Anul worms: Treatment, Prevention and When to See a Doctor
Treatment, Prevention and When to See a Doctor
Nationwide Children’s Hospital
Pinworms are small white worms about 1/2 inch long and as thin as a thread. They can sometimes be seen in and around the child’s bottom (anus) and in bowel movements. These worms live in the intestine. The adult female worm crawls out of the infected person’s anus at night and lays her eggs in the surrounding skin. This causes itching and scratching. The worms can also move to the vagina of a female child and cause itching.
Eggs can live for 2 to 3 weeks outside the body. They can be found in house dust, on toilet seats, bed linens and toys and in play areas in or outside the home. Although pinworms only grow in humans, the eggs can be carried in a pet’s fur.
Pinworms are harmless but annoying. They spread easily to others.
How the Infection is Spread
When the child scratches, tiny eggs are picked up on the fingers.
The eggs are left on any surface that is touched. They are then picked up again and swallowed when fingers, foods or anything that has eggs on it, are put in the mouth.
How Pinworms Are Diagnosed
You have to see the eggs or worms to make the diagnosis. The best way to do this is by doing a tape test. In the morning, before your child goes to the bathroom or washes up, put the sticky side of a piece of clear tape around the anus. Remove the tape and look for pinworms or their eggs. You can also buy a tape test kit at the pharmacy. If you do not find pinworms, do the tape test 2 more mornings in a row, to be sure.
How to Treat the Infection
- Your child’s doctor will prescribe a special medicine to treat pinworm infection.The dose is based on the person’s weight.Usually 2 doses of the medicine are ordered.The second dose starts 2 weeks after the first.
- Read the label on the bottle of medicine to know how to give it.
- Your child’s doctor will also treat other family members and close contacts with the same medicine, at the same time.
How to Prevent Reinfection or Spreading Pinworms to Others
It is easy to be reinfected with pinworms or spread them to others. During treatment and for 2 weeks after treatment is finished, do the following:
- Hand washing is most important.Have your child and all family members wash their hands often.They should wash before meals or eating, after using the toilet and after scratching (Picture 1). This is a good habit to do at all times.
- Keep your child’s fingernails clean and cut as short as possible.
- Teach your child to avoid touching his mouth or biting his fingernails.
- Have your child shower or bathe every morning.
- Do not let children bathe together or share items.
- Clean the anus with soap and water after each bowel movement.Use clean washcloths or paper towels each time.
- Scrub the toilet seat daily with disinfectant or soap and water (Picture 2).
- Wipe down any toys that your child usually puts in his mouth or hard surfaces that he has touched with disinfectant or soap and water.
- Change bed linens and put clean underwear and clothes on every day.Avoid shaking these things so that eggs are not put into the air.
- Promptly wash used bed linen, all clothing, towels and washcloths in hot water with detergent.
- Vacuum carpets and floors well.Wash the canister or change the vacuum cleaner’s bag after each use.Seal the bag before throwing it away.
- Wash any raw vegetables or fruits thoroughly before eating.
- Call your child’s school or childcare center so that they can take extra steps to prevent the spread to others.Your child can return to school or childcare 24 hours after treatment.
When to Call Your Child’s Doctor
- If you see pinworms
- If anal itching lasts more than 1 week
- If the skin around the anus becomes red or tender
Pinworms: Treatment and Prevention (PDF)
HH-I-56 10/78, Revised 3/19 Copyright 1978-2011, Nationwide Children’s Hospital
Pinworm infection – Symptoms & causes
Overview
Pinworm infection is the most common type of intestinal worm infection in the United States and one of the most common worldwide.
Pinworms are thin and white, measuring about 1/4 to 1/2 inch (about 6 to 13 millimeters) in length.
Pinworm
An adult pinworm generally is 1/4 to 1/2 inch (about 6 to 13 millimeters) in length. The most common symptom of infection is anal itching, particularly at night, as worms migrate to the host’s anal area to lay their eggs.
While the infected person sleeps, female pinworms lay thousands of eggs in the folds of skin surrounding the anus. Most people infected with pinworms have no symptoms, but some people experience anal itching and restless sleep.
Pinworm infection occurs most often in school-age children, and the tiny (microscopic) eggs are easily spread from child to child. Treatment involves oral drugs that kill the pinworms and thorough washing of pajamas, bedding and underwear. For best results, the entire family should be treated.
