In this article we will discuss about the dissection of earthworm. Also learn about: 1. General Anatomy of Earthworm 2. Dissection of Alimentary Canal 3. Dissection of Reproductive System.
General Anatomy of Earthworm:
For purpose of dissection, generally, fresh animals are narcotized, but preserved animals may also be used in the absence of fresh animals. The narcotized or preserved animals are placed in the dissecting tray and the two ends are stretched and pinned in such a way that the dorsal side remains above.
With the help of fine scissors make a longitudinal cut all along the length in mid-dorsal line. The scissors should not be inserted deep because that might damage the internal organs. Now, pin down the two flaps of body wall on sides by cutting the inter segmental septa with the help of scalpel.
As soon as you will properly-stretch & pin the side flaps you would see the following organs:
(I) Alimentary Canal:
The alimentary canal runs all along the length of the animal and is comprised of the following parts:
(a) The anterior most buccal cavity which is a small round sac like structure between segments 1 & 2.
(b) Just behind the buccal cavity lies the pharynx in segments 3 & 4. It is a pear-shaped highly muscular structure and is separated from buccal cavity through a transverse groove.
(c) Oesophagus is a small tube behind the pharynx and extends up to 8th segment.
(d) Behind oesophagus lies the gizzard which is a hard, thick walled, muscular & bulbous structure in the 8 & 9 segments.
(e) Behind the gizzard lies the stomach which is a narrow tube extending up to 14th segment. It is a highly vascular and glandular structure.
(f) The stomach is followed by Intestine which extends up to the last segment. In the 26th segment it gives out a pair of lateral caeca.
It is divided into three portions anteroposteriorly:
(i) The anterior most region is called pretyphlosolar region and extends from 15th to 26th segments.
(ii) Typhlosolar region continues from 26th segment to 85th segment. In this region the internal mid-dorsal mucous lining is hanging as a fold the typhlosole. This fold- is highly vascular and increases the surface area for absorption of food.
(iii) The post typhiosolar area or rectum extends from 85th segment to the last segment where it opens through the anal pore.
(II) The Circulatory System:
It is represented by 4 pairs of tubular hearts in 7th, 9th, 12th and 13th segments and the blood vessels. The dorsal blood vessel is visible just above the alimentary canal as soon as the skin flaps are pinned down on sides. The ventral blood vessel lies below the alimentary canal. The supra oesophageal vessel runs between 9-13 segments above the stomach.
The lateral oesophageal vessel run from 2nd to 13th segment on either side of alimentary canal. The sub-neural vessel extends between 14th to the last segment on ventral side & is made by the fusion of lateral vessels. Besides, there are blood glands around pharynx and lymph glands above the intestine behind intestinal caecae.
(III) Excretory System:
The excretory system is represented with nephridia as follows:
(1) The Integumentary nephridia are several in number in each segment.
(2) The pharyngeal nephridia are present in bunches in 4th, 5th and 6th segments around pharynx.
(3) The septal nephridia are found attached to inter segmental septa from 16th segment till the last segment on either side of alimentary canal.
(IV) The Nervous System:
The Nervous system is represented by a pair of cerebral ganglia situated in third segment just above the pharynx, a pair of sub pharyngeal ganglia in 3rd segment just below the pharynx and a pair of circum pharyngeal connectives running around pharynx joining the cerebral and sub-pharyngeal ganglia.
From the sub-pharyngeal ganglia starts the ventral nerve cord which runs along the length and terminates in the last segment. In each segment it dialates into a segmental ganglion.
(V) The Reproductive Organs:
The reproductive organs: are represented by 4 pairs of spermathecae one pair each in 6th to 9th segments; one pair of ovaries attached to the septum between 12th & 13th segments one on either side of ventral nerve cord; 2 pairs of testes sac one each in 10th and 11th segments.
The testis is present inside the testes sac. Besides them 2 pairs of seminal vesicles, one each in 11th and 12th segments and one pair of prostate glands extending from 17th to 20th segment. Expose all these structures and flag label them.
Dissection of Alimentary Canal:
Dissect the earthworm as described in case of general anatomy. After exposing the alimentary canal remove the blood vessels, hearts, blood glands & muscles etc. from above and around the alimentary canal. Now locate the Buccal cavity (2nd segment), Pharynx (3rd to 5th), Oesophagus (6th & 7th), gizzard (8th & 9th), stomach (10th – 15th) and intestine (16th till last).
In the intestine locate pre typhlosolar region (16th – 25th), typhlosolar region (26th till 85th) and post typhlosolar or rectum (from 85th till last). In the typhlosolar region also expose the typhlosole by making two small horizontal cuts and one longitudinal cut in the dorsal wall of intstine (as shown in diagram). Turn the flap on sides and locate a vertical ridge in the middle of the flap – the typhlosole.
Dissection of Reproductive System:
Proceed to dissect the earthworm as described in general anatomy dissection. When the alimentary canal is exposed cut it in the posterior most regions and pin it down on one side in the dissecting tray. As soon as you will lift the alimentary canal you will see the reproductive organs. Locate 4 pairs of spermathecae in segments 6th to 9th one on either side of ventral nerve cord.
These are structures belonging to female reproductive system. Below the spermathecae are present two pairs of testes sac in 10th & 11th segments and one pair of seminal vesicle in 12th segment. Also locate paired vas deference running from testes sac to prostate gland on either side of nerve cord.
The prostate gland are paired, large, lobate and elongated structures extending from 17th to 20th segment. One pair of accessory glands in 17th and other in 19th are also present near the prostate gland. All these are male reproductive organs.
The female reproductive organs are represented by a pair of ovaries in 13th segment. Each ovary is a white dot attached to the posterior side of the intersegment septum between segments 12 and 13 on either side of nerve cord.
Each ovary is a lobate structure made up of 15 elongated lobes. Below the ovary lies a small oviduct which extends from 13th to 14th segment and the two oviducts unite in 14th segment in the median line to open outside through a common female genital pore.
Fig. 261 EARTHWORM-CUT OPEN TO SHOW NERVOUS SYSTEM
Dissect the earthworm as suggested in the dissection of general anatomy. As soon as the alimentary canal is exposed locate supra-pharyngeal the cerebral ganglia above the pharynx. Now remove the alimentary canal, as suggested in dissection of reproductive system, exposing the ventral nerve cord till you are able to see the sub-pharyngeal ganglia.
Locate the circumpharyngeal connectives, and cut the alimentary canal just in front and just behind the circumpharyngeal connectives. Now, cut a small portion of ventral nerve cord and detach it from the body wall with the help of needles. Now, place the nerve ring and nerve cord in a watch glass under water and remove all extra material attached to it with the help of fine forceps and scissors.