DISSECTION OF PHERETIMA POSTHUMA (EARTHWORM)

DISSECTION OF PHERETIMA POSTHUMA (EARTHWORM)

External features of pheretima posthuma (earthworm)

It is common earthworm. For external features of pheretima posthuma (earthworm), living narcotised or preserved specimens can be examined. Observe and draw the following parts. Draw separately dorsal and ventral suifaces. Distinguish the dorsal suiface by median blood vessel.

• Shape: Body is elongated, cylindrical with pointed anterior and blunt posterior end. Body shape is nicely adapted for burrowing life (Fig. 4).
• Pigmentation: Body is dark brown in colour.
• Segmentation: Entire body is divided by transverse furrows into segments or metameres.
• Number of segments: 100 to 120 (Count in your specimen).
• Total length : 150-160 mm.
• Total breadth : 3 to 5 mm (Measure the size).
• Clitellum: It is thick, girdle like, forwardly situated region, containing only three segments of the body (14 to 16). It forms a complete ring.
• Setae : Ventrally, middle of each segment contains a ring of small curved setae which can be observed by a hand lens. Setae are chitinous and locomotory. The first segment, last segment and the clitellar region are devoid of setae.
• Mouth : It is a crescentic opening, found in the first segment and shifted ventrally by fleshy prostomium. It overhangs the mouth opening dorsally and it also occupies a small portion of the dorsal surface of the segment.
• Genital or copulatory papillae: Two pairs. One pair each is found on the ventral surface of the 17th and 19th segments respectively. (11) Female genital opening : It lies on the ventral side of the 14th segment in clitellar region.
• Male genital pores: Two in number. They are found on either side of ventral surface upon raised genital papillae.
• Spermathecal pores : 4 pairs are located ventro-Iaterally in the inter-segmental grooves of 5/6, 617, 7/8 and 8/9 segment.
• Anus lies on the last anal segment.
• Dorsal blood vessel is seen on dorsal surface extending from anterior to posterior end.

General anatomy of pheretima posthuma (earthworm)

Both freshly narcotised and killed specimens or preserved specimens can be used. Wash the worm and pindown the animal at its both ends in a dissecting dish with dorsal side upwards. Make a longitudinal mid-dorsal incision along the entire length of the worm with a very fine scissors and do not cut deep with the scissors to avoid injury to internal organs. In a bit deep incision alimentary canal is generally opened. After the longitudinal incision pin aside the flaps of the body wall. The inter-segmental septa often put resistance in opening the worm and for this, use pins to tear the septa and lossen the body wall

• Circulatory system of pheretima posthuma (earthworm) :- 4 pairs of so-called hearts in the segments 7, 9, 12 and 13 and blood vessels. Just after opening the animal, dorsal blood vessel is seen from anterior to posterior end. Lateral oesophageal hearts are very distinct in 12th and 13th segments. Various rounded and red coloured blood glands are found below the pharynx. Lymph glands are present below intestinal caeca associated with blood glands.
• Alimentary canal of pheretima posthuma (earthworm) : Discussed separately in dissection of digestive system. Various parts of alimentary canal consists of buccal cavity, pharynx, oesophagus, gizzard, stomach and intestine. Around pharynx various radial muscles are seen attached from pharnyx to body wall layer. Gizzard is a very distinct structure. The intestinal caeca are present in 25th and 26th segments.
• Excretory system of pheretima posthuma (earthworm): See different nephridia:
  • Integumentary nephridia : Several in each segment.
  • Pharyngeal nephridia : Found in three bunches in 4th, 5th and 6th segments.
  • Septal nephridia : From the 15th and 16th segments and onwards they are attached to inter segmental septa.
• Reproductive system of pheretima posthuma (earthworm): Some of the male reproductive parts are clearly observed anteriorly. The testes sacs are in 11 to 13 segments and the prostate glands are 16 to 20 segments. For details, see dissection of reproductive system.
• Nervous system of pheretima posthuma (earthworm): Discussed separately in dissection of nervous system. Nerve ring and some part of the ventral nerve cord below the alimentary canal may be exposed and seen.
• Besides the above structures, inter-segmental septum, spermathecae, accessory glands, clitellum, typhiosole, etc. can also be observed for anatomy.

