NEMERTEANS: TAXONOMY, CHARACTERISTICS, NUTRITION, REPRODUCTION - BIOLOGY - 2025

The nemertinos are a phylum of animals which are characterized by having a cylindrical body and a tube (proboscis) Ejection of great length. This edge encompasses a total of 3 classes: Anopla, Enopla, and Palaeonemertea. Between all of them, there are approximately more than 1100 species distributed throughout the world geography.

They are mostly found in predominantly marine aquatic habitats, although some are found in terrestrial environments. In general, they live in dark places such as under stones, small crevices or burrows created by themselves in the seabed.

Examples of Nemertines. Source: Bürger, Otto They were first described by the German anatomist Max Schultze in 1851. Their phylum name derives from Nemertes, which was one of the Nereids (sea nymphs) that appeared in Greek mythology.

characteristics

Nemerteans are considered multicellular eukaryotic organisms that have certain characteristics in common with both flatworms (flatworms) and annelids (segmented worms).

They are cellolated, so they do not have a coelom or internal cavity. Instead they have parenchyma-like tissue. They are also triblastic, which implies that during their embryonic development the three germ layers are present: endoderm, ectoderm and mesoderm. Continuing with the embryonic development, they are protostomated, since both the mouth and the anus are formed from the blastopore.

They present bilateral symmetry, which means that they are made up of two exactly the same halves that converge on their longitudinal axis.

Most species of nemerteans are free-living, although there are a few that parasitize other animals, mainly crustaceans.

Likewise, they are dioecious organisms because there are female individuals and male individuals. They reproduce mainly sexually, with external fertilization and indirect development.

Taxonomy

The taxonomic classification of nemerteans is as follows:

-Domain: Eukarya

-Animalia Kingdom

-Subreino: Eumetazoa

-Superphile: Spiralia

-Lophotrochozoa

-Trochozoa

-Filo: Nemertina

Morphology

- External anatomy

Nemerteans have a cylindrical body, whose size ranges from specimens as small as 0.5 mm, to others so large that they can reach a length of up to 30 meters. They have an approximate diameter of 15 mm.

This is a phylum of animals that is quite varied. They feature a wide range of colorful patterns. Interestingly, those found on coral reefs display more colorful colors than those found in other types of habitats.

The body of these animals is not segmented into regions. In the part that should correspond to the head there are two orifices, one corresponding to the mouth and the other to a cavity known as a rhincocele, in which the proboscis of the animal is housed.

The surface of the body of the adult nemertean can have several holes. First, there are the genital orifices, which, in number from 8 to 10, are arranged on the lateral edges of the body. There are also the excretory orifices, which are two and are located in its anterior part. Finally, at the terminal end is the hole corresponding to the anus.

Exemplary of Nemertino. Source: Keisotyo

- Internal anatomy

Wall

The body is protected by a wall that has several layers, a total of four, which are:

- Epithelium: this is made up of the confluence of various types of cells: ciliated (with extensions similar to small hairs), glandular (designed to secrete substances of mucous and serous consistency), undifferentiated or totipotent (with the ability to transform into any cell type).

- Basement membrane: made up mainly of connective tissue.

- Dermis: made up of collagen. It is hard in consistency.

- Muscle layer: made up of longitudinal and circular muscle fibers.

Digestive system

Nemerteans have a complete digestive system. This is made up of a mouth, which opens into the oral cavity. Immediately after this is the esophagus, which is a tube that has a very small diameter.

Later is the stomach, which is of the glandular type. This is continued with the intestine that presents in a lateral position structures similar to blind sacs called intestinal diverticula. Finally, the intestine empties into a hole that faces the outside, the anus.

Excretory system

It is made up of two collecting ducts that run along the entire lateral edge of the animal and flow into the excretory pores at the level of the front of the body.

Nervous system

It is represented by two nerve cords that run along the lateral edge of the body, surrounding the intestine. These cords have their origin in four cerebroid-type ganglia that are at the level of the rhincocele and that are joined by commissures that form a kind of ring.

Circulatory system

Your circulatory system is closed. Depending on the degree of complexity of the animal, its circulatory system will be more or less simple. In the case of the simplest nemerteans, they present two gaps, a cephalic and a posterior one. While in the more complex nemerteans, the lagoons are horseshoe-shaped.

