CHAPTER 17 Annelids Characteristics of the Phylum Annelida Diversity Exhibit segmentation or Bodies composed of units Each unit contains components of most organ systems Increased burrowing efficiency by permitting movement of segments More sophisticated system Provided a safety factor: If one segment ( /somite) failed, others could still function Characteristics About 15,000 species 2/3 are the more obscure worms. Segmentation Body segments marked by circular grooves called Setae Tiny bristles Absent in Short anchor segments in earthworms Prevent it from slipping Long setae help aquatic worms Polychaetes Primarily marine and usually (lowest level of water) Oligochaetes and leeches Freshwaters, or soils Many leeches are
Body Plan Head is composed of 2 parts: Terminal portion us the which bears the anus Head and are not considered metameres New metameres form in front of the Surface is covered with an and a thin outer layer of non-chitinous Strong and muscles underlie the body wall Coelom (mesodermal epithelium) lines body wall and forms dorsal and ventral Peritonea of adjacent segments meet to form the Gut and blood vessels extend through the septa Hydrostatic Skeleton Except in leeches, coelom is filled with fluid and serves as a skeleton Contraction of longitudinal muscles causes body to and expand Contraction of circular muscles causes body to and lengthen By separating this force into sections, widening and move the whole animal Alternate waves of contraction, or, allow efficient burrowing Swimming annelids use ( ) movements
Class Polychaeta Diversity Polychaeta is the class of annelids More than 10,000 species, mostly Vary from 1 mm to 3 long More specialization of organs than in clitellates Tolerate a wide range of Warmer regions have more polychaetes Some live in crevices, others inhabit, or are pelagic Well-differentiated with sense organs Paired appendages called on most segments No Many arranged in bundles on parapodia Polychaetes may be sedentary, free-moving,, or crawling Clamworm Nereis is an example of a predatory polychaete with jaws on an, muscular Form and Function may or may not be retractile Often bears eyes,, and sensory Surrounds mouth and may have setae, palps, or jaws Ciliary feeders may bear a tentacular that opens like a fan but can be withdrawn into the tube Most segments of trunk bear with lobes, cirri, setae, and other parts Aid crawling, swimming, and worm in a tube Usually the chief organ although the worm may also possess gills
Nutrition Polychaetes have a foregut, and hindgut Foregut has a, pharynx, and anterior esophagus lined with cuticle Midgut derived from secretes enzymes and absorbs nutrients Short derived from ectoderm and leads to anus on the pygidium Some polychaetes are or scavengers Sedentary polychaetes feed on particles or particles in sediment Circulation and Respiration Most have parapodia and gills for exchange while others use the body surface Circulation varies In Nereis a vessel carries blood forward and a vessel carries blood posteriorly Blood flows across between these major vessels in networks around the parapodia and Excretion Excretory organs vary, from to metanephridia, and mixed forms Nervous System and Sense Organs nerve cord runs length of the worm with ganglia in each metamere Sense organs include Eyes vary from simple to well-developed image-resolving eyes similar to mollusc eyes
organs are ciliated sensory pits that are probably chemoreceptive Some burrowing and tube-building polychaetes use to orient their body Reproduction and Development Polychaetes have no permanent organs Dioecious Gonads appear as simple swellings of peritoneum Gametes are shed into coelom and exit by gonoducts, metanephridia, or of the body Fertilization is and the early larva is a Clitellata Class Oligochaeta and Class Hirudinida Form reproductive structure called a Ring of cells found in a band around the body in oligochaetes but visible only during reproductive season in leeches lack (monoecious) animals that exhibit development Young develop inside a secreted by the, and emerge as small worms Class Oligochaeta Diversity Over 3000 species Occur in habitats from soil to Few are marine or Nearly all bear Fewer in number than in polychaetes
Form and Function Sometimes called crawlers Burrow in moist rich soil and usually live in branched interconnected Damp, rainy weather: Remain near Dry weather: Burrow deep underground and go coiled in a slime chamber Peristaltic movements Contractions of muscles in the anterior end lengthen the body, pushing the anterior end forward where it anchors Anchoring is accomplished by contraction of the muscles in forward segments Causes the segments to become and wide, pushing against the burrow Setae Bristlelike rods set in a sac and moved by tiny Project outward through small in cuticle Aid by digging into walls of burrow Nutrition Scavengers, feeding on decayed matter, leaves, refuse, etc. Food moistened by mouth and drawn in by a sucking action of muscular Food is stored in a thin-walled Muscular grinds food into small pieces Digestion and absorption occur in Circulation and Respiration Coelomic fluid and transport food, wastes, and respiratory gases Blood circulates in a system with main trunks running lengthwise in the body vessel contains valves and functions as a true Pumps blood anteriorly into 5 pairs of arches Aortic arches ensure steady pressure in vessel
Ventral vessel serves as an aorta, delivering blood to body walls,, and digestive tract Blood contains ameboid cells and dissolved No special gaseous exchange organs Gas exchanged across surface Excretion Each segment, except the 1 st three and terminal one, have a pair of A ciliated funnel, the, draws in wastes and leads through the septum These coil until the duct ends at a bladder that empties outside at Wastes from both the coelom and the blood beds are discharged Nervous System and Sense Organs Central nervous system and nerves Pair of cerebral connect around the pharynx to the ganglia of the nerve cord Fused in each segment contain both sensory and motor fibers Neurosecretory cells in brain and ganglia secrete Regulate reproduction, secondary sex characteristics, and Lack eyes but have many in the epidermis Free nerve endings in tegument are probably structures General Behavior Avoid bright light ( phototaxis) stimuli are important in locating food
Reproduction and Development Monoecious Reproductive structures are located in 9 through 15 Sperm produced by mature in vesicles and pass into ducts Eggs are discharged by into coelomic cavity Ciliated transport them outside Two pairs of receptacles receive and store sperm Mate at during warm, moist weather Aligning in directions, ventral surfaces together Mucus secreted by holds worms together Sperm from each worm are transported to the receptacles of the other along grooves After mutual copulation, each worm secretes a mucus tube and chitinous band to form a Cocoon passes forward and eggs,, and sperm are added Fertilization and occur in cocoon Young worms emerge from cocoon Class Hirudinida: Leeches 34 segments, lack and possess anterior and posterior suckers Diversity Mostly, few marine or live in moist terrestrial environments More common in the temperate zones Vary in color: black, brown, red, and olive green Most are Some feeding on small invertebrates Others are temporary or permanent
Hermaphroditic Form a during breeding season Secretes a for reception of eggs Form and Function Usually have a fixed number of Appear to have more due to superficial No No setae Developed for attachment Gut specialized for storage of large quantities of Most use suckers to attach and along surfaces Nutrition Not all are parasites, many are Freshwater leeches have a for ingesting small invertebrates as well as to suck blood Some terrestrial leeches feed on insect larvae, earthworms, and slugs Other terrestrial leeches climb trees or bushes to reach warm-blooded such as baby birds Most are feeders that prefer tissue fluids and blood pumped from open wounds Respiration and Excretion Some fish leeches have All other leeches exchange gases across 10 to 17 pairs of Circulation Some have a typical oligochaete system Some lack blood vessels and coelomic serve as only vascular system
Nervous and Sensory Systems Two fused ganglia form a ring around the Seven pairs of posterior fused ganglia 21 pairs of segmental ganglia in between along a nerve cord Epidermis contains free sensory nerve endings and cells Row of in central annulus of each segment. Pigment-cup are present Reproduction and practice -fertilization Sperm transferred by impregnation secretes cocoon to receive sperm and egg are buried in mud or damp soil is similar to that of oligochaetes