Annelids and Allied Taxa

Dividing up the Body The fluid-filled coelom is an efficient hydrostatic skeleton When septa divided this coelom into a series of compartments, metamerism first arose Evolutionarily, if one segment should fail, another could still function Metamerism makes independent segmental movements possible

Characteristics The three phyla are Annelida (marine and freshwater worms, leeches), Echiura, and Spincula All three phyla have a trochophore larva We will spent most of our time understanding the phylum Annelida

Annelids About 15,000 species of segmented worms 2/3 of them are obscure, marine species not noticed very easily The nervous system is more centralized and the circulatory system is fairly complicated Except for leeches (Class Hirudinea), annelids have tiny chitinous bristles called setae Short setae anchor a segment in an earthworm (for example) to prevent it from slipping backward Long setae help aquatic worms swim

The ecology of annelids They occur in the sea, freshwater, and on land Marine annelids borrow in the mud or live in tubes Some feed on organic matter in the mud, some are suspension feeders, many are predators Freshwater annelids burrow in the mud, live among vegetation, or swim freely Leeches are bloodsuckers or are carnivorous

Economic importance Many annelids are grazers and consumers of detritus in food chains; many are preyed upon by fish Burrowing on land or in oceanic mud and sand, annelids are important for drainage, aeration, mixing of soil and distribution of organic matter A study by Charles Darwin showed that the lowly earthworm brings as much as 8 -20 tons of soil to the surface each year Blood-sucking leeches are used medicinally

Anatomical Features Two part head has anterior tip containing the prostomium and peristomium Terminal portion bearing the anus is the pygidium The prostomium and pygidium are not true segments New metameres form just in front of the pygidium; thus the newest segments are at the posterior

Anatomical Features Peritoneum (mesodermal epithelium) lines the body wall and forms dorsal and ventral mesenteries Septa are between adjacent segments Except in leeches, the coelom is filled with fluid and serves as a hydrostatic skeleton The fluid volume remains constant Therefore contraction of longitudinal muscles causes the body to shorten and fatten

How annelids move Contraction of circular muscles causes the body to narrow and lengthen By separating this force into sections, widening and elongation move the whole animal Alternate waves of contraction or peristalsis result in efficient burrowing Swimming annelids use undulatory movements

Anatomy of the Annelids

More Anatomy

Class Polychaeta (the bristle worms) The largest class of annelids with more than 10,000 species, mostly marine From 1 mm to 10 feet long Some live in crevices or in tubes; some swim and are pelagic They are an important component of the food web Polychaetes have a well differentiated head with sensory organs Paired appendages called parapodia are on most segments

Characteristics of the polychaetes They have no clitellum as in earthworms Many setae are arranged in bundles on the parapodia The head or prostomium may or may not be retractable; it often bears eyes, tentacles and sensory palps The first segment or peristomium surrounds the mouth and may have setae, palps, or chitinous jaws Suspension feeders may bear a tentacular crown that opens like a fan but can be withdrawn into a tube

Segments of the trunk Most segments bear parapodia with lobes, cirri, setae and other parts Parapodia have two lobes, the dorsal notopodium and the ventral neuropodium Usually the parapodia are the chief respiratory organ although the worm may also possess gills The parapodia help the worm to crawl, swim or serve as an anchor in a tube Along with gills, they serve as chief respiratory organs Amphritite has plumelike gills

Amphitrite Builds its tubes in mud or sand Extends long tentacles over the mud to gather bits of organic matter as food

Tube-dwelling sedentary polychaetes The Christmas tree worm, Spirobranchus giganteus, lives is a calcareous tube The sabellid polychaete, Bispira brunnea, lives in a leathery tube

Sense organs of polychaetes Sense organs include eyes and statocystes that are more developed than in the class Oligochaeta The eyes vary from simple eyespots to well developed eyes that can distinguish images

Reproductive characteristics Polychaetes have no permanent sex organs and sexes are separate The gonads appear as temporary swellings of the peritoneum Gametes are shed into the coelom and exit by gonoducts, metanephridia or rupturing of the body Fertilization is external and the early larva is a trochophore

Diversity of the Polychaetes These worms are pelagic, burrowers, sometimes sedentary some live in tubes Nereis, the clam worm, is a predatory polychaete with jaws which can deliver a noticeable bite when handled Featherduster worms or fan worms live in tubes but unfurl tentacular crowns to feed Fanworms are polychaete ciliary feeders, directing small food balls along grooved radioles to the mouth by ciliary action

Sabella, a polychaete suspension feeder

Chaetopterus A sedentary polychaete which has wing-like notopodia that secrete mucous filters to strain out food particles from the water As the net fills with food, the food cup rolls it into a ball which is passed into a groove toward the mouth

Arenicola This polychaete lives in an L-shaped burow in intertidal mudflats and ingests food-laden sand

Reproductive behavior of polychaets Most of the time, polychaetes live as sexually immature worms called atokes When swollen with gametes, they are called epitokes At the beginning of the last quarter of the October-November moon, the polychaetes enter a “swarming period” The epitokes break off and swim to the surface; swarming ensures that all epitokes mature at the same time

Class Oligocheta Over 3000 species occur in habitats from soil to freshwater; a few are marine or parasitic Nearly all have setae but in general, less setae than polychaetes Earthworms burrow in moist, rich soil, emerging at night In wet weather, they stay near the surface. When it is dry, they burrow deep and become inactive

Lumbricus terrestris is often studied in biology classes More on earthworms Lumbricus terrestris is often studied in biology classes Charles Darwin studied earthworms and estimated that 10 to 18 tons of dry earth passed through earthworm intestines per acre annually The worms help churn the soil, mixing materials, and adding nutrients Giant tropical earthworms may reach 4 meters long and live in interconnected tunnels

Form and function of oligochaetes The prostomium overhangs the mouth at the anterior end In most earthworms, each segment bears four pairs of chitinous setae, some may bear over 100 Each seta is a bristle-like rod set in a sac and moved by tiny muscles The setae anchor segments during burrowing

How earthworms move Earthworms move by peristalsis Circular muscles contract, the anterior end lengthens, setae anchor the forward end Longitudinal muscles contract, body shortens, and the posterior end is pulled forward

Feeding habits of Oligochaetes Most are scavengers, feeding on decayed organic matter, leaves, refuse, etc Food is moistened by the mouth and drawn in by a sucking action of the muscular pharynx Digestion and absorption occur in the intestine; an infolded typhlosole increases the surface area Chloragogen tissue surrounds the intestine and helps in digestion The chloragogen cells also function in excretion

Circulation, respiration, and excretion Both coelomic fluid and blood carry food, wastes and respiratory gases Blood circulates in a closed system with five main trunks running lengthwise in the body The dorsal blood vessel above the alimentary canal has valves and functions as a true heart The dorsal vessel pumps blood anteriorly into five pairs of aortic arches Blood contains colorless amoeboid cells and dissolved hemoglobin

Circulation, respiration, and excretion Gaseous exchange (oxygen and carbon dioxide) occurs through the moist skin Each somite (segment) except the first three and the last one have a pair of nephridia Each nephridium occupies parts of two adjacent segments A ciliated funnel, the nephrostome, draws in wastes and leads through the septum These coil until the nephridial duct ends at a bladder that empties outside at the nephridiopore Wastes from the coelom are discharged

Nervous system and sensory organs A pair of cerebral ganglia connects around the pharynx to the ganglia of the ventral nerve cord Fused ganglia in each somite contain both sensory and motor fibers One or more giant axons are located in the ventral nerve cord to increase the rate of conduction and stimulate contractions of muscles in many segments

Reproduction and Development