What Are Setae in Earthworm?
Setae (singular: seta), also called chaetae, are tiny curved bristles present on each body segment of Pheretima posthuma. They are a defining feature studied under earthworm external morphology. Setae are composed of a horny, nitrogenous organic substance called chitin.
Setae (Chaetae)
- It is the Locomotory Bristles of Earthworm.
- Location and Arrangement of Setae.
- Each body segment carries 80 to 120 setae, arranged in a complete ring around the middle of the segment.
- Setae are absent on three specific regions: the peristomium, pygidium, and clitellum.
- Each seta is embedded within a specialised pit called a setigerous pit (or setal sac)
- Each seta is produced by a single formative cell located at the base of the sac.
Structure of a Single Seta
A seta has three distinct parts arranged from top to bottom:
- Neck— the upper narrow portion
- Nodulus— the swollen middle region; the most recognisable feature of seta structure
- Base— the lower broad portion anchored inside the setal sac
Overall, a seta is shaped like an elongated 'S' with the swollen nodulus at its midpoint. Setae have a faint yellow colour, and approximately one-third of their length projects above the skin surface when the segment is contracted.
Functions of Setae in Earthworm
- Locomotion — grip the soil surface, enabling forward movement
- Anchoring — prevent slipping during movement
- Burrowing — assist the worm in penetrating and moving through soil
2. Blood Glands in Earthworm — Haematopoietic Organs
What Are Blood Glands in Earthworm?
Blood glands are small, red-coloured, follicular bodies found in the body of Pheretima posthuma. In the study of Pheretima posthuma internal morphology, blood glands are classified as haematopoietic organs. They are closely connected with the pharyngeal (salivary) glands.
Location of Blood Glands in Pheretima posthuma
- They are found in segments 4, 5, and 6
- Positioned above the pharyngeal mass
- Directly connected with the pharyngeal or salivary glands
Structure of Blood Glands
- Each blood gland consists of a mass of loosely arranged cells enclosed within a capsule
- The capsule has a syncytial wall — formed by a continuous sheet of cytoplasm containing multiple nuclei rather than individual separate cells
Functions of Blood Glands in Earthworm
- Manufacture blood corpuscles which is primary haematopoietic function
- They produce haemoglobin which is the respiratory pigment of the earthworm
- Considered excretory in functions by some zoologists
Spermathecae (also called receptacula seminales) are 4 pairs of small, flask-shaped sperm storage organs that play a vital role in the reproductive system of Pheretima posthuma.
Location and Openings of Spermathecae
Spermathecae are positioned ventro-laterally in the body wall, with one pair present in each of segments 6, 7, 8, and 9. They open to the exterior through four pairs of spermathecal pores located in intersegmental grooves:
Location of Spermathecal Pores
- The four pairs of spermathecal pores are located in the intersegmental grooves of Pheretima posthuma as follows:
- The first pair of spermathecal pores is present between segments 5 and 6.
- The second pair is located between segments 6 and 7.
- The third pair occurs between segments 7 and 8.
- The fourth pair is situated between segments 8 and 9.
Structure of a Single Spermatheca
- Each spermatheca has three clearly defined parts:
Ampulla — It is broad, pear-shaped main body; the primary storage compartment
Neck— It is short, narrow region connecting the ampulla to the exterior
Diverticulum — It is narrow, elongated blind pouch (caecum) arising from the spermathecal duct; in Pheretima posthuma it serves as the principal sperm storage region
Function of Spermathecae
- In Pheretima posthuma, sperms are specifically stored within the diverticula.
- In other earthworm species, sperms are stored in the ampullae.
4. Path of Mature Ova— From Ovary to Cocoon
1. Shed from ovaries — Mature ova are released from the ovaries into the coelom
2. Collected by oviducal funnels— Oviducal funnels gather and direct the ova into the ducts
3. Travel along oviducts— Ova pass through the oviducal tubes towards the exterior
4. Exit via female genital aperture — Ova pass out through the female genital pore
5. Deposited inside the cocoon— Ova are enclosed within the cocoon for fertilisation and development
5. Nephridia in Earthworm
What Are Nephridia in Earthworm?
Nephridia are the excretory organs of earthworms, functionally comparable to the kidneys of vertebrates. In the study of Pheretima posthuma excretory system, nephridia are classified into three types based on location. They remove metabolic waste products from the coelomic fluid and blood.
Three Types of Nephridia in Pheretima posthuma
- Pharyngeal nephridia are located in the 4th, 5th, and 6th segments of the earthworm. They are associated with the pharyngeal region and help in excretion.
- Integumentary nephridia are distributed throughout the body wall from the 7th segment onwards. These nephridia are numerous and play an important role in removing metabolic wastes.
- Septal nephridia are found on both sides of each intersegmental septum behind the 15th segment. They are attached to the septa and contribute significantly to the excretory system of the earthworm.
A typical septal nephridium structure consists of three main parts:
Part 1 — Nephrostome (Ciliated Funnel)
- A ciliated funnel that communicates directly with the coelom
- Consists of an elliptical pore bounded by two lips
The upper lip of the nephrostome consists of one large central cell and eight to nine marginal cells. All of these cells are ciliated, which helps in the movement of coelomic fluid into the nephridium
The lower lip is composed of four to five compact cells, and all of these cells are also ciliated. |
Part 2 — Body of the Septal Nephridium
The nephrostome leads into the body via a short, narrow, ciliated neck
The body is divided into two lobes:
The straight lobe of the prostate gland is short and approximately half the length of the twisted lobe. As its name suggests, it follows a straight course without any coiling or twisting.
The twisted lobe is longer and more complex, consisting of two limbs. The proximal limb is spirally twisted and connected to the neck of the gland, while the distal limb is also spirally twisted and coiled upon itself.
Part 3 — Terminal Duct
The distal limb of the twisted lobe ends in a short, narrow terminal duct.
All excretory products are finally discharged through this duct.
Internal Structure of Septal Nephridium
The nephridium internally is a syncytial glandular mass traversed by a coiled tubule. This tubule carries four ciliated tracts.
- The neck region of the nephridium contains one ciliated tract, which helps in the regulates the the transport and processing of excretory materials within the organ.
- The body of the nephridium possesses two ciliated tracts, facilitating the transport and processing of extretory materials within the organ.
- The terminal duct contains one ciliated tract, which aids in the passage of excretory materials towards the exterior.
Number of Tubules in Septal Nephridium
- The straight lobe of the nephridium contains four tubules running parallel to one another.
- The basal part of each limb of the twisted loop possesses three tubules.
- The apical part of the twisted loop contains two tubules.
- The neck region of the nephridium has one tubule.
- The terminal duct also contains one tubule.