Each segment is separate from the others by a structure called septa. Each segment contains a complete set of organs. Each segment also has a pair of hooks and bristles and in marine species a pair of parapodia appendages used for movement.
The mouth is located on the first segment at the head-end of the animal and the gut runs through all segments to the end where an anus is located in the tail segment. In many species, blood circulates within blood vessels. Their body is filled with fluid that gives the animal shape through hydrostatic pressure. Most segmented worms burrow in terrestrial soils or sediments at the bottom of freshwater or marine waters.
The body cavity of a segmented worm is filled with fluid inside which the gut runs the length of the animal from head to tail. The outer layer of the body consists of two layers of muscle , one layer that has fibers that run longitudinally, a second layer that has muscle fibers that run in a circular pattern.
Segmented worms move by coordinating their muscles along the length of their body. The two layers of muscles longitudinal and circular can be contracted such that parts of the body can be alternately long and thin or short and thick. This enables the segmented worm to pass a wave of movement along its body that enables it to, for example, move through loose earth in the case of the earthworm.
They can make their head region thin so that it can be used to penetrate through new soil and build subterranean burrows and paths.
Many species of segmented worms reproduce asexually but some species reproduce sexually. Most species produce larvae that develop into small adult organisms. Most segmented worms feed on decaying plant materials. An exception to this are the leeches, a group of segmented worms, are freshwater parasitic worms. Leeches have two suckers, one at the head end of the body, the other at the tail end of the body.
They attach to their host to feed on blood. 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. A significant difference between leeches and other annelids is the development of suckers at the anterior and posterior ends and a lack of chaetae. Additionally, the segmentation of the body wall may not correspond to the internal segmentation of the coelomic cavity.
This adaptation possibly helps the leeches to elongate when they ingest copious quantities of blood from host vertebrates. The subclass Brachiobdella includes species like Branchiobdella balcanica sketi and Branchiobdella astaci , worms that show similarity with leeches as well as oligochaetes.
Figure 3. The a earthworm, b leech, and c featherduster are all annelids. Phylum Annelida includes vermiform, segmented animals. Segmentation is seen in internal anatomy as well, which is called metamerism.
Annelids are protostomes. These animals have well-developed neuronal and digestive systems. Some species bear a specialized band of segments known as a clitellum.
Annelids show the presence numerous chitinous projections termed chaetae, and polychaetes possess parapodia. All members of the group are to some extent segmented, in other words, made up of segments that are formed by subdivisions that partially transect the body cavity. Segmentation is also called metamerism.
Segments each contain elements of such body systems as circulatory, nervous, and excretory tracts. Metamerism increases the efficiency of body movement by allowing the effect of muscle contraction to be extremely localized, and it makes possible the development of greater complexity in general body organization. Besides being segmented, the body wall of annelids is characterized by being made up of both circular and longitudinal muscle fibers surrounded by a moist, acellular cuticle that is secreted by an epidermal epithelium.
All annelids except leeches also have chitonous hair-like structures, called setae, projecting from their cuticle. Sometimes the setae are located on paddle-like appendages called parapodia. Annelids are schizocoelous and with a large and well-developed true coelom i. Except in leeches, the coelom is partially subdivided by septa. Hydrostatic pressure is maintained across segments and helps maintain body rigidity, allowing muscle contractions to bend the body without collapsing it.
The internal organs of annelids are well developed. They include a closed, segmentally-arranged circulatory system. The digestive system is a complete tube with mouth and anus. Gases are exchanged through the skin, or sometimes through specialized gills or modified parapodia.
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