Type annelids general characteristics. General characteristics of the type annelids - Annelida
Annelids have the highest organization compared to other types of worms; For the first time, they have a secondary body cavity, a circulatory system, and a more highly organized nervous system. In annelids, inside the primary cavity, another, secondary cavity has formed with its own elastic walls made of mesoderm cells. It can be compared to airbags, one pair in each segment of the body. They “swell”, fill the space between the organs and support them. Now each segment received its own support from the bags of the secondary cavity filled with liquid, and the primary cavity lost this function.
They live in soil, fresh and sea water.
External structure
The earthworm has an almost round body in cross section, up to 30 cm long; have 100-180 segments, or segments. In the anterior third of the body there is a thickening - the girdle (its cells function during the period of sexual reproduction and egg laying). On the sides of each segment there are two pairs of short elastic setae, which help the animal when moving in the soil. The body is reddish-brown in color, lighter on the flat ventral side and darker on the convex dorsal side.
Internal structure
A characteristic feature of the internal structure is that earthworms have developed real tissues. The outside of the body is covered with a layer of ectoderm, the cells of which form the integumentary tissue. The skin epithelium is rich in mucous glandular cells.
Muscles
Under the cells of the skin epithelium there is a well-developed muscle, consisting of a layer of circular muscles and a more powerful layer of longitudinal muscles located under it. Powerful longitudinal and circular muscles change the shape of each segment separately.
The earthworm alternately compresses and lengthens them, then expands and shortens them. Wave-like contractions of the body allow not only crawling through the burrow, but also pushing the soil apart, expanding the movement.
Digestive system
Digestive system begins at the front end of the body with a mouth opening, from which food enters sequentially into the pharynx, esophagus (in earthworms, three pairs of calcareous glands flow into it, the lime entering the esophagus from them serves to neutralize the acids of rotting leaves on which the animals feed). Then the food passes into the enlarged crop and a small muscular stomach (the muscles in its walls help grind the food).
The midgut stretches from the stomach almost to the posterior end of the body, in which, under the action of enzymes, food is digested and absorbed. Undigested remains enter the short hindgut and are thrown out through the anus. Earthworms feed on half-rotted plant remains, which they swallow along with the soil. As it passes through the intestines, the soil mixes well with organic matter. Earthworm excrement contains five times more nitrogen, seven times more phosphorus and eleven times more more potassium than normal soil.
Circulatory system
The circulatory system is closed and consists of blood vessels. The dorsal vessel stretches along the entire body above the intestines, and below it is the abdominal vessel.
In each segment they are united by a ring vessel. In the anterior segments, some annular vessels are thickened, their walls contract and pulsate rhythmically, thanks to which blood is driven from the dorsal vessel to the abdominal one.
The red color of blood is due to the presence of hemoglobin in the plasma. It plays the same role as in humans - nutrients dissolved in the blood are distributed throughout the body.
Breath
Most annelids, including earthworms, are characterized by cutaneous respiration; almost all gas exchange is provided by the surface of the body, therefore the worms are very sensitive to moist soil and are not found in dry sandy soils, where their skin quickly dries out, and after rains, when in the soil a lot of water, crawling to the surface.
Nervous system
In the anterior segment of the worm there is a peripharyngeal ring - the largest accumulation of nerve cells. The abdominal nerve cord with nodes of nerve cells in each segment begins with it.
This nodular type nervous system was formed by the fusion of nerve cords on the right and left sides of the body. It ensures the independence of the joints and the coordinated functioning of all organs.
Excretory organs
The excretory organs look like thin, loop-shaped, curved tubes, which open at one end into the body cavity and at the other outside. New, simpler funnel-shaped excretory organs - metanephridia excrete harmful substances in external environment as they accumulate.
Reproduction and development
Reproduction occurs only sexually. Earthworms are hermaphrodites. Their reproductive system is located in several segments of the anterior part. The testes lie in front of the ovaries. When mating, the sperm of each of the two worms is transferred to the seminal receptacles (special cavities) of the other. Cross fertilization of worms.
During copulation (mating) and oviposition, girdle cells on the 32-37 segment secrete mucus, which serves to form an egg cocoon, and a protein liquid to nourish the developing embryo. The secretions of the girdle form a kind of mucous coupling (1).
