Reproduction involves males and females who release their gametes in the water where fertilization occurs and drifting larva form. The tube of a common tubeworm found on panhandle beaches; Diopatra. Photo: University of Michigan. Aside from parasitic tapeworms and leeches, earthworms are one of the more commonly recognized varieties of worms. Many folks actually raise earthworms for their gardens or for fish bait; a process known as vermiculture.
Earthworms differ from polychaetes in that they do not have parapodia but DO possess a clitellum, which is used in reproduction. Though most live in the upper layers of the soil there are freshwater species within this group.
The number of earthworms within the soil is dependent on several factures including the amount of organic matter, the amount of moisture, soil texture, and soil pH. Earthworms can significantly improve soil conditions by consuming soil and adding organics via their waste, or castings.
Unlike polychaetes, earthworms lack gills and take in oxygen through their skin, one reason why they most live in moist soils. Another difference between them and polychaetes is in reproduction. Aquatic polychaetes can release their gametes into the water where they are fertilized but terrestrial earthworms cannot do this. Instead two worms will entangle and exchange gametes; there are no male and females in this group. Tube-dwellers, such as the fan worms, utilize their large feather-like tentacles to filter small food particles from the water.
Tubes vary in complexity of construction. Some are made up of sand or mud particles cemented together with digestive secretions. In others the worms also contribute calcium carbonate or proteins and polysaccharides.
Examine the pictures and video below Watch carefully to see particles move down the pinnule. Annelids have a closed circulatory system. This means that the blood is contained entirely within hearts and blood vessels and does not drain into the body cavities. In annelids, there are two major blood vessels which run down the length of the body and are connected to one another in most segments by interspersed capillary beds that lie along the digestive tract.
In the anterior end, the two longitudinal blood vessels are connected by larger vessels that contract rhythmically, thus serving as "hearts". The close association of the circulatory system with the digestive tract allows for absorption of nutrients from the gut into the blood, which then transports them to the various parts of the body. In oligochaetes, oxygen diffusing through the body wall can be picked up from the coelomic fluid by the blood and transported to wherever it is needed, with carbon dioxide following the reverse route.
Thus the coelom also serves as a transport system, albeit to a limited extent. The nitrogenous waste products of metabolism diffuse from the body's cells into the fluid of the coelom, which thus plays a role in waste transport.
The coelomic fluid is slowly drained into small tubular organs, called metanephridia singular metanephridium. As the fluid passes through the metanephridia it can exchange some materials with the network of blood vessels that are intertwined around the tubule. Accordingly, the content of the fluid may become somewhat modified. The ability of annelids to modify their excretory products allows for osmoregulation to occur. For example, certain polychaetes can live in estuaries where the salinity of the water fluctuates widely.
Finally, the fluid with its contained nitrogenous wastes exits the body through tiny pores in the body wall. The basic annelid nervous system consists of a single or double nerve cord running along the ventral side of the body with an enlarged region ganglion in each segment. Active annelids, such as the earthworm, have larger ganglia in the head region that serve as simple "brains".
The segmentally organized nervous system of an annelid, with the longitudinal connectives and ganglia fused along the midline. There is an elongated ganglion for each segment. Annelid sense organs then are simple and vary according to life style of the worm. Oligochaetes do not have discrete sensory organs, although the skin, particularly in the anterior region, has cells with sensory functions.
Some are light-sensitive, others chemosensory and still others are sensitive to touch. The active polychaetes have a well developed head region bearing tentacles and palps that are sensitive to chemicals and touch. The head also bears eyes that range from simple eyespots to large conspicuous eyes on stalks.
Oligochaetes are hermaphroditic and practice a form of mutual copulation. The sperm from each worm's body are transferred to the other worm some species have penises and then move to the spermatheca in the other worm, where it is stored until fertilization. A complex cocoon is eventually secreted by the clitellum into which eggs and sperm are secreted, so fertilization is external.
The worms separate and form cocoons; the cocoon moves forward, picking up eggs at the 14th segment; at the 9th and 10th segments it picks up the sperm deposited by the other earthworm. The embryo develop in this "cocoon" which will be shed by the worm. There is no larval form in oligochaetes, even in marine species. Leeches are hermaphroditic and most of them have reproductive habits similar to earthworms. They have a clitellum which produces a cocoon that slides off the animal, collecting the gametes as it does so.
However, other leeches have a more innovative way of reproducing. One leech deposits a packet of sperm onto the body of its partner. The sperm then bore through the skin and fertilize the eggs. In this case, fertilization in truly internal! Although most leeches deposit their cocoons, then leave, a few species carry the cocoons with them until the young leeches emerge.
Polychaetes have separate sexes and fertilization is externa l. Polychaetes are unique in not having well-defined gonads. Rather the gametes are produced by germ cells located in the lining of the coelom or in the walls of the septa between segments. Like most aquatic invertebrates, polychaetes reproduce by shedding their gametes into the water, and fertilization occurs in the aqueous environment. Some polychaetes can also reproduce asexually by budding off individual body segments that then grow into complete worms.
Polychaetes include bristle worms, while oligochaetes include earthworms. Polychaetes are typically marine, while oligochaetes live in both aquatic and terrestrial habitats.
Polychaetes have a well-developed head, while oligochaetes have a less-developed head. Polychaetes have parapodia with many chaetes, while oligochaetes have few chaetes with no parapodia. Polychaetes are mainly free-living, while oligochaetes can be either free-living, commensals or parasites. Polychaetes are dioecious with temporary or seasonal gonads, while oligochaetes are monoecious with permanent gonads.
Polychaetes undergo external sperm transfer and external fertilization. But, oligochaetes undergo no external sperm transfer but their fertilization occurs in the clitellum or cocoon.
Polychaetes are a subclass of annelids, having many bristles arranged in parapodia. They include bristle worms that are dioecious with temporary and seasonal gonads. Typically, polychaetes exclusively live in marine habitats. Meanwhile, oligochaetes are another subclass of annelids, having few bristles but no parapodia.
They live in both terrestrial and aquatic habitats. Phylum Annelida includes segmented worms. These animals are found in marine, terrestrial, and freshwater habitats, but a presence of water or humidity is a critical factor for their survival, especially in terrestrial habitats. The name of the phylum is derived from the Latin word annellus , which means a small ring.
Animals in this phylum show parasitic and commensal symbioses with other species in their habitat. Approximately 16, species have been described in phylum Annelida. The phylum includes earthworms, polychaete worms, and leeches.
Figure 1. The clitellum, seen here as a protruding segment with different coloration than the rest of the body, is a structure that aids in annelid reproduction. Annelids display bilateral symmetry and are worm-like in overall morphology. Annelids have a segmented body plan wherein the internal and external morphological features are repeated in each body segment. This metamerism is thought to arise from identical teloblast cells in the embryonic stage, which give rise to identical mesodermal structures.
The overall body can be divided into head, body, and pygidium or tail. The clitellum is a reproductive structure that generates mucus that aids in sperm transfer and gives rise to a cocoon within which fertilization occurs; it appears as a fused band in the anterior third of the animal Figure 1.
The epidermis is protected by an acellular, external cuticle, but this is much thinner than the cuticle found in the ecdysozoans and does not require periodic shedding for growth.
0コメント