WEEK 9 HANDOUT FOR INVERTEBRATE ZOOLOGY, BIOLOGY 03050: THE ANNELIDS
10/25/99 Text: 7th ed., Ch. 22. Dr. W. Crone (303 FTZ, 629-7439, cronewil@hvcc.edu, http://www.hvcc.edu/academ/faculty/crone/index.html)
possible web sites: http://www.yucky.com/worm/index.ssf?/earthworms.html
(cartoonish yet informative site on earthworms and other worm-like organisms)
http://www.york.biosis.org/zrdocs/zoolinfo/grp_worm.htm (lots of annelid and other worm links)
1. Phylum Annelida ("little-ringed"): Elongate, segmented, worm-like phylum of around 15,000 species. Three classes: Polychaeta--marine clam worms and tube worms, Oligochaeta--earthworms, Hirudinea--leeches. Annelids contain a true coelom (eucoelom), a complete gut with mouth and anus, a closed circulatory system, and segmented nervous and excretory systems.
2. Formation of true coelom and segments in annelids. Ectoderm (skin and nerves), mesoderm (muscle, skeleton, and internal organs such as kidneys, gonads, and circulatory organs), and endoderm (gut lining) are germ layers in the forming embryo. Cavities form in segments within mesoderm, so that the adult worm has septa (like bulkheads) and mesenteries (organ attachment sheets) formed of the (mesodermally-derived) lining of peritoneum that lines the body cavity or coelom.
Metamerism: "condition of being made of serially repeated parts (metameres); serial segmentation" (Glossary, p. 716)
Aspects of functional morphology. What is the use of the metameric design?3
A) For locomotion: use of a hydrostatic skeleton to move a worm form. Remember, muscles can only contract. So muscles not only act as antagonists, but can present a thrust against the environment. Also different segments can be expanded--the worm is not easily pulled out by the bird.
B) For repair: if a part of the metameric body gets damaged, the worm can easily repair or at least minimize damage. Earthworms have a good regenerative capacity.
C) For possible specialization or tagmatization: Not seen so much in the earthworm as in the polychaetes, so that can have physiological specialization along the way, e.g. head for eating, respiration segments, excretion segments, etc.
Class Oligochaeta ("few long hairs"): the earthworms. Highlights of the earthworm, which many of you have seen before, especially with comparisons to human systems--i.e., there's enough complexity here to make more direct comparisons than with previous organisms!
3. External and locomotion:
Human skin for protection; locomotion from attachments of muscles to internal skeleton (endoskeleton). Earthworm skin moist for respiration, with a thin cuticle for protection secreted by the skin. The earthworm is segmented throughout, externally seen as annuli or rings. Swollen segments near the anterior end make up the clitellum (see below under reproduction). Locomotion involves the hydrostatic skeleton in conjunction with circular and longitudinal (lateral) muscles, with setae (bristles) for traction.
4. Digestive system:
Human digestive system, from mouth to anus. Stomach main storage organ, omnivorous diet. Earthworm digestive system runs the length of worm, with soil-based diet. A muscular pharyx brings in food from the mouth. The esophagus is followed by a crop (storage) and gizzard (grinding). Calciferous glands along esophagus helps with Ca+2 intake and pH regulation.1 A long, straight intestine (digestion and absorption) contains an internal dorsal fold of typhlosole (increased surface area) and external covering of chloragogen tissue (functions as liver), before castings produced at the posterior anus.
5. Circulatory system:
Human closed circulatory system, with one heart actively pumping. The earthworm also has a closed circulatory system, with blood entirely contained within dorsal and ventral vessels. Several pumping sites in dorsal vessels around esophagus may act as"hearts," although more pumping comes from the main dorsal ventral vessel.2
6. Excretory system:
Human kidneys filter blood from the circulatory system. In the earthworm, there are a pair of"kidney-like" structures per segment called nephridia that filter the coelomic body fluid. Each nephridium has a funnel-like tubule and its own pore to the outside.