The worm crawls out of it with its back end first, laying eggs in the mucus. The edges of the coupling stick together and a cocoon is formed, which remains in the earthen hole (2). Embryonic development of eggs occurs in a cocoon, from which young worms emerge (3).
Sense organs
The sense organs are very poorly developed. U earthworm there are no real organs of vision, their role is played by individual light-sensitive cells located in skin. The receptors for touch, taste, and smell are also located there. Earthworms are capable of regeneration (easily restore the back part).
Germ layers
The germ layers are the basis of all organs. Annelids have ectoderm (outer layer of cells), endoderm ( inner layer cells) and mesoderm (intermediate layer of cells) appear at the beginning of development as three germ layers. They give rise to all major organ systems, including the secondary cavity and the circulatory system.
These same organ systems are subsequently preserved in all higher animals, and they are formed from the same three germ layers. This is how higher animals repeat in their development evolutionary development ancestors
Annelids (ringworms) are a large type (about 9 thousand species) of higher free-living marine, freshwater and soil animals, having more complex organization than flatworms and roundworms. This primarily applies to marine polychaete worms, which are a key group in the evolution of higher invertebrates: mollusks and arthropods descended from their ancient ancestors.
The main progressive features of the ring structure are as follows: :
1. The body consists of numerous (5-800) segments (rings). Segmentation is expressed not only in external, but also in internal organization, in the repeatability of many internal organs, which increases the survival rate of the animal with partial damage to the body.
2. Groups of segments similar in structure and function in polychaete worms are combined into body parts - head, trunk and anal lobe. The head section was formed by the fusion of several anterior segments.
3. Secondary body cavity (whole) filled with coelomic fluid.
Figure 11.7. Head end of the Nereid: I1-eyes; 2 - tentacles; 3 - antennae; 4 - parapodia with tufts of setae.
4. Skin-muscle bag consists of a thin elastic cuticle, a single-layer epithelium located underneath it and two muscle layers: the outer - circular, and the inner - highly developed longitudinal.
5. Specialized ones that appeared for the first time organs of movement - parapodia- are lateral bilobed outgrowths of the body walls of the trunk segments into which the coelom extends. Both lobes (dorsal and ventral) bear more or less number of setae (Fig. 11.7). In oligochaete worms there are no parapodia, there are only tufts with a few setae.
6. B digestive system Having three sections, the foregut is highly differentiated into a number of organs (mouth, pharynx, esophagus, crop, stomach).
7. First developed closed circulatory system. It consists of large longitudinal dorsal and abdominal vessels, connected in each segment by annular vessels (Fig. 11.8). The movement of blood is carried out due to the pumping activity of the contractile areas of the spinal cord, and less commonly of the annular vessels. Blood plasma contains respiratory pigments similar to hemoglobin, thanks to which ringworms have populated habitats with very different oxygen content.
8. Respiratory system in polychaete worms - gills; these are thin-walled, leaf-shaped, feathery or bushy outer outgrowths of part of the dorsal lobes of the parapodia, penetrated by blood vessels. Oligochaete worms breathe over the entire surface of their body..
9. Excretory organs- metanephridia located in pairs in each segment, removing the final waste products from the cavity fluid.
10. Nervous system ganglion type. It consists of paired suprapharyngeal and subpharyngeal ganglia, connected by nerve trunks into a peripharyngeal nerve ring, and many pairs of ganglia of the ventral nerve cord, one pair in each segment (Fig. 11.8, a). Sense organs are diverse: vision (in polychaete worms), touch, chemical sense, balance.
11. Overwhelming majority kolchetsov- dioecious animals, less often hermaphrodites. In most aquatic ringlets, fertilization is external, while in soil forms it is internal. Development with metamorphosis (in polychaete worms) or direct (in polychaete worms, leeches). Some types of ringworms, in addition to sexual reproduction, also reproduce asexually (by fragmentation of the body with subsequent regeneration of the missing parts).
12. Phylum Annelids are divided into three classes - Polychaetes, Oligochaetes and Leeches.
Class Oligochaetes
Its representatives are mainly soil dwellers, but freshwater forms are also known. The structure of the oligochaetes is largely determined by the soil way of life, due to which the organization of the worms has been simplified. The head section has a simple structure and is devoid of sensory organs. Parapodia are absent, although a limited number of setae are preserved. All oligochaetes are hermaphrodites. The reproductive system is concentrated in a few segments of the anterior part of the body, fertilization is internal.
