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Chapter

BIO2231: Phylum Annelida characteristics

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Department
Biology
Course
BIO2231
Professor
Various
Semester
Spring

Description
PHYLUM ANNELIDA Characteristics of Phylum Annelida: o Unique annelid head and paired epidermal setae present (lost in leeches); parapodia present in the ancestral condition o Marine, freshwater and terrestrial o Most free-living, some symbiotic, some ectoparasitic o Body bilaterally symmetrical, metameric, often with distinct head o Tripoblastic body o Coelom (schizocoel) well developed and divided by septa, except in leeches; coelomic fluid functions as hydrostatic skeleton o Epithelium secretes outer transplant, moist cuticle o Digestive system complete and not segmentally arranged o Body wall with outer circular and inner longitudinal muscle layers o Nervous system with a double ventral nerve cord o Sensory system of tactile organs, taste buds, statocysts (in some), photoreceptor cells, and eyes with lenses (in some); specialization of head region into differentiated organs, such as tentacles, palps, and eyespots of polychaetes o Asexual reproduction by fission and fragmentation; capable of complete regeneration o Hermaphroditic or separate sexes; larvae, if present, are trochophore type; asexual reproduction by budding in some; spiral cleavage and mosaic development o Excretory system typically a pair of nephridia for each segment; nephridia remove waste from blood as well as from coelom o Respiratory gas exhancge through skin, gills, or parapodia o Circulatory system closed with muscular blood vessels and aortic arches (hearts) for pumping blood, segmentally arranged; respiratory pigments often present; amebocytes in blood plasma Phylum Annelida: Annelida are worms whose bodies are divided into similar segments (also called metamers) arranged in linear series and externally marked by circular rings called annuli. Body segmentation (metamerism) is a division of this body into a series of segments, each of which contains similar components of all major organ systems. In annelids, the segments are delimited internally by septa. Annelids are sometimes called ‘bristle worms’ because, with the exception of leeches, most annelids bear tiny chitinous bristles called setae. Short, needlelike setae help anchor segments during locomotion and long, hairlike setae aid aquatic forms in swimming. Since many annelids burrow or live in secreted tubes, stiff setae also aid in preventing the worm from being pulled out or washed out of its home. Body Plan: The annelid body typically has a two-part head, composed of a postomium and a peristomium, followed by a segmented body and a terminal portion called the pygidium bearing an anus. The head and pygidium are not considered to be segments. New segments differe
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