Products & Services
Symptoms
Symptoms of pinworm infection may include:
- Itching of the anal or vaginal area
- Insomnia, irritability, teeth grinding and restlessness
- Occasional stomach pain and nausea
Pinworms often cause no symptoms.
When to see a doctor
Consult your doctor if you have severe anal itching, especially at night.
Causes
Accidentally swallowing or breathing in pinworm eggs causes a pinworm infection. The tiny (microscopic) eggs can be carried to your mouth by contaminated food, drink or your fingers. Once swallowed, the eggs hatch in the intestines and mature into adult worms within a few weeks.
Female pinworms move to the anal area to lay their eggs, which often results in anal itching. When you scratch the itchy area, the eggs cling to your fingers and get under your fingernails. The eggs then get transferred to other surfaces, such as toys, bedding or toilet seats. The eggs can also be transferred from contaminated fingers to food, liquids, clothes or other people.
Pinworm eggs can survive for two to three weeks on surfaces.
Risk factors
Risk factors for pinworm infection include:
- Being young. Pinworm infections are most likely to occur in children ages 5 to 10. The tiny (microscopic) eggs are easily spread to family members, caregivers, or other children at school or child care centers. Pinworm infections are uncommon in children younger than age 2.
- Living in crowded spaces. People who live in institutions are at higher risk of developing pinworm infections.
Pinworm infections are most likely to occur in children ages 5 to 10. The tiny (microscopic) eggs are easily spread to family members, caregivers, or other children at school or child care centers. Pinworm infections are uncommon in children younger than age 2.Complications
Typical pinworm infections don’t cause serious problems. In rare circumstances, heavy infestations can cause infection of female genitals.
The parasite can travel from the anal area up the vagina to the uterus, fallopian tubes and around the pelvic organs. This can cause problems such as inflammation of the vagina (vaginitis) and inflammation of the inner lining of the uterus (endometritis).
Although rare, other complications of a pinworm infection may include:
- Urinary tract infections
- Weight loss
- Infection of part of the abdomen (peritoneal cavity)
Prevention
Pinworm eggs can cling to surfaces, including toys, faucets, bedding and toilet seats, for two weeks.
So besides regular cleaning of surfaces, methods to help prevent the spread of pinworm eggs or to prevent reinfection include:
- Wash in the morning. Because pinworms lay their eggs at night, washing the anal area in the morning can help reduce the number of pinworm eggs on your body. Showering may help avoid possible re-contamination in bath water.
- Change underwear and bedding daily. This helps remove eggs.
- Launder in hot water. Wash bedsheets, pajamas, underwear, washcloths and towels in hot water to help kill pinworm eggs. Dry on high heat.
- Don’t scratch. Avoid scratching the anal area. Trim your child’s fingernails so there’s less space for eggs to collect. Suggest that your child avoid biting his or her nails.
- Wash your hands. To reduce your risk of getting or spreading an infection, wash your hands well after using the toilet or changing a diaper and before eating.
Type annelids, preparation for the exam in biology
Annelids, or annelids (from Latin annulus – ring) – a class of worms with external and internal segmentation.
All of them have ring ledges,
usually corresponding to the internal division of the body. The type has about 18 thousand species.
They belong to primary animals, the body is divided into segments, the number of which in some species reaches several hundred. Let’s start studying annelids
with classification.
The appearance of annelids (annelids) was accompanied by large, significant aromorphoses.
For the first time in annelids from the mesoderm, a whole appears – a secondary body cavity. It is a space (cavity) between the internal organs and the walls of the body.
The surface of the internal organs is lined with a special epithelium – the whole body. The number of sections into which the whole is divided corresponds to the number of segments of the body.
The presence of a secondary body cavity is the most important anatomical feature of annelids.
The functions of the whole are varied:
- Protective – generally filled with liquid containing amoeba-like cells, apparently performing a protective function by phagocytizing foreign particles
- Support – the whole is a hydroskeleton, just like the primary cavity in roundworms. The whole is a support for the skin-muscle bag
- Transport – through the walls of the intestine, nutrients enter the coelom cavity, which are then delivered to the tissues
- Excretory – coelomic fluid is involved in the excretion of metabolic products
The whole is a support for the skin-muscle bag The body of annelids is divided into segments (metamers, segments). Metamerism of internal organs is directly related to body segmentation – the structure of most segments is the same,
in them you can find the lateral branches of the intestine, the nerve node, blood vessels, excretory organs, sex glands.
