|Earthworm (Pheritima posthuma)
It is found in soil, which is rich n humus. It
is found abundantly in rainy season. Other days it is found in soil. The earthworm is usually active at night and so called nocturnal. They feed on soil; digest organic matters and undigested matter are excreted out called castings. The earthworm plough soil, makes soil fertile, and so called farmers friend.
Body is long elongated and cylindrical. Anterior and posterior ends are blunted. Body is bilaterally symmetrical. A mature worm is 150 mm long and 4-5 mm width. Body is segmented. There are about 100-120 segments. All the segments are similar and so called metamericlly segmented.
The first segment is called peristomium and the last is called anal segment. In front of peristomium there is a lobe like sensitive structure called prostomium. The 14th 15th and 16th segments are thick and united to form clitellum. The region before clitellum is called pre-cliteller and the region after clitellum is called post-clitellar region. At ventral surface on 14th segment there is a mid pore called female genital pore. On ventral surface, at 18th segment there is pair of pore called male genital pores. There are two pairs of outgrowths on 17th and 19th segments called genital papillae. It helps in reproduction.
At each 5-6, 6-7, 7-8, 8-9th segment there is a pair of opening called spermathecal pore. Below each spermathecal pore, small sac like structure is resent called spermathecae. Except one and two segments, minute pores are scattered throughout the body called nephridiopores, which are excretory organs. On the dorsal surface after 12th segment, small pore is present at each segment called dorsal pore, which communicates body cavity to the environment. The locomotion is carried out by setae. The setae are S shaped structure, which penetrate the body wall and embedded into saetal sac. The setae are made up of chitin, which is complex polysaccharide. At the mid portion of setae there is round structure called nodule.
The process of ingestion, digestion, absorption, and excretion is called digestion. The digestive system is divided into following heads.
The alimentary canal is long tube, which starts from mouth and ends to anus.
Mouth: Mouth is small cresentric aperture. It is situated below prostomium at first segment.
Buccal Cavity: It is thin walled, short tube behind mouth, which extends upto 3rd segment. It helps to ingest food.
Pharynx: It is pear shaped muscular chamber, which extends up to 4th segment. On inner dorsal side of the pharynx, there is salivary gland. Two lateral enfolding are also present called shelves. Pharynx helps to pump the food inside the mouth.
Oesophagus: It is narrow, elongated; thin walled tube extends up to 7th segment. It acts as food pipe.
Gizzard: It is oval, hard, thick walled, highly muscular organ, which extends up to 9th segment. It helps to grind the food particles.
Stomach: It is a narrow tube, which extends from 9th to 14th segment. It is longitudinally internal folded.
Intestine: It is very long, wide tube, which extends from 15th segment to anus. In 26th segment two lateral finger like projections move forward up to 22nd segment, which is called intestinal Intestinal caeca. Internally intestine is highly folded. The foldings are called villi. On mid dorsal side there is a large vilus called typhlosole. Based on presence of typhlosole intestine is divided into 3 parts.
It is the region between 14th segments to 26th segment where typhlosole is absent.
It is the region from 26th segment to 23-25th segment in front of anus where typhlosole is present.
Post tyhlosolar region
It is the region form last 23rd segment to 26th segment where typhlosole is absent. This region is also called rectum.
Anus: It is circular aperture situated at last segment through which undigested food materials is thrown out.
Feeding mechanism/physiology of digestion
The earthworm feeds upon dead organic matter in the soil. Sometimes it feeds up on grasses, microorganisms etc. the food is ingested by pumping and sucking activities of pharynx.
In the pharynx
In pharynx, food is mixed with saliva. The saliva is secreted by salivary gland. The saliva is composed of mucin and proteolytic enzymes. The mucin present here lubricates food and proteolytic enzymes digest proteins.
In gizzard, food is grinded up in fine state. Physical digestion or mastication takes place in gizzard due to the contraction of circular muscles of gizzard.
The stomach secretes proteolytic enzymes from glandular cells present on stomach wall, which digest proteins present in the food, and calcium, which is also present, neutralizes acidic food.
The intestine secretes juice called intestinal juice where following enzymes are present
Amylase: Digests carbohydrate into glucose
Lipase: Digests fat into fatty acid and glycerols.
Pepsin: Digests protein into peptones
Trypsin: Digests peptones into amino acids.
Cellulase; Digests cellulose.
Chitinase: Digests chitin present in food.
The digested food is absorbed through villi of intestine and undigested food is thrown out through anus.
The earthworm is hermaphrodite or monocious i.e. male and female reproductive organs are found in same individual.
Male Reproductive System
Testes: there are two pairs of testes. One pair is situated at 10th segment and another pair at 11th segment. Each testis consists of 4 to 8 digits.
Testes Sacs: Testes are enclosed by fluid filled sacs called testes sacs. One testes sac is found at 10th segment and other is at 11th segment. Two testes sacs communicate each other.
