dark median mid-dorsal line (dorsal blood vessel) along the longitudinal axis of the body
• The ventral surface is marked by the genital pores
• The anterior end consists of mouth and prostomium
• The prostomium is a lobe that acts as the covering of the mouth. It also acts as a wedge to open up the soil when the earthworm crawls
• The first segment is called the peristomium. It contains the mouth and buccal cavity
• In a mature earthworm, the segments 14-16 are covered with a dark band of glandular tissue called Clitellum
• Therefore, the body of the earthworm can be divided into pre-clitellar, clitellar and post-clitellar region
• Clitellum can be defined as the thick and non-segmented region towards the head that secretes a viscid sac in which the eggs are deposited
• Hence, the main function of the clitellum is to store the eggs of the earthworm
• It is visible when the earthworm is sexually active
   

Morphology of Earthworm - Genital Openings and Setae

Genital Openings:

• Four pairs of spermathecal aperture are present on the ventrolateral sides of the intersegmental grooves of 5th-9th segments.
• A single female genital pore is on the mid-ventral line of the 14th segment.
• A pair of male genital pores are present on the mid-ventral line of the 18th segment.

Setae:

• In each body segment, except the first, last and the clitellum, there are rows of S-shaped setae embedded in the epidermal pits in the middle of each segment.
• Setae can be extended or retracted.
• Their principal role is in locomotion.

Anatomy of Earthworm - Body Wall

• The outermost layer of the body wall of an earthworm is thin and non-cellular layer called cuticle.
• The cuticle is followed by the epidermis. It is made up of a single layer of columnar epithelial cells in which secretory gland cells are also present.
• Below the epidermis, there are two muscular layers present. The outer one is circular and the inner one is longitudinal.
• The muscular layer is followed by innermost coelomic epithelium called the peritoneum.