Most of the investigations into annelids have been carried out on polychaetes, particularly several species of nereids. It has been shown that growth, reproduction, regeneration and metamorphosis are controlled by neurosecretions from the brain.
Normal growth is regulated by a growth hormone, and when the mature form is achieved, either the amount of secretion wanes, or the tissues become less sensitive to it. Maturation of the sex cells is inhibited by a “juvenile” hormone from the brain, but when the level of this is sufficiently reduced, maturation begins. Later, for full development of the oocytes, a very low level of the hormone is necessary. There is some justification for regarding this “juvenile” hormone as being identical with them, may secrete a hormone which stimulates the final development of ripe eggs.
Regeneration of segments cut off, or lost naturally in annelids, takes place first by the formation of a growing point or blastema at the site of loss. This blastema is particularly sensitive to growth hormones and new segments are formed, at first slowly and then much more rapidly. It has been shown that both the blastma and the hormone are necessary for regeneration to take place.
Metamorphosis from the Nereid to the heteronereid condition is also hormonally controlled. Again, this may depend on the concentration of the growth hormone, or, if it exists, of a low enough, metamorphosis will proceed. There is some possibility that the developing oocytes may produce a hormone influencing metamorphosis.
The facts of hormone control of certain processes are clearly established, but as yet there is no certainty as to how many hormones are involved. Some workers have suggested that the growth hormone controls all the above processes by its varying concentration. Others suggest that there may be at least three different hormones, two produced by neurosecretion from the brain and one by secretion from the oocytes.
Role of Hormones in Annelids
Annelids are soft, segmented invertebrates that occur in freshwater or marine environments. They are hermaphrodites with a clitellum and have a triploblastic body characterized by a large coelomic cavity.
The cerebral ganglion of the polychaete Nereis produces a small peptide hormone that inhibits precocious sexual development. A complex beneath the brain functions as a neurohemal organ, with epithelial cells that secrete neurohormones into the coelomic fluid.
Growth of Annelids
The members of Phylum Annelida, which include leeches, worms and woodlice, are bilaterally symmetrical with two cell layers, a through gut and a closed circulatory system. They live in a wide range of marine, freshwater and terrestrial environments. Unlike other invertebrates, most annelids reproduce asexually. The earliest fossil known to be an annelid was the polychaete Burgessochaeta from the Canadian Burgess Shale, which formed about 505 million years ago in the early Cambrian period. The worm Myoscolex, which was discovered in Australia and is older than Burgessochaeta, has also been referred to as an annelid.
Hormones are chemical messengers that control many processes in cells, tissues and organs by binding to receptors that are produced in these areas of the body. They switch on or off specific genes in the cells, altering existing proteins or stimulating them to build new ones. The resulting proteins carry out the hormones’ instructions.
For example, the estrogen-sensitive ERs of mollusks and cephalochordates (see below) are activated by hormones that are structurally related to estradiol. The same is likely true for the annelid ERs PdER and CcER, which are very similar to human steroid and other estrogen-sensitive ERs. The conservation of the estrogen-binding sites in these receptors suggests that they existed before the divergence between the deuterostomes (including chordates and echinoderms) and the protostomes.
In addition to regulating gene expression, the ERs of these invertebrates are also sensitive to certain xenobiotics. For instance, the phytoestrogen genistein and the pharmaceutical DES are strong activators of PdER, while bisphenol-A, which is found in plastics and detergents, inhibits it. This suggests that annelid ERs can be disrupted by a large class of chemicals known as endocrine-disrupting substances.
Reproduction in Annelids
Annelids are bilaterally symmetrical, triploblastic invertebrates that reproduce both sexually and asexually. They are segmented and have parapodia for locomotion. They are found in most wet environments and range in length from less than a millimeter (the earthworm Lumbriculus) to over three meters (seep tube worms, such as Laminellibrachia luymesi). They are classified as members of the Lophotrochozoa, which also contains molluscs, brachiopods and nemerteans.
Annelids can reproduce asexually by cell division or fragmentation. This method of reproduction is useful for annelids that inhabit stagnant water or soil such as planaria and hydra. It allows the offspring to mature without consuming a large amount of the parent organism’s body fluid. However, asexual offspring lack genetic variation.
Sexual reproduction involves a complex series of events leading to fertilization and the formation of new gametes. The process is typically biparental and involves meiosis for gamete formation. Male and female gametes, which are haploid cells, are then physically brought together to facilitate fusion, known as fertilization. Fertilization introduces genetic variations into offspring and plays a key role in evolution of organisms.
Most annelids are polychaetes, which include the familiar earthworms and leeches. The class Oligochaeta includes both freshwater and marine species, while the class Polychaeta comprises marine, sedentary, and burrowing species such as nereids and siboglinids. The class Hexapoda is a sub-group of the class Polychaeta and contains both leeches and a variety of aquatic, terrestrial and symbiotic species. Cladistic studies have radically changed the traditional division of annelids into polychaetes, oligochaetes and hexapoda. These changes have placed the Pogonophora, Echiura, and Sipuncula groups in the polychaete phylum rather than their own phyla. The hexapoda now also includes the nereids and the Siboglinidae.
