Earthworm Reproduction Earthworms are hermaphrodites where each earthworm contains both male and female sex organs. The male and female sex organs can produce sperm and egg respectively in each earthworm. Although earthworms are hermaphrodites, most need a mate to reproduce. During mating, two worms line up inverted from each other so sperm can be exchanged.
The earthworms each have two male openings and two sperm receptacles, which take in the sperm from another mate. The earthworms have a pair of ovaries that produce eggs. The clitellum will form a slime tube around it, which will fill with an albuminous fluid. The earthworm will move forward out of the slime tube. As the earthworm passes through the slime tube, the tube will pass over the female pore picking up eggs. The tube will continue to move down the earthworm and pass over the male pore called the spermatheca which has the stored sperm called the spermatozoa.
The eggs will fertilize and the slime tube will close off as the worm moves completely out of the tube. The fertilized eggs will develop and become young worms. Figure 2. This schematic drawing shows the basic anatomy of annelids in a cross-sectional view.
Annelids possess a closed circulatory system of dorsal and ventral blood vessels that run parallel to the alimentary canal as well as capillaries that service individual tissues.
In addition, these vessels are connected by transverse loops in every segment. These animals lack a well-developed respiratory system, and gas exchange occurs across the moist body surface. Annelids show well-developed nervous systems with a nerve ring of fused ganglia present around the pharynx.
The nerve cord is ventral in position and bears enlarged nodes or ganglia in each segment. Annelids may be either monoecious with permanent gonads as in earthworms and leeches or dioecious with temporary or seasonal gonads that develop as in polychaetes. However, cross-fertilization is preferred in hermaphroditic animals. These animals may also show simultaneous hermaphroditism and participate in simultaneous sperm exchange when they are aligned for copulation. Phylum Annelida contains the class Polychaeta the polychaetes and the class Oligochaeta the earthworms, leeches and their relatives.
The many chetae of polychaetes are also arranged within fleshy, flat, paired appendages that protrude from each segment called parapodia , which may be specialized for different functions in the polychates. The subclass Hirudinea includes leeches such as Hirudo medicinalis and Hemiclepsis marginata. The class Oligochaeta includes the subclass Hirudinia and the subclass Brachiobdella. Flatworms have a surprisingly elaborate system to rid the body of wastes Fig.
This network runs the length of the animal on each side and opens to the outside through small pores in the posterior region of the body. Connected to the tubes are tiny cells that move wastes and water from the tissues into the tubes.
These cells contain flagella that beat back and forth, creating a current of fluid that constantly moves toward the excretory pores. Under a microscope the flagellar movement looks like a flickering fire, and the structure is called a flame bulb. Flatworms have no circulatory system.
Animals without a circulatory system have limited abilities to deliver oxygen and nutrients to their body cells because of the way that molecules behave.
As molecules spread through water, they become less concentrated as they move away from their source. This is known as diffusion. But cnidarians have no problem with diffusion because most cells of their bag-shaped bodies are in direct contact with the water, making the exchange of oxygen and nutrients easy Fig.
Flatworms, bag-shaped but flattened, also get oxygen and nutrients to their body cells easily because all their cells are close to either their outer surface or their digestive cavity Fig. As animals become larger and more complex, diffusion is often no longer an option, and then we begin to see the development of circulatory and respiratory systems. Species in the phylum Nematoda from the Greek root word nema meaning thread are better known as the roundworms Fig.
There are about 25, species of nematodes formally described by scientists. Nematodes are found in almost every habitat on Earth. One species was first discovered living inside felt beer coasters in German alehouses. Studies of farmlands have found as many as 10, nematodes in cubic centimeters cm 3 of soil. Nematodes are similarly abundant in marine and freshwater sediments where they serve as important predators, decomposers, and prey for other species like crabs and snails.
Like flatworms, roundworm species adopt either a free-living or a parasitic lifestyle. Parasitic nematodes Fig. Many nematodes that are parasitic on plants can devastate crops. Some nematodes are cryptobiotic and have demonstrated a remarkable ability to remain dormant for decades until environmental conditions become favorable.
Like the flatworms, nematodes are bilaterally symmetrical. They take their name from their round body cross-sectional shape. Unlike the flatworms in which food and waste enter and exit from the same opening, nematodes have a complete digestive system. An animal with a complete digestive system has a mouth at one end, a long tube with specialized parts in the middle, and an anus at the other end.
With a complete digestive system an animal can eat while its previous meal digests. Parts of the digestive system can specialize to do different jobs, digesting food in stages Fig.
As the food moves along, it is broken into molecules and absorbed by the cells lining the tube. Muscles surrounding the tube contract, squeezing the food and pushing it along in a process called peristalsis.
Indigestible wastes pass out through the anus. Unlike flatworms, nematodes are slender, and they are covered by a protective cuticle. A cuticle is a waxy covering secreted by the epidermis, or outermost cellular tissue. Because of this covering, gas exchange cannot occur directly across the skin as in flatworms. Rather, gas exchange and waste excretion in nematodes occurs by diffusion across the wall of the gut. Although nematodes do have a space in the body between the digestive tract and the body wall, it is not lined with tissue and is not considered to be a true coelom.
Thus, nematodes are sometimes referred to as pseudocoelomates Fig. Most worms have two bands of muscles: longitudinal muscles that run the length of the body and circular muscles that form circular bands around the body. Unlike other worms that have two bands of muscles, nematodes only have longitudinal muscles.
This explains their characteristic thrashing movement, as they can move only by contracting the long muscles on either side of their body and wriggling forward. The nervous system of nematodes consists of a set of nerves that run the length of the body and connect to anterior ganglia. Segmentation can help the earthworm move. Each segment or section has muscles and bristles called setae. The bristles or setae help anchor and control the worm when moving through soil. The bristles hold a section of the worm firmly into the ground while the other part of the body protrudes forward.
Annelid, phylum name Annelida, also called segmented worm, any member of a phylum of invertebrate animals that are characterized by the possession of a body cavity or coelom , movable bristles or setae , and a body divided into segments by transverse rings, or annulations, from which they take their name.
The segmented body plan of an annelid worm with both circular and lengthwise muscles is the ideal structure for active burrowing. A one-way gut runs through the body, and they have both a circulatory system and nervous system. A segmented body is when the body is divided to segments were the segments piggyback on one another. Sometimes, sets of segments are grouped into a larger unit, such as the thorax or the abdomen. Some examples of animals with a segmented body include: crayfish, worms, grasshoppers, arthropods, annelids, etc.
Segmented things are divided into separate parts, and segmented animals have bodies made up of distinct sections. An earthworm is one example of a segmented animal.
Segmentation provides the means for an organism to travel and protect its sensitive organs from damage. The ability to divide functions into different portions of the body allows an organism to perform increasingly complex activities and use different segments to perform varying functions.
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