Segmentation is homonomous and heteronomous. Homonomous (from the Greek homos – similar, identical) segmentation, or metamerism – a type of segmentation in organisms whose segments
have the same structure and perform similar functions – for example, in annelids. Heteronomous (from Greek heteros – different) segmentation is characteristic of organisms in which
segments differ sharply from each other in structure and function, for example, in arthropods.
Such a design can be imagined as compartments in a submarine, each of which has everything necessary for life. With mechanical damage
the ringed worm is capable of regenerating the whole organism from a small fragment of the body.
In annelids (more specifically, in the polychaete class), parapodia limbs first appear (from the Greek pará – “near” and pódion – “leg”) – muscular
outgrowths that are located on the sides of each segment and serve for movement.
Annelids have a circulatory system for the first time! This evolutionary moment is very important. The circulatory system of the rings is closed, a real heart
is absent, the blood moves through the vessels due to the pulsation of their walls.
I want to note that a closed circulatory system is a type of circulatory system that is distinguished by blood flow exclusively in the vessels (arteries, veins,
capillaries) – exit from the vessels occurs only when they are damaged.
In an open circulatory system (in arthropods), blood flows into the cavity, washes
internal organs diffusely.
In rings, blood flows only in the vessels and does not pour out into the cavity.
The nervous system is represented by a large head ganglion – the brain, from which the abdominal nerve chain departs, represented by nerve nodes (ganglia)
in each segment.
The main details of the structure of annelids will be studied by us using the example of a typical representative – an earthworm (in the oligochaete section).
© Bellevich Yury Sergeevich 2018-2023
This article was written by Yury Sergeevich Bellevich and is his intellectual property. Copying, distribution
(including by copying to other sites and resources on the Internet) or any other use of information and objects
without the prior consent of the copyright holder is punishable by law. To obtain the materials of the article and permission to use them,
please refer to Yuri Bellevich .
› Back to | ||
AuthorV.V. Starunov EditorO.V. Zaitseva | Annelids or annelids (phylum Annelida) are a fairly large phylum of protostomes invertebrates, numbering about 18,000 species. Sizes vary from 0.3 mm ( Diurodrilus sp.) up to 3 meters long ( Eunice aphroditois ). For the most part, these are small and medium-sized animals, from 1 to 10 centimeters long. Annelids live mainly in marine biotopes, but many species inhabit fresh water (oligochaetes, leeches), as well as soil (earthworms). | Contents
“ The body of annelids is divided into prostomium (head lobe), a number of trunk segments and pygidium (anal lobe). “ On the sides of the trunk segments there are locomotor appendages—parapodia bearing setae (chaetae). “ Anterior to the pygidium is the growth zone, separating anteriorly new trunk segments. |
Annelid System | The annelids system has undergone quite a lot of changes in recent years. According to traditional ideas, all annelids were divided into two subtypes: girdle (Clitellata) and girdleless (Aclitallata). The class of polychaete worms (Polychaeta) was classified as girdleless, and the group of girdle worms included oligochaete worms or oligochaetes (Oligochaeta) and leeches (Hirudinea). In recent years, the development of methods of molecular phylogenetics has provoked a new round of research into the annelids system. The modern system is very different from the traditional one. So, Oligochetes and Leeches, according to new data, turn out to be a daughter taxon within the Polychaeta group. At the same time, taxa that were previously considered as separate families within the class of polychaetes found themselves outside of it in the position of sister groups (for example, Oweniidae, Chaetopteridae and Mageloniidae). | |
Body segments | An important feature of annelids is their amazing morphological diversity. This diversity also applies to the structure of the nervous system, which demonstrates a high level of evolutionary plasticity. In terms of morphological diversity, this group is extremely heterogeneous. However, most of the living representatives of this type have a certain set of characteristic features. First of all, it is a segment organization. The entire body of annelids can be divided into three main sections: prostomium (or head lobe), a number of trunk segments, and pygidia (or anal cavity). The cephalic lobe contains the cerebral ganglion or cerebrum and may also bear a number of sensory organs: eyes, antennae, palps, and nuchals (occipital organs). Trunk segments are the main structural unit of the body of annelids. On the sides of the trunk segments are locomotor appendages – parapodia, equipped with setae (chaetae). The structure of the parapodia and setae is species-specific and is often used to identify annelids. Each segment can bear no more than one pair of parapodia. In some representatives of annelids, the parapodia are reduced to parapodial ridges