Earthworms live in moist, humus-rich soil. The body is elongated, the segmentation is homogeneous. On each segment, the remaining eight setae are arranged in two rows on the sides of the body. Clinging to uneven soil, the worm, with the help of the muscles of a powerful skin-muscular sac, moves forward.
The digestive system has a number of significant structural features. Its anterior section is differentiated into the muscular pharynx, esophagus, crop and muscular stomach. The ducts of the calcareous glands open into the cavity of the esophagus. Their secretions neutralize the acids that the food consumed by worms is rich in. In the midgut, food is digested and absorbed.
The movement of blood in a closed circulatory system is carried out by contraction of the five anterior Maltsev vessels (“hearts”).
Breathe earthworms the entire surface of its wet body due to the presence of a dense subcutaneous network of blood vessels.
Earthworms are hermaphrodites. Cross fertilization. To do this, two worms are applied with their ventral sides to each other, as a result of which an exchange of seminal fluid occurs, which enters the sac-like skin invaginations - the seminal receptacles. After exchanging sperm, the earthworms disperse. After this, the girdle areas (segments 32-37) of each individual begin to form a mucous membrane into which the worms lay eggs. As the coupling moves through the segments containing the spermatheca, the eggs are fertilized by sperm belonging to another individual. The clutch with fertilized eggs is thrown off the front end of the body by the movement of the worm's muscles, becomes compacted and turns into an egg cocoon, where young worms develop.
Characteristic of earthworms high ability to regeneration.
Soil annelids- useful animals. Even Charles Darwin noted their importance for soil fertility. By dragging fallen leaves into holes, they enrich the soil with humus, and by making passages in the soil, they loosen it and facilitate the penetration of air and water to the roots of plants. The amount of soil passed through the digestive tract of worms in Europe ranges from 6 to 84 t/ha, and in Cameroon it can reach 210 t/ha.
Freshwater oligochaetes play a significant role in the nutrition of bottom-dwelling fish.
Annelids apparently originate from lower segmented worms with parenchyma. The most ancient of the annelids are the marine polychaetes. From them, during the transition to a freshwater and terrestrial way of life, oligochaetes evolved, and from them leeches.
Class Polychaetes
This class is represented by marine animals. Many of them lead an active lifestyle, crawling along the bottom, burrowing into the ground or swimming in the water column. There are attached forms that live in protective tubes. The body is usually divided into the head, trunk and anal lobe. Annelids are often predators. Their pharynx is equipped with grasping appendages, sharp thorns or jaws. Parapodia are present, having a variety of shapes depending on the habitat and method of movement. They breathe with gills. Polychaetes are dioecious, fertilization is external.
Typical representatives of this class are the nereid (see Fig. 11.7) and the sand vein. They are food items for a number of commercial fish. Nereid has been successfully acclimatized in the Caspian Sea.
Related information.
Class Polychaetes, Class Oligochaetes, Class Leeches
Question 1. Describe the structural features of annelids.
Characteristic features of the annelid type:
The body is always segmented (segmentation and internal structure– repeatability of many internal organs).
They have a secondary body cavity - the coelom.
The circulatory system is closed.
The nervous system consists of the peripharyngeal nerve ring and the ventral nerve cord. The suprapharyngeal node is the “brain”.
The sense organs are located on the head segments.
The organs that facilitate locomotion are setae (in polychaetes there are 8 on each segment) and parapodia with tufts of setae (in polychaetes).
Question 2. What are parapodia? What do you think is their evolutionary significance?
Parapodia are lateral outgrowths of the body in polychaete worms, arranged in pairs and serving as organs of movement. Evolutionarily, parapodia are the predecessors of limbs.
Question 3. Describe the structure of the circulatory system of annelids.
The circulatory system is closed, consists of vessels, some of which have contractile walls (“hearts”), which ensures blood circulation. Some groups do not have a circulatory system. The blood of a number of forms contains hemoglobin (a red blood protein that contains iron and carries oxygen from the respiratory organs to the tissues).
Question 4. Describe the ring secretion organs.
The excretory system is represented by segmentally located metanephridia. Their funnel faces the body cavity, and the other end opens outward.
Question 5. How does the reproduction process occur in an earthworm?
