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Q, ANNELIDA : CHARACTERS, CLASSIFICATION AND EXAMPLES 5, a, on the dorsal surface of anterior sucker in leech) and statocysts. Eyes of Nerei# have, transparent, gelatinous and refractile lens which cannot form an image on the retinal cells, ® but can detect only changes in the light intensity., 19. Asexual reproduction is reported in s, fission in Syllis. Pe in some polychaetes e.g. fragmentation and transverse, , i 20. Sexual reproduction, Sexes are either unisexual or dioecious (e.g. polychaetes like Nereis), : or bisexual or monoecious or hermaphrodite (e.g. oligochaetes and hirudines). Gonads are, temporary in the polychaetes while these are permanent in the oligochaetes and hirudines., Fertilization is internal (e.g. Hirudinaria) or external (in an ootheca in earthworm and in sea, : water in Nereis). Development is either direct in monoecious forms like earthworm and leech, oe includes a free-swimming and ciliated trochophore larva in dioecious forms like, 21. Regeneration. Annelids have less regeneration power than the planarians e.g. in majority of, annelids, only a few body segments at either ener both ends of body can Be regenerated., Number of segments regenerated is species specific e.g. only about fifteen segments can be, regenerated in earthworm. Among annelids, a very interesting regenerative phenomenon is, found in Eunice (Polalo-worm) in which posterior sexual part called epitoke is regenerated, , a number of times to increase the chances of sexual reproduction., , PECULIAR FEATURES OF ANNELIDA, 1. These show metameric segmentation., 2. Presence of schizocoelic true coelom., 3, These have tube-within-tube plan., | 4. Body wall has both circular and longitudinal smooth muscles., 5. These are first animals to have circulatory system. Haemoglobin is dissolved in blood, , 6. These have special structures, called nephridia, for excretion and osmoregulation., 7. Nerve ring is circumpharyngeal., 8. Development is either direct or includes trochophore larva., CLASSIFICATION, Phylum Annelida is divided into four Classes : Polychaeta, Oligochaeta, Hirudinea and, ' Archiannelida on the basis of habitat, cephalization, organs of locomotion, clitellum, segmenta, tion and reproduction and development (Table 1) :, Table 1. Four Classes of Phylum Annelida., , , , Characters Class Polychaeta | Class Oligochaeta Class Hirudinea Class Archiannelida, , , , , , : eC . a, , > | Habitat ivelymarine | Mostly terrestrial | Mostly fresh water and | Exclusively marine, org fosso- | (e.g. earthworm) |ectoparasitic (e.g. e.g. Polygordius., , $ rial (e.g. Aphrodite), while some fresh | Hirudinea), some ma, tubicolous (e.g. | water (e.g. Aelo- |rine (e.g. Pontobdella)., terus)orun- | soma, Tubifex)., , , , , , , , , , , , , , , , , , , , , , , , , A der the stones (e.g., Nereis)., : Head with eyes, |No cephalization., $ and | palpsandtentacles, and|is absent. Only, , si tentacles, | prostomium, d palps and eyes. | present., , ous bristle- | Locomotion by |, ! muscular ', 0S