and small processes equipped with setae. In oligochaetes, the parapodia are completely reduced, with only a small number of setae on the lateral sides of the segments. In leeches, in most cases, the bristles are also reduced. At the posterior end of the body of annelids, the segments are usually smaller than in the middle of the body. This is mainly due to the fact that here, in front of the anal lobe, there is a growth zone that separates the new trunk segments anteriorly from itself. Thus, the anterior segments are the oldest in the body of the worm, and those located at the posterior end of the body are the youngest. Located at the posterior end of the body, the anal lobe or pygidium bears the anus of the digestive system, and may also have various sensory appendages (pygidial antennae), best developed in free-living representatives with an active lifestyle. | |
Body wall | The body of annelids is covered with a layer of cuticle consisting of collagen fibers located in mutually perpendicular layers and giving the body of these animals a characteristic iridescent sheen. Usually the cuticle fibers are located at an angle of 45 degrees to the anterior-posterior axis of the body of the worm. Muscular layers are located under the layer of the integumentary epithelium. The structure of the musculature of annelids is extremely diverse and strongly depends on the lifestyle of a particular animal. In worms living in the thickness of the soil, the musculature of the body wall is well developed and consists of an outer layer of annular and an inner layer of longitudinal muscles, usually divided into four well-defined strands. Sometimes a layer of diagonal muscles can be located between the annular and longitudinal muscle layers. In annelids living on the ground surface, the annular muscles are poorly developed or completely absent. | Selected species of annelids (polychaetes)Chrysopetalum occidentale Johnson, 1897 (family Chrysopetalidae). Photo by V.V. Starunova Syllis sp. (family Syllidae). Photo by V.V. Starunova Oxydromus pugettensis (Johnson, 1901) (family Hesionidae). Photo by V.V. Starunova Phyllodoce sp. (family Phyllodocidae). Photo by V.V. Starunova |
Overall | Between the body wall and the intestine in most annelids there is a secondary body cavity or coelom. Each body segment bears a pair of coelomic sacs – left and right. The partitions between the left and right coelomic cavities are called mesenteries, and between the cavities of adjacent segments they are called dissepiments. | |
Circulatory andexcretory systems | Many representatives of annelids have a developed closed circulatory system. The most important vessels are the dorsal and ventral, located respectively in the dorsal and ventral mesentery. Blood flows through the dorsal vessel to the anterior end of the body, and through the abdominal vessel to the rear. There are also lateral vessels running in dissipations and a peri-intestinal plexus. The dorsal vessel or some lateral vessels most often act as the propulsator organ. The excretory system is represented by paired nephridia arranged in segments. | |
Digestive system | Digestive system of annelids through. Usually the digestive tract can be divided into several sections: the pharynx, esophagus, intestines and hindgut. In some species of annelids, the pharynx is able to completely or partially turn inside out to capture food, in addition, chitinous jaws can often be located in it. | |
Nervous system | The central nervous system of annelids consists of the brain (cerebral ganglion), peripharyngeal connectives, and trunk brain (ventral nerve cord). The peripheral nervous system is represented by nerves and nerve plexuses in the body wall, as well as the stomatogastric nervous system. Despite the general plan of organization, the structure of the nervous system of various representatives of annelids can vary significantly. These differences apply to all parts of the ventral nerve chain: ganglia, connectives, commissures, segmental and longitudinal nerves. The brain is located in the prostomium. In general, it consists of two bilaterally symmetrical parts. The connection between the right and left parts is carried out through a large number of commissures, of which two main ones can be distinguished, connecting the roots of the peripharyngeal connectives and occurring in most annelids. Conventionally, the brain of annelids can be divided into three sections. The anterior section is associated with the palps, the middle section with the antennae and eyes, and the posterior section with the nuchal organs. In addition, up to 26 pairs of different cell clusters, called brain nuclei, can be distinguished in the brain of various representatives of annelids. Depending on lifestyle, the degree of brain development can vary greatly. In sedentary annelids leading a sedentary lifestyle, spending their whole lives in their own tubes, it is developed to the least degree and is only a paired ganglion in which it is impossible to distinguish any departments or nuclei. The cerebrum is connected to the trunk via a pair of peripharyngeal connectives. The trunk brain is usually located on the ventral side of the animal along the midline and consists of several pairs of longitudinal nerve