Morphology of Earthworm
Earthworm consists of a cylindrical body of 10 to 20 cm length and 0.5 cm diameter. The body is divisible into 100-200 segments by circular grooves and external segmentation is corresponds to internal segments (metameric segments). Fourteen to sixteen segments have a circular band (no grooves) or reproductive girdle called clitellum of cingulum. First segment of periostomium has a central mouth with a dorsal lobe or prostomium.The last segment of the body contains anus. A couple of Male genital pores are situated ventrolaterally on segment 18 and female genital pore is situated on 14 th segment. Two pairs of genital papillae are found ventrolaterally on 17 th and 19 th segments. Four pairs of permathecal pores are formed in grooves from 5/6 to 8/9. In the body of earth worm dorsal pores are present mid dorsally behind 12 segments up to last one groove. Nephridiopores occur on all segments except the first, last and clitellar segments. Body wall contains five layers – cuticle, epidermis, circular muscle cells, longitudinal muscle layer, peritonium.
Coelom – First true coelom is observed. Coelom contains fluid that is made up of cells- phagocytes, circular leucocytes, myocytes and yellow amoebocytes.
Digestive system of Earthworm: Earth worm swallowed food along with soil by sucking action. Alimentary canal consists of -mouth in the first segment, protrusible buccal cavity, a pear shaped muscular pharynx in 3rd and 4th segments with roof containing a pharyngeal mass or bulb with some chromophil cells. Pharynx of earthworm is divided into a dorsal salivary chambersfor partial protein digestion and a ventral conducting chambers. Narrow oesophagus between 4-8 segments, muscular gizzard in 8th or 9th segments for crushing of food. Narrow stomach between 9-14 segments with sphincter at either end for storage of food. Calciferous glands are present which neutralizeacidity of hmic acid present in the food. Some kinds of proteolytic enzymes are secreted. Beyond the stomach alimentary canal which is called intestine is present. It can be divided into three parts –pre typhlosolar,typhlosolar and post typhlosolar. Pre typhlosolar extends between 15 to 26 segments. Two intestinal caeca open in 26thsegment. They reach up to 22nd segment. This conical out growths secret various digestive enzymes like amylase, lipase, cellulase,chitinade, invertase etc.The typhlosolar region of intestine is extended from 27th to 75th /95th segment . It contains a large typhlosole for increasing absorptive area. In last 25 segments of earthworm stored food and solidified for passing out worm casting through anus.
Respiration of Earthworm: Earthworm performs cutaneous respiration as mucus, coelomic fluid keep the skin moist.
Blood – Blood contains red coloured plasma with dissolved haemoglobin and amoeboid nucleated white corpuscles produced by blood glands present in 4th, 5th and 6th segments.
Blood vascular system of Earthworm: Blood circulation of earth worm is closed type in the first thirteen segments as compared to rest of the body. Dorsal vessels- it collects blood in each segment from two pair of dorsiventral – intestinal and one pair of commissural vessels also distribute blood to intestine through the depth intestinal vessels. Ventral vessels –it is without valve which receives blood from the hearts . In the hinder part ventral vessels contains – ventrotegmentary, septonephridial, ventro intestinal. Latero-oesophageal vessels- it receives blood from segmental organs from integumentary vessels. Subneural vessels – it forms due to union of two latero oesophageal vessels which collects blood and flows it back wardly.
Supra oesophageal vessels of Earthworm: This is unpaired which is situated between the 9th and 13thsegments receive blood from latero-oesophageal oesophageals through anterior loops pours it into ventral vessels through latero oesophageal hearts.
Lateral hearts – It pumps blood from dorsal vessels to ventral vessels and situated by pair in each segment of 7th and 9th.
Latero - oesophageal hearts –This send blood from dorsal and supra oesophageal vessels into ventral vessels.
Excretory system of Earthworm: It is ureotelic by nature but it excretes ammonia and creatinine in extra moist soils. Excretory organs can be divided into – pharyngeal nephridia – this are closed and hence occur in paired groups. They are enteronephric (peptonephridia open in buccal chamber). Septal nephridia –nephrostome collect excretory material from coelomic fluid. Terminal ends of nephridia open into septal excretory canal that enters alimentary canal and thus it is enteronephric. Integumentary nephridia- it is exonephric and remain attached to body wall in groups of 200 to 250 except in first two segments. Forests or clouds of integumentary nephridia occur in clitellar segments. This are open outside through nephridiospores and nephrostome is absent.
Nervous system of Earthworm: Nervous system of earth worm can be divided into three parts. They are - central, peripheral and autonomous nervous system. Central system consists of brain – ring/ circum pharyngeal ring and a ganglionated double ventral nerve cord. It contains a brain consisting of two supra pharyngeal ganglia, two circum pharyngeal connectives and a pair of subpharangeal ganglia. From subpharangeal ganglia nerve cord is being originated. The peripheral nervous system consists of nerves that develop from central nervous system and supply to different parts of the body.These are mixed nerves. Autonomous nervous system is made up of nerve plexus which is present in the wall of alimentary canal.
Sense organs of Earthworm: Group of specialised cells that are found in skin and lining of the buccal cavity. Different types of receptors are – photoreceptors, tactile receptors, olfactory receptors and gustatory receptors.
Reproduction of Earthworm: This animal is hermaphrodite and protandrous. It has a power of regeneration. Cross fertilization takes place between two animals by mutual insemination.
Male reproductive system of Earthworm: Male reproductive system is consists of two pairs of testes, two pairs of seminal vesicles, two pairs of seminal funnels, prostate glands, accessory glands.
Spermatheca of Earthworm: This animal have four pairs of flask shaped sacs, each of them have diverticulum for storage of sperms and a large ampulla for their nourishment.
Female reproductive systems of Earthworm: It contains a pair of ovaries, a pair of oviducts, pair of ciliated oviductal funnels for collecting ova.
Copulation of Earthworm: At first the two worms layed in head tail pattern, then the sperm of one worm are transferred to spermatheca of other and then separated. Each of them produces clitelum in which the eggs are layed then the sperms are deposited. The separated girdles produce coccon where the fertilization and development take place.