List of three important members of Phylum Annelida:-1. Polychaetes 2. Oligochaetes 3. Hirudinea.
Member # 1. Polychaetes:
This group includes heterogeneous forms where variation is marked not only in size but also in habits and structural organisation.
Following examples will illustrate the range of diversities:
Arenicola:
Burrows in sand; body is distinctly divisible into three regions—anterior region (or pre-branchial region) with seven segments bearing setae and with no eyes and tentacles; a small trilobed prostomium is fused with peristomium; pharynx protrudes as proboscis covering papillae; middle region or branchial region of thirteen segments with branched gills and setae; the gills are the modified notopodial ciri; the posterior region or post branchial region of several segments without gills and seate; nephridia are six pairs; possesses statocyst; build L or U-shaped tubes within the burrows; tubes cannot be separated from burrows; gills are red in colour; ventral branch of parapodia has pads; reproductive cells are liberated through nephridia; maturity reaches after two years and the reproductive phase lasts for two weeks (Fig. 17.42A).
Sabella:
They are marine, benthic and commonly called peacock worm; usually 10- 25 cm in length; number of segments vary from 100-600; live within tubes which are made up of mucin and encrusted with mud; gill filaments arise from a semi-circular base; filaments are of equal sizes; operculum is absent; long-ciliated faecal groove runs from the anus to the opening of the tube to throw out faecal matters; faecal groove runs dorsally along the thorax; prostomium is reduced and encircled by whorls of tentacles which are arranged like a funnel; blood is green (Figs. 17.42C-C1).
Chaetopterus:
They are marine, found in Europe and U.S.A. and commonly called ‘paddle worm’; usually 15-25 cm in length; lives within parchment like U-shaped tube and is luminiscent; body is divisible into— anterior region, middle region and posterior region; anterior region includes 15-20 segments with a wide, and funnel-shaped mouth but tentacles and palps are absent; gills are absent.
One pair peristomial cirri present; peristomium is long and collar- shaped; parapodia of anterior region are uniramous with reduced notopodia; three pairs of biramous parapodia situated in the middle region which are modified as wing and fans to draw water inside the tube (Fig. 17.42); the 1st pair of parapodia in the middle region is produced great wings which are directed forwards; the next segment of the great wings bears a pair of suckers and a food cup; the posterior region includes about 30 segments with unmodified and biramous parapodia.
Reproduction is usually asexual and by transverse fission; several worms may live within one tube and also other animals may stay within the tube as commensals. The animal can regenerate the whole worm from a single isolated segment.
Amphitrite and Terebella:
Both of them are tube-dwellers but can leave their tube to move like snakes; prostomium bears incompletely retractile multi-grooved tentacles; gills are three pairs and in Terebella, the third pair is small.
Serpula and Spirorbis:
Live within tubes, which are formed by mucin impregnated with calcareous bodies; possess a crown of many tentacles derived from the prostoinial palps; tentacles are feathery on both side and one tentacle acts as operculum; ciliary feeding takes place; tentacles are richly vascularised and act as gills; Serpula is 5-7 cm in length and has 200 segments. In Spirorbis, the body is grooved on one side and the groove acts as brood pouch.
Aphrodite (= Aphrodita):
It is popularly known as ‘seamouse’ (Fig. 17.44G); body is usually broad and compact; hard jaws are absent in the proboscis; dorsal cirri are modified to form plate-like structure, called elytra which act as gills; number of elytra varies from 15-20 and in addition to their function as respiratory structures the elytra produce water-current within the burrows; setal threads arise from notopodium and form an anastomosing network on the dorsal side; capable of digging burrows from 5-50 m deep.
Eunice. Elongated body having eversible proboscis with tooth plates and teeth; presence of cirrus in place of dorsal parapodium; notopodium bears a branched gill; setae are straight; length varies from 3- 70 cm in different species and some may contain more than 500 segments.
Polynoe:
Body is short and dorsoventrally compressed (Fig. 17.42E); provided with bioluminiscent elytra like those of Aphrodites; anterior end bears head with tentacles and two pairs of peristomial cirri; proboscis bears four jaws.