Earthworms are hermaphrodites, but they undergo cross-fertilization. The two worms approach and exchange sperm, which enter their spermatic receptacles. Then a mucous muff is formed on the body of each worm. By contracting the muscles, the worm moves it to the anterior end of the body. When the muff passes by the openings of the ovarian ducts and spermatic receptacles, eggs and sperm enter it. Then the muff slides off the worm and closes into a cocoon, where small worms develop from the fertilized eggs.
Question 6. What classes are united by the type of annelids?
The phylum Annelids unites several classes, of which three main ones are Polychaetes, Oligochaetes and Leeches.
Question 7. Why are some annelids called polychaetes, while others are called oligochaetes? How do oligochaetes differ from polychaete worms?
Oligochaetes are one of the subclasses of Annelids. The most striking and familiar representative of the taxon is the most trivial Earthworm.
Polychaetes are one of the subclasses of Annelids. The most famous representatives of the taxon are the sandworm and the nereid. Sometimes the animals are called polychaetes, which means “many hairs” in Greek.
Difference between oligochaete and polychaete worms
There are fewer species of oligochaete worms than polychaetes. There are only 3 thousand species of the first, about 10 thousand of the second.
The maximum size of polychaetes exceeds the maximum size of oligochaetes, reaching 3 meters.
Animals have different habitats. Oligochaete worms live mainly in the ground; most polychaete worms prefer warm and salty water bodies.
Oligochaetes perceive oxygen through the entire surface of the skin, while polychaetes breathe using pseudo-gills-setae.
Oligochaetes are hermaphrodites, polychaetes are dioecious animals.
Oligochaetes, emerging from eggs, are similar to their parents. Polychaetes go through a larval stage.
Oligochaetes devour dead foliage and corpses; most polychaetes are active predators.
Question 8. When and from whom did the first annelids originate? What major changes accompanied the emergence of the type? Discuss as a class what these transformations mean. Write down the results of the discussion in your notebook.
Annelids evolved from free-living flatworms. From the common ancestors of worms, under the influence of evolutionary factors, annelids also evolved. An important point in their evolution is the division of the body into segments (rings). Due to active movement, annelids have developed a circulatory system that supplies the body with nutrients and oxygen. Ancient annelids had more complex structure compared to other worms.
Question 9. Make a table " Comparative characteristics structure of organs and systems in flat, round and annelid worms” (work in small groups).
Comparative characteristics of the structure of organs and systems in flat, round and annelid worms
FEDERAL EDUCATION AGENCY
TAMBOV STATE UNIVERSITY
NAMED AFTER G.R.DERZHAVIN
Test
in biology
on the topic of: general characteristics and classification of annelids
Completed by a student
1st year correspondence course
Faculty of Geography
Petropavlovskaya Olesya Sergeevna
(For verification to V.V. Koryakin)
TYPE RINGED WORMS
( ANNELIDA )
The annelids include primary annelids, polychaete and oligochaete worms, leeches and echiurids. In the phylum of annelids there are about 8 thousand species. The most primitive marine primary ringlets are archiannelids. Polychaetes and echiurids are inhabitants of the sea. Oligochaete ringlets and leeches are the main inhabitants fresh water and soil.
Structure. Annelids are the most organized representatives of worms. The sizes of the rings range from fractions of a millimeter to 2.5 m. These are predominantly free-living forms. The body of the ringlets is divided into three parts: the head, the body, consisting of rings, and the anal lobe. Animals that are lower in their organization do not have such a clear division of the body into sections.
The ringlet's head is equipped with various sensory organs. Many ringlets have well-developed eyes. Some have particularly acute vision, and their lens is capable of accommodation. Eyes can be located not only on the head, but also on the tentacles, on the body and on the tail. Ringworms also have developed senses of taste. On the head and tentacles of many of them there are special olfactory cells and ciliary fossae that perceive various odors and the effects of many chemical irritants. The ringed birds have well-developed hearing organs, arranged like locators. Recently, hearing organs have been discovered in sea ringed echiurids, very similar to the lateral line organs of fish. With the help of these organs, the animal subtly distinguishes the slightest rustles and sounds, which are heard much better in water than in air.
The body of the ringlets consists of rings, or segments. The number of rings can reach several hundred. Other ringlets consist of only a few segments. Each segment to some extent represents an independent unit of the whole organism. Each segment includes vital parts important systems organs.
