Segmentation: The Annelid Worms (1961)

the earthworm has the distinction of belonging to a group of animals that show an important evolutionary development they and many other animals in the phm anela are clearly and obviously segmented anela means ringed or grooved the ringed condition is more often known as segmentation each ring is called a segment we shall use these blocks to illustrate one of the theories of how segmentation may have evolved in the distant past there may have been an ancestors of the anelids that reproduced by merely pinching in two as some of the modern lower animals do among these ancestors perhaps some fail to divide completely this failure may have occurred repeatedly in the same animal with each segment retaining many representative parts of each organ system in the modern analid body each segment has in some ways the construction of a whole whole animal each has a protective outer covering muscle tissue a camic cavity lined with a layer of cells called the peritoneum part of the digestive tube excretory organs neia in most segments and a part of the nerve cord segmentation helps an animal to act in a more coordinated way in this diagram of a longitudinal section the brain is connected to the ganglia of the nerve cord which runs through segments of the body each gangion controls that segment of the body in which it is located the signal from the brain move along the chain of ganglia at a high rate of speed as each ganglion receives its signals It Takes Over Control of its own segment the brain can stop the signals as well as start them thus by dividing the body into segments all of the body functions may be more effectively carried out than in an unsegmented body some nervous impulses coordinate the movement of the body and extension and retraction of the bristles the bristles enable the earth worm to hold on to the slippery sides of the burrow and withdraw into it other nervous impulses coordinate the movements of the digestive system the digestive tube of the earthworm is specialized along its length each part has a special function the mouth leads into a muscular FX next surrounded by three pairs of glands that secrete calcium salts is the esophagus followed by the crop the gizzard and intestine muscular contractions move food through the digestive system in a continuous Passage we will follow the food to show how the system functions in the mouth salival like secretions moisten the food strong muscular contractions of the ferx action push the food into the esophagus where secretions of glands neutralize organic acids that may be present pistolic movements Force the food into the crop which seres serves as a temporary storage sack some of the food moves into the gizzard where strong muscular contractions thoroughly grind it in the intestine digestive juices break down the food into simpler substances which can be absorbed into the blood vessels that carry it throughout the body to burn the food the earthworm requires oxygen which it absorbs through the moist epidermis such absorption called respiration requires a moist skin that is why they live in damp soil and come out of their Burrows only at night when the air is humid another member of the Pham anela is the hudia or the leeches they live in fresh and salt water and in damp places on land the number of external rings does not indicate the true number of internal segments they have no bristles suckers at each end of the body enable the leech to hold onto surfaces as it moves or feeds the head or or anterior sucker surrounds the mouth most leeches suck the blood of vertebrates as this leech is doing on a fish leeches are used by some people for the treatment of black eyes the largest and most generalized class of anelids is the polyes poly means many bristles polys live in the sea some are free swimming others live in tubes or Burrow under rocks or in the mud or sand this is nerus The Familiar sandworm of the New England coast paired flap-like extensions called parapod occur on each segment of its body except for the head and last segment bristles extending from the parapod aid in crawling and in swimming the flattened parapod are gills and represent one of the earliest types of breathing organs in the animal kingdom on the inner wall of the FX are two large Jaws by extending the fings through the mouth the Jaws can grasp food the reproductive system of naras is very simple and is similar to that in many Marine worms sex cells are budded off from the lining of the celum eggs from the female and sperms from the male are shed into the water where they unite the precise timing for the shedding of eggs and sperm coincides with a phase of the moon here's another seaw worm it has many tent at its head which reach out in all directions and pick up tiny particles of food it builds a tube of sand grains cemented together with mucus under a rock the worm also uses its tenticles to pull itself along it has brightly colored spongy gills for breathing the painful sting of this worm's bristles when handled has given given it the name fireworm the body surface of an analy is sensitive to light the reaction of the two rows of gills demonstrates its sensitivity to light one of the most unusual Marine anals is the one that lives here the ends of its tube are spaced about 10 to 12 in apart we'll dig one out of the sand the parchment like u-shaped tube is formed by a hardening of a mucus secretion from the worm's body here we have placed a glass window in front of a cutaway section to show how the tube dweller kopus lives this poly has a distinct head and tentacles the parapod are highly specialized for various functions these form a food Gathering cup these fan-like parapod produce a flow of water through the tube most of the other body segments have rows of parapod used in breathing and Locomotion a net of mucus invisible in the water traps tiny food particles the food cup picks up the mucus net with the entrapped food and forms it into a small pellet reversing the current flow the worm passes the food ball to its mouth and swallows it most colorful of the anelids are the flower worms these dainty polys live within various kinds of tubes built by secretions from their body IES when Disturbed the worms pop back into their tubes they breathe through their feathery gills which also filter minute food particles from the water beside earthworms leeches and polys there is another group of anelids one example is poly Gord while this worm appears to be unsegmented its larvae clearly shows internal segmentation the polygordius is either a primitive or a degenerate form that has lost its parapod and bristles it does not possess a retractable ferx yet it is a true anald with most analid characteristics the larvae of the polygordius look very much like that of a mollusk this indicates that anelids and mollusks are closely related a study of successive stages in the growth of a typical poly tror larvae shows how the segments develop the earthworm as a land anald differs greatly in method of reproduction from the Marine forms earthworms are hermaphroditic each individual possesses both both male and female sex organs since there is no water in which to shed sex cells the worms must transfer their sex cells directly during mating mucus from a collar-like ring called the clitellum is secreted by both worms to form a slime tube in preparation for the exchange of sperm this exchange is necessary because cannot fertilize its own eggs the sperms leave their respective seminal vesicle openings and travel along separate grooves to enter storage sacks the seminal receptacles of the opposite worm after mating the worms separate and return to their Burrows later in its burrow when the eggs are mature and ready for discharge the worm forms a slime tube over the clitellum and interior segments then it secretes nourishment into the tube next the worm discharges eggs from the female pore into the tube as the worm withdraws from the tube the tube passes over the openings of the seal receptacles and receives the stored sperms the Slime tube and now fertilized eggs slide off the worm's body to the floor of the the burrow there it dries into a hard cocoon from which the young earthworms will later emerge in certain tropical regions of the world giant earthworms can be found here is one from South America compared with an ordinary American earthworm one of these two from Australia is over 30 in in length the Burrows of these giant worms have volcano shaped openings earthworms are among the most important animals of the world other animals use them as food we use them as bait for fishing their greatest contribution to man however comes from the their burrowing activities by passing the soil through their bodies it is chemically enriched and is loosened and air rated whatever their kind or use worms of the philm anela are and the seom