trunks and paired ganglia located in segments on them. In each segment, the ganglia of the trunk brain are connected by commissures, the number of which varies in different representatives from one to seven. Only in some cases (in representatives of the families Nerillidae and Protodrillidae) can the number of commissures be disordered and indefinitely large. The number of longitudinal nerve trunks is also variable, it can be from two to five. The ganglia of the ventral nerve cord are usually well defined and well demarcated. Only in some primitively organized polychaetes is it impossible to draw clear boundaries between the ganglia of adjacent segments. Segmental nerves depart from the ganglia of the trunk brain, carrying out innervation of the body wall and parapodia. The number of these nerves varies widely. In species with developed parapodia, a thicker parapodial nerve can be distinguished among the segmental nerves, which often contains a small cluster of nerve cells called the parapodial ganglion. In addition to the segmental nerves, the body wall may contain peripheral longitudinal nerves, the number of which varies from family to family. An important part of the peripheral nervous system is the stomatogastric nervous system. It is a collection of nerves, ganglia and nerve plexuses of the pharynx and intestinal tract. It connects with the central nervous system through paired stomatogastric nerves extending from the brain and from the subesophageal ganglion, and also, probably, through segmental nerves stretching to the dorsal side and reaching the intestine through the mesenteries. The detailed structure of the stomatogastric nervous system in annelids has been studied relatively poorly and only on a few representatives. | “ The central nervous system of annelids consists of the brain, peripharyngeal connectives, and the trunk brain (ventral nerve cord). “ The brain of annelids can be conventionally divided into an anterior section associated with palps, a middle section with antennae and eyes, and a posterior section with nuchal organs. “ The trunk brain consists of several pairs of longitudinal nerve trunks and paired ganglia located in segments on them. “ Annelid ganglia consist of a central fibrous part with a large number of synaptic endings, and a peripheral part containing nucleated bodies of nerve cells. “ The peripheral nervous system is represented by nerves and nerve plexuses in the body wall, as well as the stomatogastric nervous system. |
Sensory Organs | The sensory structures of annelids are diverse and are represented by both unicellular photo-, chemo-, and mechanoreceptors, as well as multicellular sensory organs. Most of the receptor structures are concentrated at the anterior end of the body. A large number of mechano- and chemoreceptor cells are concentrated on sensory appendages: antennae, tentacle-like, parapodial and pygidial antennae, as well as on palps. Usually these are bipolar cells. The sensitive process of each such cell is located in the epithelium and usually bears a sensitive cilium at the distal end. The axonal process can reach a considerable length and extends to the neurons of the central nervous system. Sensory cells can be clustered into specialized ciliary sensory organs. Usually they are considered to be organs of chemoreception. These include the nuchal (or occipital) organs located on the prostomium, as well as the dorsal and lateral ciliary organs located on the trunk segments. One or more pairs of eyes can be distinguished on the head lobe of many annelids. Such cerebral eyes can be extremely diverse in structure. In representatives of plank polychaetes of the Alciopidae family, the eyes also have a light-collecting lens and occupy most of the head lobe. Eyes can be located not only on the head lobe. Representatives of some families have eyes located on the trunk segments and even on the pygidium. In addition, individual photoreceptor and photoreceptor-like cells can be found in the integumentary epithelium of the body wall. Representatives of certain families of annelids have statocysts – organs of gravitational sense. They represent an epidermal vesicle, supporting, glandular and sensory cells. There are several statoliths inside the bubble. Such sensory organs are characteristic, first of all, of animals leading a burrowing lifestyle. | “ Sensory structures of annelids include unicellular photo-, chemo-, and mechanoreceptors and multicellular sensory organs. – Multicellular ciliary sensory organs include nuchal (or occipital) organs in the prostomium and dorsal and lateral organs on the trunk segments. “ The head lobe of many annelids bears cerebral eyes. In addition, representatives of some families have eyes located on the trunk segments and even on the pygidium. “ Most annelids, with the exception of oligochaetes and leeches, have separate sexes. Fertilization is external in most cases. “ Cleavage in annelids is spiral, gastrulation is epiboly or invaginated. Most have an actively swimming larva (trochophore). “ Some species of polychaetes (eg Platynereis dumerilii and Capitella teleta ) are model objects in developmental biology. |
