• Presence of tactile receptors (receptors of touch),
gustatoreceptors (receptors of taste), photoreceptors
(sensitive to light) and statocysts (balancing organ).
ClassifiCation of annelida
• Annelida are divided into four main classes, primarily on
the basis of the presence or absence of parapodia, setae,
metameres and other morphological features: Polychaeta,
Oligochaeta, Hirudinea and Archiannelida.
Class Polychaeta
• It is the largest class which includes marine worms. Some
are fresh water.• As their name implies, each body segment bears numerous
setae on a pair of lateral appendages, the parapodia.
• Head distinct with eyes, palps and tentacles.
• Clitellum absent.
• Sexes separate. Gonads temporary and in many segments.
• Development includes trochophore larva.
• Examples: Aphrodite, Nereis, Chaetopterus, Arenicola, etc.
Class oligochaeta
• It includes terrestrial earthworms and some other species
that live in fresh water.• The members of this class possess few locomotory setae
borne directly by body segments which are devoid of
parapodia.• Burrowing, crawling, nocturnal and detritivorous in habit.
• Head indistinct and without sensory organs.
• Glandular clitellum present for cocoon formation.
• Hermaphroditic. Testes anterior to ovaries.
• Fertilisation external (in cocoon), development direct without
any larval stage.
• Examples : Pheretima, Tubifex, etc.Chaetopoda is a super group which includes both Polychaeta
and Oligochaeta. This creation is because of the fact that both
the classes are provided with setae.Class hirudinea
• It comprises leeches which are perhaps the most specialised
annelids without parapodia and setae but with suckers.
• About 300 species of leeches are known to occur in the
tropical and temperate parts of the globe.
• Most of the leeches are freshwater, while some are marine
or terrestrial.
• Most are ectoparasitic, living on the blood of vertebrates,
while some are predaceous, feeding on worms, snails,
insects, larvae etc.
• Body with fixed number of segments (33). Each segment is
subdivided externally into annuli.
• Coelom much reduced due to its filling by botryoidal tissue
and forms haemocoelomic sinuses.
• Hermaphroditic, with one male and one female gonopore,
but cross fertilisation occurs. Fertilisation internal. Segments
9, 10 and 11 develop a temporary clitellum during breeding
season.
• Development in cocoons, direct without larval stages.
• Examples : Hirudo, Hirudinaria etc.