The blood vessels are covered by a resistant endothelium and blood circulates through them. The color of this depends on molecules such as hemoglobin and hemeritrin. They do not have a main organ like a heart.

Nutrition

Nemerteans are organisms that can have two types of lifestyles: parasitic or free. In the case of those that have a parasitic life, they need to live attached to or attached to other animals, obtaining their food from them. In this sense, some nemerteans parasitize certain crustaceans and take advantage of the moment when they release their eggs to feed on them.

Predators

However, the vast majority of nemertean species are considered carnivorous animals. This means that they feed on other living things. It is a known fact that nemerteans are highly effective predators that have certain infallible mechanisms to capture their prey.

The main element that these animals use to provide themselves with food is their proboscis, which, as specified, can become large.

There are species whose proboscis is provided with a tip, known as a stylet, which is used by the animal to inoculate its prey with a harmful neurotoxin-type substance. This has a paralyzing effect on the prey, thus facilitating the ingestion process.

In species that do not have the poisonous stylet, the proboscis is also part of the feeding process, since they use it to completely surround their prey and keep it immobilized to be able to ingest it.

Digestion

The nemertean ingests the entire prey. Once in the oral cavity, it begins to undergo the action of certain chemical substances known as enzymes, which contribute to the start of food processing.

Subsequently, the food is transferred to the esophagus and from this to the stomach. Here the digestion process continues, the food being subjected again to the action of chemical substances, making it easier to absorb.

From the stomach, food passes into the intestine, which is where the absorption process takes place and nutrients pass into the circulatory system. The part of the food that is not absorbed, continues its transit towards the terminal part of the intestine, to be carried to the outside through the anus.

Exemplary of Nemertino. Source: «gbhone» on the page. Signed on the photo

Reproduction

Nemerteans present the two types of reproduction that exist: asexual and sexual.

Asexual

Asexual reproduction is one in which there is no exchange of genetic material or fusion of gametes. In this type of reproduction, the individuals that are generated are exactly the same as the parent from the genetic point of view and, therefore, physically as well.

There are many processes that are integrated in asexual reproduction. In the case of nemerteans, they reproduce asexually through a process known as fragmentation. This process consists in that starting from a fragment of the animal a complete individual can be generated. It is also called Autotomization.

This occurs mainly when the animal is under stress or irritation. Also when he feels threatened by some element of the external environment.

It is important to specify that regeneration from a fragment of the animal does not occur in open space, but within a kind of mucous cyst.

Sexual

In this type of reproduction, two gametes fuse, one female and the other male. Some of the families that belong to this phylum have a copulatory organ, a kind of penis. In these, fertilization is internal. On the contrary, in the vast majority of species, fertilization is external.

After fertilization occurs, the eggs are generally laid together by a gelatinous substance, a kind of adhesive gel that holds them together. There are also species that do not follow this pattern, but instead release their eggs into ocean currents or immersed in plankton.

After the required time elapses, a larva that is known as a pilidum hatches from the egg. This is a free swimmer and moves calmly in the ocean currents, until, after experiencing a series of transformations, it becomes an adult individual. There are also species in which development is direct.

References

1. Brusca, RC & Brusca, GJ, (2005). Invertebrates, 2nd edition. McGraw-Hill-Interamericana, Madrid
2. Curtis, H., Barnes, S., Schneck, A. and Massarini, A. (2008). Biology. Editorial Médica Panamericana. 7th edition.
3. The giant slimy worm. Obtained from:
4. Hickman, CP, Roberts, LS, Larson, A., Ober, WC, & Garrison, C. (2001). Integrated principles of zoology (Vol. 15). McGraw-Hill
5. Junoy, J. and Herrera, A. (2010). The nemerteans of the National Maritime-Terrestrial Park of the Atlantic Islands of Galicia. In the book: National Parks Research Project 2006 - 2009. National Parks Autonomous Organization.
6. Moretto, H. and Scelzo, M. (2004). Nermetin worms. Chapter of the book "Life between tides: plants and animals of the coasts of Mar de Plata, Argentina". INIDEP special publications, Mar del Plata.