Regeneration is a fundamental biological process that occurs in annelids at many levels, including cell repair, germline restoration and regrowth of structures. Annelids have impressive regenerative abilities, although the mode of regeneration and the cellular sources are often unclear. It is likely that NS plays an important role in annelid regeneration, but its precise mechanism of action remains unknown. It may secrete a cocktail of factors that induce proliferation and promote growth of the regeneration tissue, or it may act as an organizer by directing cell movements and regulating the formation of organs and tissues.
Annelids can regenerate parts or entire body segments, and they can regain lost functions following injury. The most remarkable examples of segmental regeneration are observed in leeches (Parionyx excavatus) and dinophilids such as the sandworm Arenicola marina, which can restore their head or tail after amputation. This ability is largely dependent on the structure of their mesodermal tissues, which are highly vascular and have an efficient metameric coelom. It also depends on the availability of a sufficient amount of spare nutrients and an appropriate microenvironment for regeneration to occur.
A number of annelids can regrow their gut after amputation, but the exact mechanisms are not fully understood. Gut regeneration is accompanied by de novo formation of the foregut and hindgut, each of which has a different embryonic origin.
High-throughput expression profiling has identified genes that are up-regulated during regeneration in several annelids. The repressive chromatin regulator protein CdER, for example, is up-regulated during the anteroposterior process in the syllis worm P. dumerilii, and the transcriptional activity of this gene is inhibited by a low dose of estrogens. The annelid ER binds classic estrogen response elements with high affinity and is responsive to estrogens, similar to vertebrate ERs.
Annelids have provided classic experimental studies for metamorphosis, regeneration and cell differentiation. For example, a segment of Capitella that has been cut off regenerates with the formation of a growing point or blastema at the site. This growing point is sensitive to growth hormones and produces new segments rapidly. Whether these cells originate at the site of amputation or migrate from an adjacent area has been shown to depend on the concentration of this growth hormone, suggesting that it influences the pattern of regeneration.
Similarly, metamorphosis of amphibian larvae is controlled by the actions of two classes of hormones. The molting hormones of amphibians function like the thyroid hormones of insects, and an increase in their relative concentration leads to a rapid change in gene expression that causes the resorption of larval tissues and the differentiation of adult organs. The relative ease with which these changes can be triggered suggests that hormones can dramatically alter the course of an animal’s evolution.
Regeneration is another characteristic shared by the annelids and other animals. Depending on the species, they can regenerate lost segments of their bodies or even replace entire segments. Regeneration in annelids involves remodeling of pre-existing tissues concordant with the reorganization of internal structures and is typically accompanied by cell proliferation. The ability to regenerate is limited by the availability of nutrients, which is why most polychaetes and lower oligochaetes can regenerate only a portion of their anterior ends.
The annelids are a large phylum of soft extensile triploblastic worms with true coelomic fluid, a closed circulatory system and a hydraulic skeleton made of chitin. They occur in a variety of habitats, including marine environments such as tidal zones and hydrothermal vents, fresh water and moist terrestrial soil. They have a segmented body broadly divided into rings and a pair of highly developed parapodia or chaetae for locomotion.
Digestion in Annelids
The annelids (or polychaete worms) are a large phylum of over 15,000 species. They are protists, which means they do not have a digestive tract and rely on other systems to absorb nutrients. This phylum contains earthworms and leeches among others. They are to some extent segmented, with a central coelom that is divided into separate and repeted segments called metameres. Each metamere holds elements of the circulatory, nervous, and excretory systems. This segmentation makes movement of the worm very efficient by allowing for extremely localized muscle contraction.
Digestion in annelids takes place through a complex series of chemical reactions. A variety of hormones regulate the digestive process. Gastrin stimulates the stomach gland to secrete pepsinogen and hydrochloric acid. Cholecystokinin is secreted in the duodenum and stimulates pancreatic enzyme secretion, bile secretion in the gall bladder, and increases gastrointestinal motility. Gastric inhibitory peptide decreases stomach secretion, and slows stomach emptying and motility.
Chemical digestion is also used by unicellular organisms such as amoebae, which surround and engulf food particles before bringing them inside the cell for nutrient absorption. The worms in the phylum Annelida evolved jaws during the early Ordovician, and the first polychaete annelids appeared around 488 million years ago.
Terrestrial annelids can be invasive pests in some environments. The earthworm Amynthas agrestis, for example, has been introduced to glaciated regions of the United States and has had devastating effects on forest ecosystems by causing soil erosion, loss of leaf duff, changes in soil chemistry, and decreased biodiversity. As a result, this species has been banned in some states. The worms are also a nuisance in urban areas where they build burrows that block sewers and drains.