7. Nervous system:
Human dorsal central nervous system. The earthworm has a ventral nerve cord with segmental ganglia, and lateral nerves along the body rings (annuli) extending from ganglia. Connectives reach up from ventral nerve cord to dorsal cerebral ganglia that form a"brain".
8. Reproductive system:
Humans, separate sexes, internal fertilization and fetal maturation. Earthworms are hermaphroditic but reproduce by cross mating and fertilizing. Internally, testes are associated with seminal vesicles for own sperm storage and ovaries are associated with oviducts for egg deposition out the female pore. Female pores are found in segment (somite) # 14, male pores in # 15, seminal grooves from # 15 back towards clitellum. Order of events: Two worms meet up and meet up so that the clitellum of one is opposite the seminal receptacles of the other in segments # 9, 10. Sperm comes out of the male pore in # 15 of one worm, down seminal grooves towards clitellum, where it is deposited into the seminal receptacles at # 9, 10 of the other worm. After two or three hours of copulating, the worms separate and each wiggles off a slime tube from around the clitellum toward the head. First the eggs are taken up into the slime tube, and then the sperm in the seminal receptacles in # 9, 10 is also included. The slime tube acts as a cacoon for the eggs to hatch. Earthworms, as do leeches, show direct development, with no separate larval stage (as vs. polychaetes below).2
9. The burrowing life. An earthworm uses its prostomium as a burrowing probe, since can elongate with contraction of circular muscles. Earthworms don't really have eyes, rather scattered photoreceptors, since live in ground. What do earthworms eat? Earthworms burrow in soil (or on surface at night) to ingest soil along with all kinds of organic matter especially leaves and other dead plant material. As Charles Darwin showed, earthworms modify the soil structure by passing soil through their bodies and casting it towards the surface. Furthermore, earthworms aerate soil and improve drainage and bring organic materials down into the soil and bring inorganic material from the subsoil up into the vegetation.1,2
10. Class Polychaeta ("many long hairs"): clam and tube worms. These are inconspicuous but common marine animals, living burrowed in sand, under rocks, or within tubes. Lateral appendages on each segment called parapodia have setae attached to their ends, so that movement in polychaetes different than in earthworms. Large bands of longitudinal muscle will assist in the clam worm"wriggle." Bands of oblique muscle are associated with parapodium function. Also in contrast to the earthworms, polychaetes like the clam worm have much more of a head, including a prostomium with four eyes, two tentacles, two palps, and the peristomium has four pairs of tentacles in addition to the oral apparatus. These tentacles can become ciliated in tube worms for filter feeding.
11. Class Hirudinea (" leechlike"): leeches. Mostly fresh water, leeches have dorsal-ventrally flattened bodies with suckers on each end. Instead of internal septa, many are full of connective tissue and muscle in oblique as well as circular and longitudinal bands. Leeches can swim with dorsoventral undulations. On a solid surface, leeches move as one unit, first attached by the larger posterior sucker, then reaching out with smaller anterior sucker. While some scavenge for food or eat other invertebrates, leeches are better known for bloodsucking, attacking snails, fish, turtles, and other aquatic (or bathing!) animals. Use of anticoagulant in saliva keeps blood flowing.
12. Evolutionary relationships of annelids. Generally agreed to be close to the arthropods (especially the Uniramia, insects and centipedes/millipedes). Similar patterns to the arthropods with metamerism, pattern, a secreted external cuticle (which gets quite thick in the arthropods!), and a ventral nerve cord. Differences between the arthropods and annelids can be explained in terms of the cuticle characteristics and locomotion.2,4
1 R Buchsbaum et al., Animals Without Backbones, 3rd ed. (U of Chicago Press, Chicago, 1987), pp. 295-296
2 CP Hickman et al., Biology of Animals, 7th ed. (WCB McGraw-Hill, Boston, 1998), pp. 480, 481, 486
3 SA Miller, JP Harley, Zoology, 2nd ed. (WC Brown, Dubuque, IA, 1994), p. 333
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This page updated on October 25, 1999