Seminal Vesicle: there are two pairs of seminal vesicles. One pair is situated at 11th and other at 12th segment. Each seminal vesicle communicates with testes sac. The spermatogonia are passed from testes into testes sacs and then they are passed into seminal vesicles where they develop into sperms. The mature sperms again go back into testes sac and pass into seminal funnel.
Seminal Funnel: There are two pairs of seminal funnel. One pair is found in 10th segment and another pair in 11th segment. They are ciliated and found below testes.
Vasa differentia: There are four tubes, which start running from 12th segment to 17th segment. In 17th segment, they unite with the duct of prostate gland to form common prostatic and spermatic duct.
Prostate gland: There are two prostate glands. They are larger, irregular, flat, and wide. They extend from 16th or 17th segment to 20th to 21st segment. Each prostate gland has small curved duct, which joins with spermatic ducts and opens through male genital aperture at 18th segment.
Accessory gland: There are two pairs of accessory gland. One pair in 17th segment and another pair are in 19th segment. They are small and round in structure. They bear small ductules, which open through genital papillae.
Female Reproductive Organ
Ovaries: There is a pair of ovary situated at the 13th segment hanging on septum between 12th and 13th segment. Each ovary has finger like projections, which contained ova in series. The mature ova are found toward end and immature ova are found on the base of the lobules of the ovary.
Oviducal Funnel: One pair funnel like structures at 13th segment found below ovary with ciliated mouth called oviducal funnel. Each funnel opens into short conical tube called oviduct. Two oviducts fuse together at 14th segment and open outside through female genital aperture.
Spermathaceae: There are four pairs of spermathecae. Each pair is found between 5-6, 6-7, 7-8, and 8-9th segment. Each spermatheca is flask shaped. The main body of spermathaca is called ampulla and short small lobe found attached at its side is called diverticulum. They store sperms.
In rainy season, two earthworms copulate at night. During copulation, two earthworms get attached through their ventral surface with their anterior end pointing in opposite direction.
The male genital aperture become erected and inserted into the spermathecal pore of each other. During the process sperms are exchanged together. Then they separate after about an hour.
After copulation, a membrane is secreted around clitellum by membrane secreting glands. The membrane starts to move towards anterior end of earthworm whereas the worm starts withdrawing itself backward. During the process, the membrane receives ova coming from female genital aperture and sperms coming from spermathecal pores. Lastly, the membrane is laid out on the ground. The elastic opening of membrane becomes closed. The structure is called cocoon (ootheca). Within cocoon, one of the sperm fertilized with ovum to form zygote and young worm is developed that comes out of cocoon about 2-3 weeks. A dozen of cocoons is formed after each copulation by each conjugant.
The nervous system of earthworm consists of three parts
a) Central Nervous system
It includes following structures:
There are two pear shaped cerebral ganglia fused together at 3rd segment called brain.
Two circumharyngeal connectives arise from each cerebral ganglion laterally. They encircle pharynx and fuse at 4th segment. The fused portion is called sub pharyngeal ganglia.
Ventral nerve cord
It is white rod like structure, which starts running from sub- pharyngeal ganglia towards posterior end. In each segment, ventral nerve cord swells which is called segmental ganglia. Actually, there are two cords fuse together to form single ventral nerve cord. Ventral nerve cord is composed of nerve cells and nerve fibers. There are 4 giant fibers o mid dorsal side of nerve cord which conducts impulses rapidly. The outer covering of ventral nerve cord is called peritoneum.
b) Peripheral nervous system
It includes nerve fibers or nerve, which arises from central nervous system.
From cerebral ganglia, 8-10 nerves arise and supply to prostomium, buccal chamber, and pharynx.
From circumpharyngeal connectives, two pairs of nerves arise and supply to 1st and 2nd segment.
From subpharyangeal ganglia, three pairs of nerve arise and supply to 2nd, 3rd and 4th segment.
From each segmental ganglion, three pairs of nerves arise and supply to respective segment.
c) Sympathetic nervous system
It consists of nerve plexuses extensively branched and distributed beneath epidermis, alimentary canal that is connected to circumpharyngeal connectives.
Working of nervous system
The sensory cells from different parts of the body receive impulse. Sensory fibers (afferent fibers) carry impulse to central nervous system. Motor fibers (efferent fibers) carry impulse back to effective organs or different parts of body from central nervous system. In earthworm, impulse is also travel from one ganglion to another ganglion through ventral nerve cord.
It makes the burrow in soil. The soil become porous and helps the plants to grow well. So they are considered as natural plough.
They are used as food.
They are used as medicines to cure stone in bladder, diarrhoea, jaundice, piles etc.
are used as baits for fishing.
There are used in laboratory for dissection.
They distract the fields and gardens as a result soil erosion takes place.
They damage different kinds of plants in field and gardens.
Class – Oligocheata
Genus – Pheretima