8.3 Detailed study of earthworm
Pheretima posthuma (earthworm)
- Kingdom: Animalia
- Phylum: Annelida
- Class: Oligochaeta
- Genus: Pheretima
- Species: posthuma
- New name: Metaphire posthuma
a) Habit and Habitat
- It is cosmopolitan in distribution. It is very common in the moist soil of garden, irrigated farms, which are rich in decaying organic matters. It is cold-blooded, nocturnal. The process of burrowing is peculiar.
b) External features
i) Shape and size
It is a bisymmetrical animal. It is cylindrical and elongated. A mature worm measures about 150 mm in length and 3 to 5 mm in width. The dorsal surface of the body is recognised by its darker colour and a mid-dorsal line.
ii) Colouration
- It is quite slimy to touch and appears brown due to the presence of porphyrin pigment in body
iii) Segmentation
- The body of earthworm consists of about 100 - 120 small tiny.
iv) Setae
- Each segments except first, last and clitellum bears. Locomotion takes place by setae.
v) Clitellum
- It is a lighter coloured pigmented ring. It is situated about two centimeters behind the front end of the body. The clitellum occurs around the segment 14, 15 and 16. Due to presence of clitellum the body is divisible into pre clitellar in 13 segments, clitellar in 14, 15, 16 segments and post-clitellar from 17th segments up to last segments.
vi) Body wall
- The body wall consists of cuticle, epidermis, musculature.
vii) Body cavity
- The body cavity of an earthworm has true coelom, containing milky white alkaline coelomic fluid. First septa starts from \(4^{th}\) and \(5^{th}\) segments.
c) External appearances
i) Mouth
It is large, crescentic and terminal aperture at the anterior end.
ii) Genital openings
It is Bisexual. Male genital pores upon the \(18^{th}\) segment, female genital pore upon the \(14^{th}\) segment.
iii) Anus
It is a small and vertical slit-like aperture at the end of last segment as anal segment. It is the exit of the alimentary canal by which undigested wastes are removed.
iv) Spermatheca pores
There are four pairs of small ventro-lateral segments of 5/6, 6/7, 7/8, 8/9.
v) Genital papillae
The \(17^{th}\) and \(19^{th}\) segments bear a pair of small and conical ventro-lateral genital papillae.
vi) Dorsal pores
These are situated after the \(12^{th}\) segment to all segments except last segments.
vii) Nephridiopores
A large number of very minute nephridiopores are found situated throughout the body except anterior first three segments. These pores are the apertures of integumentary nephridia through which metabolic wastes of the body are removed.