|
|
| Labs |
|
Labs
|
Phylum Annelida Common Name: Segmented worms (earthworms, leeches, and marine worms) Etymology: "Ringed", from the Latin annellus, a little ring, referring to the ring-like segments of their bodies. Representative examples:
Additional Information: The phylum Annelida is a group of triploblastic, eucoelomate protostomes in which the coelom is partitioned by septa into segments with continuous longitudinal blood vessels, nerve cords, and a complete digestive tract penetrating the septa. The segmentation first observed in this group allows for a high degree of specialization of different regions of the body and will be seen to be exploited to an even greater degree in the arthropods. The true coelomic body cavity acts as a hydrostatic skeleton, supporting new kinds of movements in this group and providing a space for storage and development of complex organ systems. The annelids can distinguished from most molluscs and all arthropods in that segmented worms have a closed circulatory system, usually consisting of large dorsal and ventral vessels connected by a series of muscular pumping hearts. Annelids are generally hermaphroditic, but practice cross-fertilization in which individuals exchange sperm, store it, and use it to fertilize their own eggs. Although superficially similar to roundworms in their basic body shape, along with segmentation annelids posess both longitudinal and circular muscle groups, allowing them to control their movements and use modes of locomotion not available to roundworms. Although traditionally divided into three large subgroups (oligochaetes or earthworms, polychaetes or marine worms, and hirundineans or leeches), recent molecular systematics work now groups the leeches and earthworms together in a clade, the Clitellata. This clade may in fact fall within the polychaete group, though the situation is unresolved at this time (for more on annelid phylogeny see the Annelida page on the Tree of Life website). The leeches can be distinguished by a lack of internal segmentation (they still retain superficial external segments), a reduced coelom, loss of a closed circulatory system, and (of course) by their parasitic ecology. Polychaete worms are marine worms that generally posess distinct, unjointed extensions of the body wall associated with each segment. These parapodia can act as paddles or hooks for locomotion and sometimes incorporate a gill-like element. Polychaete worms also exhibit a greater degree of cephalization than earthworms, having a well-defined head bearing eyes, antennae, and heavy jaws enclosed in a muscular, eversible pharynx. Earthworms are terrestrial and do not have parapodia or an obvious head. Oligochaetes do have a distinctive set of mid-body segments modified to form the clitellum, a region that functions in reproduction, secreting a mucus cocoon in which eggs are fertilized and housed during their development outside the parent's body. Another distinctive feature of annelids in general are bristle-like structures called chaetae (or setae). These may be used to give the body traction during locomotion and are made of chitin. The name polychaete refers to the fact that marine worms have many of these bristles (which are often associated with their parapodia), while oligochaete, meaning "few bristles", refers to the relatively lower numbers of these structures in the earthworm. As a group, annelids have a well-developed excretory system in the form of pairs of metanephridia associated with most segments. Annelids lack a formal respiratory system, instead exchanging gases through their skin by diffusion. As mentioned previously, many polychaetes have a well-vascularized region of each parapodium devoted to gas exchange. These "gills" serve to increase the surface area of the marine worm's epidermis. Links:
|
||||||||||||||
|
|