Special organs of movement are very characteristic of ringlets. They are located on the sides of each segment and are called parapodia. The word "parapodia" means "foot-like". Parapodia are lobe-shaped outgrowths of the body from which tufts of bristles protrude outward. In some pelagic polychaetes, the length of the parapodia is equal to the diameter of the body. Parapodia are not developed in all ringlets. They are found in primary ringworms and polychaete worms. In oligochaetes only the setae remain. The primitive leech Acanthobdella has bristles. Other leeches move without parapodia and setae. Echiurids do not have parapodia and have setae only at the posterior end of the body.
Parapodia, nodes of the nervous system, excretory organs, gonads and, in some polychaetes, paired intestinal pouches are systematically repeated in each segment. This internal segmentation coincides with the external annulation. The repeated repetition of body segments is called by the Greek word "metamerism". Metamerism arose in the process of evolution in connection with the elongation of the body of the ancestors of ringlets. The elongation of the body necessitated the repeated repetition of first the organs of movement with their muscles and nervous system, and then internal organs.
Extremely characteristic of ringlets is the segmented secondary body cavity, or coelom. This cavity is located between the intestines and the body wall. The body cavity is lined with a continuous layer of epithelial cells, or coelothelium. These cells form a layer covering the intestines, muscles and all other internal organs. The body cavity is divided into segments transverse partitions– dissepiments. A longitudinal septum, the mesenterium, runs along the midline of the body, dividing each compartment of the cavity into right and left parts.
The body cavity is filled with fluid, which chemical composition very close to sea water. The fluid filling the body cavity is in continuous movement. The body cavity and abdominal fluid perform important functions. Cavity fluid (like any fluid in general) does not compress and therefore serves as a good “hydraulic skeleton”. The movement of the cavity fluid can transport various nutritional products, secretions of the endocrine glands, as well as oxygen and carbon dioxide involved in the respiration process inside the body of the ringlets.
Internal partitions protect the body in case of severe injuries and ruptures of the body wall. For example, an earthworm cut in half does not die. The septa prevent cavity fluid from flowing out of the body. The internal partitions of the rings thus protect them from death. But not all annelids have well-developed septa in the body cavity. For example, in echiurids the body cavity does not have partitions. A puncture in the body wall of an echiurid can lead to its death. In addition to the respiratory and protective role, the secondary cavity acts as a container for reproductive products that mature there before being excreted.
Ringlets, with few exceptions, have a circulatory system. However, they have no heart. The walls of large vessels themselves contract and push blood through the thinnest capillaries. In leeches, the functions of the circulatory system and the secondary cavity are so identical that these two systems are combined into a single network of lacunae through which blood flows. In some rings the blood is colorless, in others it is colored green color a pigment called chlorocruorin. Often ringlets have red blood, similar in composition to the blood of vertebrates. Red blood contains iron, which is part of the hemoglobin pigment. Some ringworms, burrowing into the ground, experience an acute deficiency of oxygen. Therefore, their blood is adapted to bind oxygen especially intensively. For example, the polychaete Magelonapapillicornis has developed the pigment hemerythrin, which contains five times more iron than hemoglobin.
In ringlets, compared to lower invertebrates, metabolism and respiration are much more intense. Some polychaete ringlets develop special respiratory organs - gills. A network of blood vessels branches out in the gills, and through their wall oxygen penetrates into the blood and is then distributed throughout the body. Gills can be located on the head, on the paropodia and on the tail. The through intestine of ringlets consists of several sections. Each section of the intestine performs its own special function. The mouth leads into the throat. Some ringbills have strong horny jaws and teeth in their throats, which help them grasp live prey more firmly. In many predatory ringlets, the pharynx serves as a powerful weapon of attack and defense. The pharynx is followed by the esophagus. This section is often supplied with a muscular wall. Peristaltic movements of the muscles slowly push food into the next sections. In the wall of the esophagus there are glands whose enzyme serves for the primary processing of food. Following the esophagus is the midgut. In some cases, goiter and stomach are developed. The wall of the midgut is formed by epithelium, very rich in glandular cells that produce digestive enzymes. Other cells of the midgut absorb digested food. Some ringlets have a midgut in the form of a straight tube, in others it is curved in loops, and still others have metameric outgrowths on the sides of the intestine. The hindgut ends at the anus.