Alimentary canal of pheretima posthuma (earthworm)

Dissect the worm as in the case of general anatomy. In the anterior region remove the inter segmental septa and musculature to clear the parts of alimentary canal. See the following parts

1. Buccal cavity: Between segments 1 and 2. It is a sac-like structure.
2. Pharynx: It is highly muscular and found in the segments 3 and 4. It is a pear-shaped structure and is demarcated from the buccal cavity by a transverse groove.
3. Oesophagus: It lies behind pharynx and extends upto the 8th segment.
4. Gizzard : It is hard, bulbous and thick-walled, situated in the 8th and 9th segment. It is highly muscular and it becomes very clear after removing the visceral layer over it.
5. Stomach It is narrow tube, found in the segments 9 to 14. The stomach is glandular and vascular structure.
6. Intestine It starts from the 15th segment and continues up to rectum. It is a thin-walled structure occupying major part of the body. In the 26th segment, intestine gives a pair of intestinal caeca. Often the wall of the intestine is cut and mud is seen in the lumen. Intestine is divided into 3 regions :
   • Pre-typhlosolar region : It extends from the 15th to 26th segments. No median fold.
   • Typhlosolar region : It continues from 26th segment to backwards leaving last 23 to 25 segments. It is characterised by having an internal mid-dorsal fold or typhlosole which can be exposed by cutting open the intestine from the side. It is meant for absorption of the food material. It is extra absorptive area.
   • Post-typhlosolar region : Found in last 23 to 25 segments. It is called as rectum and opens to the exterior by anus.

Reproductive system pheretima posthuma (earthworm)

Procedure

Dissect the animal as in the case of general anatomy. First expose male reproductive system. For exposing the vasa deferentia, tease the body wall and insert black paper below sperm ducts.

Male reproductive system of of pheretima posthuma (earthworm)

1. Testes: Two pairs, one pair in the 10th segment and another in the 11th segment. Each testis is enclosed in a testis sac. Testes are generally degenerated in adult specimens. Each is associated with a spermiducal funnel, which communicates with sperm duct or vas deferens, which runs up to the 18th segment to join with prostatic duct.
2. Seminal vesicles: Two pairs, lying in the 11th and 12th segments.
3. Prostate glands: One pair of lobulated and irregularly-shaped prostate glands are found from 16-17 to 20-21 segments.
4. Accessory glands: Two pairs are present. One pair in the 17th segment and another pair in the 19th segment. The glands look like rounded or coiled circles.

Female reproductive system of of pheretima posthuma (earthworm)

• Ovaries: 2 in number, in the form of whitish masses found attached to the posterior face of 13/14 septum below the gut on either side of the ventral nerve cord. Locate the heart region, invert the intestine and ovaries will be seen as whitish dots.
• Oviducts: 2 short ducts with oviducal funnels. Oviducts converge to open on the ventral surface of the 14th segment.
• Spermatheceae: 4 pairs found in 6th, 7th, 8th and 9th segment. Each spermatheca consists of a pear-shaped ampulla and a narrow duct.

Nerve ring of of pheretima posthuma (earthworm)

Dissect the animal as in the case of general anatomy. Try to expose the complete nerve cord from anterior to posterior end. Remove the intestine and nerve cord will be seen as whitish cord. Nerve ring is found encircling buccal chamber and cut the buccal chamber and remove it leaving nerve ring. Insert small pieces of the black paper under nerve cord and study the following parts.

1. Supra-pharyngeal ganglia: It forms a bilobed mass lying dorsal to the pharynx. It is also called as cerebral ganglia.
2. Sub-pharyngeal ganglia : It is also a bilobed mass, found below the pharynx.
3. Circum-pharyngeal connective: It connects the above two kinds of ganglia forming a complete ring around pharynx.
4. Ventral nerve cord: Found ventral to digestive tube. It starts from the sub-pharyngeal ganglia and extends up to the last segment. In each segment it forms a swollen ganglion.
5. Small nerves from ganglia and nerve cord supply to various organs.