2. Introduction
•Includes elongate, vermiform creatures characterized by segmented
bodies
•Usually called segmented worms which distinguishes them from
Platyhelminthes and Nematoda
•Includes familiar leeches, earthworms and nereis, besides a large
multitude of marine and freshwater species
•Show metamerism: linear repetition of similar body parts along the main
axis called metameres.
•Annelids are the first to indicate cephalization(modification of the
anterior end of the body to form a head)
•Annelids are the first to have a true coelom
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4. I. Polychaeta general reproductive features
•Gonads are unspecialized and serially repeated
•Sexes usually separate
•Type: Neanthles or Nereis
•Testes and ovaries are very simple consisting of masses of cells formed
only during breeding season
•By rapid proliferation and modification of coelomic epithelial cells
surrounding the principle blood vessels
•Reproductive cells are formed in each segment except near anterior end
•Sexual cells detach from the coelomic epithelium and remain in the
coelom till they are ripe
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5. Reproductive system
•In males, the sperm mother cells undergo active cell division forming
smaller cells which eventually will develop into a mature sperm with a rod-
shaped head and a long vibratile flagellum or tail
•In females, the ovaries develop similarly and the relatively large rounded
ova become fille with yolk globules
•No gonoducts in Neanthes, the ripe ova and sperms escape into the sea
through nephridia or temporary ruptures of body-wall
•Larval typically a free swimming trochophore
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6. Swarming (occurs at night during certain phase of the moon)
•Reproduction occurs at definite periods(late July and August)
•Sexually mature worms, well-filled wit eggs or sperms leave their
burrows and swim to the surface of the sea in great numbers
•Both sexes swim actively liberating eggs and sperms in the form of white
clouds in
the surrounding water
•Eggs of gravid females give off a substances which triggers the
males to shed sperms
•Both sexes die soon after shedding their reproductive cells
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7. Fertilization
•As ripe ova and sperms are discharged into the sea, fertilization occurs
externally into the surface layers
•Concentration of breeding activities within a short time insures
fertilization of huge numbers of eggs
Heteronereis Phase- sexual phase
•During a particular phase of the moon, the individuals leave their burrows
and come out in large swarms for spawning
•They swim actively in surface waters by undulating movements of the
body with active paddling of the parapodia
•The females attract the male and they shed ova and sperms like white
cloud into the seawater fllowed by the death of the sexual individuals.
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8. Embryogeny
1)Eggs
•Surrounded by a thick, transparent gelatinous coat
•Zona radiate disappears in fertilized egg(zygote)
•Yolk granules accumulate at the vegetal pole making the egg
telolecithal
•Two polar bodies are extruded out at the pole and cleavage sets in
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9. 2)Cleavage
•Unequal and spiral type, but first two cleavage of the zygote are equal
and vertical, resulting in four blastomeres lying in the same plane(picture)
•After the 4 -celled stage, each blastomere shall give rise to only one
quadrant of the embryo
•Cells D and F will form the lateral quadrants
•E the ventral quadrant
•G (larger than the others) will form the dorsal quadrant
•All succeeding cleavages are unequal and of the spiral type, so that the
cells of one quarter are not placed directly over or below the cells of the
adjacent quarters but somewhat displaced to the left or right.