Special organs - metaniphridia - serve to release germ cells - sperm and eggs. Metanephridia begins as a funnel in the body cavity; from the funnel there is a convoluted channel, which opens outward in the next segment. Each segment contains two metanephridia.
Reproduction. Rings reproduce asexually and sexually. Common in water rings asexual reproduction. At the same time, their long body breaks up into several parts. After some time, each part restores its head and tail. Sometimes a head with eyes, tentacles and a brain forms in the middle of the worm's body before it splits into parts. In this case, the separated parts already have a head with all the necessary sensory organs. Polychaetes and oligochaetes are relatively good at restoring lost body parts. Leeches and echiurids do not have this ability. These ringlets have lost their segmented body cavity. This is partly why, apparently, they lack the ability to reproduce asexually and restore lost parts.
Fertilization of eggs in ringed fish most often occurs outside the body of the mother's body. In this case, males and females simultaneously release reproductive cells into the water, where fertilization occurs.
In marine polychaetes and echiurids, the crushing of fertilized eggs leads to the development of a larva, which is not at all similar to adult animals and is called a trochophore. Trochophore a short time lives in the surface layers of water, and then settles to the bottom and gradually turns into an adult organism. Freshwater and terrestrial ringnecks are most often hermaphrodites and have direct development. Dense membranes here protect the eggs from mechanical damage and from drying out under the scorching rays of the sun.
Practical significance. In Russia, for the first time in the history of world science, the acclimatization of some invertebrates was carried out to strengthen the food supply of the sea. For example, the polychaete Nereis, acclimatized in the Caspian Sea, became the most important food item for sturgeon and other fish.
Earthworms not only serve as fishing bait and food for birds. They bring great benefits to humans by loosening the soil, making it more porous. This facilitates the free penetration of air and water to the roots of plants and increases crop yields. While burrowing in the ground, worms swallow pieces of soil, crush them and throw them to the surface well mixed with organic matter. The amount of soil brought to the surface by worms is amazingly large. If we were to distribute the soil plowed by earthworms every 10 years over the entire surface of the land, we would get a layer fertile land 5 cm thick.
Annelids are a fairly large type of invertebrate (about 9,000 species). They are classified as free-living animals that have a very complex organization, unlike flat and roundworms. These include primary ringlets, multi- and oligochaete worms, echiurids and leeches. The most primitive species are considered to be archiannelids, which live in the marine environment.
Echiurids and polychaetes also live in the sea, but their organization is more complex. And leeches and oligochaete worms are inhabitants of soil and fresh water bodies.
Highly organized type
Annelids can reach 2.5 meters in length. Most species are free-living forms. Any ringworm has a body consisting of 3 parts: the trunk (consists of rings), head and anal lobe. The head of the ringlets houses various sensory organs. Most annelids have well-developed eyes. Some of them have specifically acute vision, their lens is capable of accommodation. Eyes can be located in different areas: on the head, body, tentacles and even the tail. In addition, annelids have a highly developed sense of taste. Many of them have special olfactory cells on their heads or tails that perceive all kinds of odors or the effects of chemical irritants. Hearing organs are also present, and they work on the principle of locators. Not so long ago, hearing organs were discovered in many echiurids, very similar to the lateral line organ in fish. This allows them to always be ready: to hear all the rustles and splashes, because underwater audibility is much better than on land. Annelids are named for their body, which is made up of segments (rings). The number of these rings can reach a couple of hundred. However, most ringlets have only a few segments.
In principle, each ring represents an independent particle of the whole organism. Annelids move thanks to specific organs of movement - parapodia, which are located on the sides of each segment. They look like lobe-shaped outgrowths of the body, from which tufts of bristles protrude outward. However, not every ringlet has parapodia. Polychaete worms and some primary ringworms have them. In oligochaete individuals, only the setae are the organs of locomotion. The body cavity of annelids is filled with a liquid similar in composition to chemical water. This liquid is constantly moving, due to which the transport process occurs nutrients throughout the body of ringlets, secretions of endocrine glands, oxygen and carbon dioxide, which are directly involved in the respiration process of any organism.
Reproduction of annelids
Annelids reproduce sexually and asexually. However, asexual reproduction dominates in aquatic ringworms. Fertilization of the egg occurs outside the female's body. The male and female synchronously release their reproductive cells into the water, where fertilization actually occurs.
