Annelida Lecture Notes PDF

Taxonomic Summary Phylum: Annelida Class: Polychaeta Class: Oligochaeta Class: Hirudinea – the leeches Class: Pogonophora Subclass: Perviata Subclass: Obturata (or Vestimentifera) Class: Echiura Phylum Annelida Defining Characteristic: One or more pairs...

Taxonomic Summary Phylum: Annelida Class: Polychaeta Class: Oligochaeta Class: Hirudinea – the leeches Class: Pogonophora Subclass: Perviata Subclass: Obturata (or Vestimentifera) Class: Echiura Phylum Annelida Defining Characteristic: One or more pairs of chitinous setae (= chaetae). Phylum Annelida - diversity Introduction: There are about 12,400 described annelid species. All adult annelids possess at least 1 pair of chitinous bristles, called setae (= chaetae) and all are vermiform (worm- like). These animals are soft-bodied, reasonably circular in cross section and longer than they are wide. Phylum Annelida General Characteristics : The bodies of most annelids consist of a series of repeating segments. This serial repetition of segments and organ systems (skin, musculature, nervous system and excretory system) is known as metamerism, or metameric segmentation (repeatation and linear) (Fig.13.1). Phylum Annelida General Characteristics: The annelid outer body wall is generally flexible and can play an active role in locomotion. The thin body wall can serve as a general surface for gas exchange. Epidermis secretes a protective cuticle, the cuticle remains permeable to both water and gases.  Phylum Annelida General Characteristics: Respiration – skin, gills & modified parapodia Excretion of wastes generally occurs by means of structures called nephridia (= kidneys). Most annelid segments contain 2 nephridia, each of which is open at both ends, This type of nephridium is called a metanephridium. In addition to providing an outlet for metabolic waste products, nephridia may be used to regulate the water content of the coelomic fluid. Class Polychaeta Defining Characteristic: Paired lateral outfoldings of the body wall (parapodia). Many setae Introduction: 63% of annelid species are placed in the class Polychaeta. Nearly all polychaetes live in salt water. Class Polychaeta General characteristics: Polychaetes generally possess at least 1 pair of eyes and at least 1 pair of sensory appendages (tentacles) on the anteriormost part of the body (the prostomium). Generally, the body wall is extended laterally into a series of thin, flattened outgrowths called parapodia. Parapodia are useful for polychaete identification. Class Polychaeta General characteristics: The parapodia function in gas exchange and locomotory in many species. Siliceous, chitinous or more rarely calcareous bristles called setae protrude from each parapodium; setal morphology also differs substantially among polychaete species. Class Polychaeta General characteristics: Polychaetes divided into 2 general groups. One group includes the generally active, mobile species (i.e. the errant species, which were formerly placed in a separate subclass, the Errantia). Other species (Sedentaria) typically spend their entire lives in protective tubes – that is, the animals live sedentary lives. Their tubes vary in construction from simple organic secretions mixed with sand grains or mud to tubes composed of calcium carbonate and/or complex mixtures of proteins and polysaccharides. Class Polychaeta General characteristics: Feeding habits are highly diverse among polychaetes and include predation, scavenging, suspension feeding and deposit feeding. Class Oligochaeta Defining characteristics: 1. Pronounced cylindrical glandular region of the body (clitellum) that plays important roles in reproduction. 2. Permanent gonads. Class Oligochaeta Introduction: Approximately 3,500 oligochaete species have been described. Only 6.5% of oligochaete species are marine, most are found in freshwater or terrestrial habitats. Class Oligochaeta Introduction: Oligochates have a more streamlined appearance. Parapodia are lacking and the prostomium (head) lacks conspicuous sensory structures, such as eyes and tentacles. Oligochaetes do have setae, but the setae are less densely distributed along the body. There are usually no specialized respiratory organs; gas exchange is accomplished by diffusion across a moist body wall. Class Oligochaeta General characteristics: To move, oligochaetes generate a continuous series of localized contractions and relaxations of the circular and longitudinal musculature. These cycles of contraction, known as peristaltic waves. Peristaltic wave are especially effective in burrow fromation and in movement within a burrow or tube. Class Oligochaeta General characteristics: All oligochaete are hermaphroditic and generally only a few segments of each individual produce gametes. Sperm are generally exchanged simultaneously between 2 mating individuals. The sperm are stored for later use in specialized organs called spermathecae. Class Oligochaeta General characteristics: eggs and sperm are extruded from separate openings into a complex cocoon secreted by the clitellum, so that fertilization occurs externally. The embryos develop in and feed upon a nutritive fluid found within the cocoon. When development is completed, miniature worms emerge from the cocoons; oligochaetes lack free-living larval stages, even in marine environments. Class Oligochaeta General characteristics: Asexual reproduction is more common among oligochaetes, particularly in freshwater species. Strobilation The process of asexual reproduction involves the transverse division of the adult into a number of separate sections and the subsequent regeneration of each section into a complete individual. Class Hirudinea Defining characteristic: Posterior sucker. Class Hirudinea Introduction: 500-630 described species. Most leeches occupy freshwater or terrestrial habitats, only a small proportion of species is marine. Conspicuously absent are parapodia, other specialized respiratory appendages and head appendages, as in oligochaetes. Class Hirudinea General characteristics: Most leeches lack setae. The body is generally not separated into compartments by septa, and the continuous coelomic space is largely filled with connective tissue, or mesenchyme (Fig. 13.17). Class Hirudinea General characteristics: The circulatory medium is the coelomic fluid. Leeches do not move by generating peristaltic waves; instead, they locomote over solid substrates by using suckers as temporary anchors. Class Hirudinea General characteristics: These suckers are formed from groups of segments at the body’s anterior and posterior ends. Leeches are generally ectoparasitic, feeding either on the blood of other invertebrates or more commonly, on the blood of vertebrates. Most parasitic species possess 3 toothed jaws within the mouth; the leech uses these jaws to make an incision in the host.  Class Hirudinea General characteristics: Leeches are simultaneous hermaphrodites and a free-living larval stage is absent from the life cycle. Only a few segment of each individual are directly involved in gametogenesis (gamete production). Fertilization among leeches occurs internally, either through copulation, or through penetration of the body wall by the spermatophores themselves. Sperm exchange is mutual between two mating individuals , and fertilized eggs of each individual generally develop within external cocoons. Class Hirudinea General characteristics: The clitellum generally functions in producing the cocoon and a nutritive fluid. A miniature leech eventually emerges from the cocoon.