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10. •The micromeres give rise to the ectoderm and the macromeres to
the mesoderm
•One of the macromeres called somatoblast produces the mesoderm
of the larva and the adult
•Gastrulation occurs by epiboly
•The blastopore forms the mouth and the stomodaeum
Anus forms at a new site below the closed blastopore
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11. 3) Trochophore
•Usually minute, unsegmented, ciliated and almost pear-shaped pelagic
creature with a protruded equator and the head end broader than the anal
end
•The front end shows the apical plate bearing apical cilia, sometimes a
pair of pigmentes larval eyes and the cerebral ganglion
•The ventral mouth, lying in the middle of the body, opens into a short
ectodermal stomadaeum (oesophagus) leading into a wide endodermal
mid-gut(stomach), followed by the ectodermal proctodaeum which opens
to the exterior anus, lying at the narrow posterior end of the body
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12. •Coelom absent at this stage, but a large space present between the
alimentary canal (endoderm) and the ectoderm called the blastocoels, with
a pair of larval excretory organs(protonephridia)
•The body is surrounded by two circlets of cilia: prototroch and metatroch,
sometimes telotroch, mesotroch and polytroch and rarely atroch
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14. Metamorphosis of trochophore
•While drifting in the sea, the
trochophore undergoes
metamorphosis to change into the
adult
•Pre-oral part becomes the
prostomium
•Post-oral region elongates and
becomes constricted into segments
which develop the parapodia with
long setae that can either be
provisional or larval
•First segment becomes the
peristomium and last the pygidium
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15. •Mesodermal band divides into
three pairs of somites, each
developing a cavity
•Coelom later obliterates the
blastocoels
•Ciliary band disappear
•Larva grow in size and length
with the addition of new segments
•After metamorphosis the young
worm sinks to the bottom to
become the burrowing adult worm
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17. Asexual reproduction in polychaetes
• Show various regenerative capabilities such as regenerating lost
appendages like palps, tentacles, cirri and parapodia
• Most can also regenerate posterior body segments however most cannot
regenerate lost head
• Regeneration is controlled by neuroendocrine secretions released by the
central nervous system at sites of regrowth
• Normal growth and addition of segments in young worms takes place
immediately anterior to the pygidium in the growth zone
• When the trunk is severed, the cut region heals over and then a patch of
generative tissue, blastema forms
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18. • The process occur simultaneously with the growth of the gut, which
contributes part of the ecodermal origin
• Undifferentiated cells from mesenchyme-like layers of the body also
migrate to injured areas and contribute to the regenerative process
• Neoblast cells which are ectomesodermal in origin contribute to tissues
and structures associated with true mesoderm and other germ layers
• Macroseptum is also produced by the ingrowth of the epidermis
• Asexual reproduction results in a variety of regeneration patterns,
including chain of individual, budlike outgrowth, or direct growth to new
individuals from isolated fragments
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Reproductive individuals budding from parents
parapodia in syllis
21. oligochaeta general reproductive features(Type: Pheretima)
• Monoecious or hermaphroditic
• Possess gonads which are limited within a few body segments only
Male reproductive organs includes testes, testis sacs, seminal vesicles, vasa
diferentia, prostate glands and accessory glands
• Testes -minute, white and lobed, made up of a compact narrow base, from
which arise 4 to 8 small digitate processes containing rounded cells called
the spermatogonia
• Testis sacs- encloses a pair of testes and behind each testis a ciliated
spermiducal funnel and also communicates behind with a pair of seminal
vesicles of the succeeding segment
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22. • Seminal vesicles- also referred to as septal pouches are large and white, lies in the 11
and 12 segments
• Spermiducal funnels- there are two pairs of ciliated spermiducal funnels(sperm-
rosettes), one of them lying behind each testis in the same segment and enclosed
within the same testis sac
• Vasa deferentia- each spermiducal funnel leads behind into a slender, ciliated and
thread-like sperm duct also known as the vas duct
• Prostate glands-upon emergence, enclosed immediately in a common muscular
sheath, along with the two vasa deferentia of the side, forming the common spermatic
and prostatic duct which curve to open to the exterior by a pair of male genital pores
• Accessory glands- -open to the exterior by a number of ducts on two pairs of genital
papillae, situated externally
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23. From the testes the spermatogonia are shed into the testis sacs,
wherefrom they enter the seminal vesicles to undergo maturation and
develop into spermatozoa. The mature sperms move back to the testis sacs,
enter the spermiducal funnels, travel along the vasa deferentia and finally
pass out through the male genital pores
Female reproductive organs include ovaries, oviduct and spermathecae
•Ovaries-Each ovary is a white compact mass made up of finger-like
processes in which the ova are arranged in a linear series
•Oviducal funnels- usually large saucer-shaped with much folded and
ciliated margins lying behind each ovary in the 13th
segment leading into the
oviduct
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24. • Oviducts-- two oviducts form short common oviduct, which opens to the
exterior through the female genital aperture situated mid-ventrally on the
14th
segment
• Spermathecae-the spernathecae receive sperms from another worm
during copulation which are stored in their diverticula in Pheretima and in
the ampullae in other worms
The mature ova shed from the ovaries are entangled by the oviduct funnels,
travel along the oviducts, and pass out to the exterior through the female
genital aperture to be laid inside the cocoon
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25. Fertilization
Earthworms are bisexual however self-fertilization does not occur. This is
because they are protandrous, that is their testes ripen earlier than their
ovaries.