Reproductive system | Most annelids, with the exception of oligochaetes and leeches, have separate sexes. For some species, the phenomenon of epitoky is characteristic – the transformation of the worm’s body prior to reproduction for life in the pelagic zone. In a number of cases, only the posterior part of the animal’s body undergoes epitonic rearrangement. | |
Ontogeny | Cleavage in annelids is spiral, gastrulation usually follows the type of epiboly or invagination. Most annelids have a larva called a trochophore that actively swims in the water column. In the future, the larva acquires the features of a segmental organization and becomes a metatrochophore. In the course of further transformations, the metatrochophore turns into a nektochaete, vaguely resembling an adult worm with a reduced number of segments. Further transformations are mainly associated with the activation of the work of the growth zone, an increase in the number of trunk segments and the final transformation of the head and pygidial parts of the body. | |
Conclusion | Representatives of polychaetes occupy a special place in the collections presented on our website. Polychaetes are currently considered one of the nodal groups on the phylogenetic tree of the animal world. The results of the latest research in the field of molecular developmental biology suggest that polychaete-like animals (Urbilateria) could be a common ancestor for all bilaterally symmetrical animals, which means that the nervous system of annelids can bear the features of a primitive organization that is characteristic of the last common ancestor of all other groups bilaterally. symmetrical animals. It should also be noted that certain types of polychaetes, such as Platynereis dumerilii or Capitella teleta have now become one of the standard model objects for various branches of biology, and mainly for developmental biology. | |
Recommended reading |
| |
Our site currently uses the traditional version of the annelids system with division into classes Polychaetes, Oligochetes and Leeches.
The number of segments in different groups of annelids can vary greatly. It can be either indefinite and depend only on the size and age of the animal, or strictly fixed. The first trunk segment usually differs in its structure from the subsequent ones. It bears the mouth opening and is called the peristomium. In addition, sensory appendages can be located on it – peristomial or tentacle-shaped antennae, which perform mainly a sensory role.
Their arrangement in the best way provides both strength and elasticity of the integument of the body. The cuticle is secreted by cells of the integumentary epithelium, which also contains a large number of glandular cells that secrete mucus. Representatives of some families have specialized skin glands, with the help of which animals build a protective tube around themselves.
The entire skin-muscle sac consists of a set of individual muscle bundles. In addition to the musculature of the body wall, there is a rather complex set of muscles that move the parapodia and setae, as well as the dessipiment muscles, oblique and dorsoventral muscles.
Coelomes are filled with coelomic fluid. Often in the coelomic cavity, own cellular elements, coelomocytes, are found. The functions of the coeloms are diverse: it is part of the musculoskeletal system of the animal, acting as a hydroskeleton, the coelomic fluid can duplicate or even completely take over the functions of the circulatory system, distributing oxygen and nutrients, in addition, excretory organs are located in the coelomic cavities and sex products develop .
The brain is slightly more strongly developed in the burrowing annelids. The brain reaches its greatest development in free-living species that lead an active benthic lifestyle and have well-developed sense organs.
The ganglia consist of a central fibrous part, which has a large number of synaptic endings (neuropiles), and a peripheral part, containing the nucleated bodies of nerve cells. The ganglia of the anterior segments of the body merge to form large suboesophageal ganglia. In addition, the paired ganglia located within the same segment may partially or completely merge. In leeches, there is also a fusion of the ganglia of the last trunk segments in the region of the posterior sucker. Finally, in misostomids, a complete fusion of the entire ventral nerve chain into a single nerve center is observed.
Together with segmental nerves, they can form a regular lattice of peripheral nerves, similar to the orthogon of flatworms, but this similarity is secondary, since it is exclusively functional in nature and is associated with the development of the peripheral parts of the nervous system.
There are also sensitive organs located segmentally and on the pygidium.
Simply arranged eyes are arranged according to the inverted principle. Large eyes are usually everted (straight).
She grows a new head for herself, separates from the mother’s body and swims independently, dispersing the reproductive products. Fertilization in most cases is external, sometimes there are cases of care for offspring, for example, representatives of leeches, as well as some polychaetes of the Syllidae family, bear juveniles on the ventral side of the body.
T. The comparative embryology of the Polychaeta // Acta Zool. 1966. T. 47. S. 1-42.