CHAPTER 17 Annelids 17-1
Characteristics of the Phylum Annelida Diversity Exhibit segmentation or metamerism Bodies composed of repeated units Each unit contains components of most MAJOR organ systems Increased burrowing efficiency by permitting independent movement of segments More sophisticated nervous system Provided a safety factor: If one segment (metamere/ somite) failed, others could still function 17-2
Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Characteristics About 15,000 species 2/3 are the more obscure marine worms. Segmentation Body segments marked by circular grooves called annuli Setae Tiny chitinous bristles Absent in leeches Short setae anchor segments in earthworms Prevent it from slipping backward Long setae help aquatic worms swim Polychaetes Primarily marine and usually benthic (lowest level of water) Oligochaetes and leeches Freshwaters, or terrestrial soils Many leeches are predators 17-3
Body Plan Head is composed of 2 parts: Prostomium Perstonium Terminal portion us the pygidium which bears the anus Head and anus are not considered metameres New metameres form in front of the anus Surface is covered with an epidermis and a thin outer layer of non-chitinous cuticle Strong circular and longitudinal muscles underlie the body wall 17-4
Coelom Peritoneum (mesodermal epithelium) lines body wall and forms dorsal and ventral mesenteries Peritonea of adjacent segments meet to form the septa Gut and longitudinal blood vessels extend through the septa 17-5
Hydrostatic Skeleton Except in leeches, coelom is filled with fluid and serves as a hydrostatic skeleton Contraction of longitudinal muscles causes body to shorten and expand Contraction of circular muscles causes body to narrow and lengthen By separating this force into sections, widening and elongation move the whole animal Alternate waves of contraction, or peristalsis, allow efficient burrowing Swimming annelids use (wavelike) movements 17-6
Class Polychaeta Diversity Polychaeta is the largest class of annelids More than 10,000 species, mostly marine Vary from 1 mm to 3 meters long More specialization of sensory organs than in clitellates Tolerate a wide range of salinity Warmer regions have more freshwater polychaetes Some live in crevices, others inhabit tubes, or are pelagic 17-7
Well-differentiated head with sense organs Paired appendages called parapodia on most segments No clitellum Many setae arranged in bundles on parapodia Polychaetes may be sedentary, freemoving, burrowing, or crawling Clamworm Nereis is an example of a predatory polychaete with jaws on an eversible, muscular pharynx 17-8
Form and Function Prostomium may or may not be retractile Often bears eyes, tentacles, and sensory palps Surrounds mouth and may have setae, palps, or chitinous jaws Ciliary feeders may bear a tentacular crown that opens like a fan but can be withdrawn into the tube Most segments of trunk bear parapodia with lobes, cirri, setae, and other parts Aid crawling, swimming, and anchor worm in a tube Usually the chief respiratory organ although the worm may also possess gills 17-9
Nutrition Polychaetes have a foregut, midgut and hindgut Foregut has a stomodeum, pharynx, and anterior esophagus lined with cuticle Midgut derived from mesoderm secretes enzymes and absorbs nutrients Short hindgut derived from ectoderm and leads to anus on the pygidium Some polychaetes are predators or scavengers Sedentary polychaetes feed on suspended particles or particles in sediment 17-10
Circulation and Respiration Most have parapodia and gills for gaseous exchange while others use the body surface Circulation varies In Nereis a dorsal vessel carries blood forward and a ventral vessel carries blood posteriorly Blood flows across between these major vessels in networks around the parapodia and intestine Excretion Excretory organs vary, from protonephridia to metanephridia, and mixed forms 17-11
Nervous System and Sense Organs Ventral nerve cord runs length of the worm with ganglia in each metamere Sense organs include Eyes vary from simple eyespots to well-developed image-resolving eyes similar to mollusc eyes Nuchal organs are ciliated sensory pits that are probably chemoreceptive Some burrowing and tubebuilding polychaetes use statocysts to orient their body eyes 17-12
Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Reproduction and Development Polychaetes have no permanent sex organs Dioecious Gonads appear as simple temporary swellings of peritoneum Gametes are shed into coelom and exit by gonoducts, metanephridia, or rupturing of the body Fertilization is external and the early larva is a trochophore 17-13