As a rule, cross fertilization takes place accompanied by copulation, after
which eggs are laid in the cocoon.
In earthworms, due to hermaphroditism, all individuals can lay eggs.
However, have no specific role except increasing the rate of their
reproduction
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26. Copulation
• The spermatozoa of one worm are transferred to another during a process
termed copulation
• Generally takes place at night or in the morning before sunrise, during
rainy season, from July to October, and lasts for about one hour
• During copulation, two worms apply on each other by their ventral
surfaces with the head ends pointing in opposite directions, so that the
male genital pores of each lie against the spermathecal pores of the other.
• The areas surrounding the male genital apertures are raised into papillae,
which are inserted successively from behind forwards, into the
spermathecal pores of the other worm and discharge the spermatic and
prosthetic fluid containing spermatozoa which are stored in the
spermathecae, thus there is a reciprocal cross-fertilization
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27. • After this interchange of sperms, the two worms separate and later lay
their eggs in cocoons; fertilization is external, taking place in the cocoons
Cocoon Formation (Type : Eisenia and Rhynchelmis)
• The cocoon is Secreted as a vicid and gelatinous substance, forming a
broad membranous band or girdle around the clittellum
• It hardens gradually on exposure to the air into a tough but elastic tube
which becomes the cocoon(egg capsule)
• Slime tube is produced by the epidermal mucous cells around the clitellum
over the cocoon
• As the worm wriggles behind the slime tube, the cocoon is slipped forward
towards the head
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28. • On its way, the cocoon receives several ova from the female genital
aperture and spermatozoa and an albuminous fluid is also deposited inside
• Finally, when the cocoon is thrown off the head, its elastic ends close up
and a yellowish and rounded cocoon is formed
• Fertilization occurs after the cocoon has been deposited in moist earth
For example: cocoon formation in Pheretima takes place in summer, during
and after monsoon
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29. • A cocoon may contain many fertilized eggs, but only one embyo develops
further, growing at the expense of the others, serving as the nurse cells
• Cleavage is holoblastic and unequal and the development is direct without any
free larval stage
• A hollow blastula is formed and later a gastrula by invagination
• The mesoderm develops from two cells of blastula, called mesoblasts. They divide
to form two mesoblastic bands, which later give rise to the coelomic epithelial
lining
• Swimming larval stage does not occur unlike the polychetes
• The young worm, when fully grown crawls out of the cocoon in about 2 or 3
weeks
• The baby worm resembles the adult at hatching, except for size and the absence
of clitellum
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Development
30. Regeneration
• Earthworms do not reproduce asexually but have high power of
regenerating segments removed at the ends of the body
• When a worm is cut into 2, the front half will regenerate the tail but
the posterior will not regenerate the head if 14 or more segments
are cut off. The head will only regenerate if only 4 or 6 segments are
cut off at the anterior end
• Regeneration of a tail instead of a head sometimes occur, the
posterior half may regenerate a tail instead of a head. Such a double
tailed worm cannot feed and slowly starve to death
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31. Grafting
• Occurs in planaria normally
• Experimentally, worms with two tails were produced by grafting the
terminal parts of two short worms and long worms, by joining parts of
3 or more worms
• Earthworms probably do not survive due to regeneration in the same
way as planarians or sea-stars
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