Tomopteris:
Pelagic; body is transparent; presence of one pair of lateral appendages at the anterior end; proboscis is unarmed and can be everted; parapodia are large and paddle-like, biramous and without setae; length usually 1.2-8.7 mm; one pair of eyes are present.
Autolytus:
Size is small (Fig. 17.42D), usually 5-30 mm in length; teeth are present on the proboscis; parapodium is uniramous; presence of alternation of asexual and sexual generations—asexual forms develop from zygote by a process called budding.
The process includes proliferation and construction at the posterior end and such buds develop into sexual generation. Males and females are different and both are dissimilar to the parent as regards the appearance of parapodia and setae.
Member # 2. Oligochaetes:
The oligochaetes are simple compared to the polychaetes they are less varied. The aquatic members are much smaller than the terrestrial forms; in aquatic form male genital aperture lies in front of the female aperture.
Some well-known forms are discussed below:
Megascolex:
Terrestrial and large sized; presence of several nephridia in each segment.
Tubifex:
Length varies from 2.5-8.5 cm; red in colour; anterior part of the body remains within a tube of mucus and the posterior end waves rapidly. With the slightest disturbance in the environment, the worms withdraw the body within the tube. With the decrease of dissolved oxygen in water, the worm increases its extension from the tube; reproduction is asexual.
Allolobophora:
Length varies from 2.5- 17 cm; usually ringed with maroon and yellow; some species are polyploid; one species (A rosea) is parthenogenetic.
Eisenia:
Usual length is 6-13 cm; each segment with a red or brown band; presence of a stinky odour.
Chaetogaster:
Usual length is 0.1-1.5 cm; possesses distinct segmental grooves and septa; carnivorous; internal structures are distinctly visible through the transparent integument; presence of asexual reproduction at regular intervals, resulting into the appearance of chains of zooids which after separation becomes sexually matured.
Aeolosoma:
Prostomium large and bears cilia (Fig. 17.43A); cuticle is without segmental groove and septa are confined to loose muscles; reddish-yellow droplets are seen in the epidermis; organs for sexual reproduction are present, but usually asexual reproduction takes place.
Member # 3. Hirudinea:
Nearly 300 species of leeches exhibit a uniformity in their appearance and structure. All are hermaphrodites and are devoid of typical annelid appendages like parapodia. Body is usually adapted for living as ectoparasite. Some interesting members of this group are discussed below (Fig. 17.43).
Haemadipsa:
Length is variable—2.5-8 cm; terrestrial; possesses three jaws; can move very rapidly.
Haemopis:
Usual length is 10-30 cm; colour is greenish-black; body is cylindrical with eyes; presence of 14 blunt teeth on the jaws in two rows; usually parasitic on horses. Some species of Haemopis are predators and can swallow an earthworm (Fig. 17.43C).
Branchellion:
Marine and parasitic on ray fishes; cylindrical body with bell-like anterior sucker; one part of the body is narrow and the other part is broad; presence of foliaceous respiratory plates on the lateral sides of the body.
Pontobdella:
Marine and parasitic on bony fish; body is cylindrical and club-shaped with anterior sucker and truncated posterior sucker; skin is leathery with knob-like warts; eyes and gills are absent (Fig. 17.43D).
Acanthobdella:
Length is usually 2-37 cm; lives as a parasite on Salmon fish and in addition to blood, eats skin and fin tissues; anterior sucker is absent but posterior sucker is well-developed; anterior part is flattened and here each segment bears two pairs of setae (Fig. 17.43E).
Glossiphonia:
Body is dorso-ventrally flattened but oval in outline; length is 0.5-3 cm; parasite on fresh-water snails and aquatic insects; anterior sucker i£ flat but posterior sucker is cup-like; three pairs of eyes are present; intestine bears four pairs of lateral caeca; a ventral groove carries the eggs and young ones (Fig. 17.43F).
Hirudo:
Common medicinal leech is known as Hirudo medicinalis; dark green body with six red, yellow or brown coloured elongated bands; presence of 100 sharp teeth; size of cocoon is 2-3 cm in length (Fig. 17.43B).