Quiz 17: Annelids and Allied Taxa

The distinguishing characteristics of phylum Annelida are: • They have a unique annelid head and paired epidermal setae present (lost in leeches); parapodia were present in the ancestral condition. • They are marine, freshwater and terrestrial. • Most of them are free-living; some are symbiotic while some are Ectoparasites. • They possess a bilaterally symmetrical Body which is metameric and often has a distinct head. • They possess a triploblastic body with the 3 layers-ectoderm, mesoderm and endoderm. • The Coelom (Schizocoel) is well developed and divided by septa, except in leeches; the coelomic fluid functions as a hydrostatic skeleton. • The Epithelium secretes an outer transparent moist cuticle. • Their Digestive system is a complete one and not segmentally arranged. • They possess a body wall with outer circular and inner longitudinal muscle layers. • Their Nervous system has a double ventral nerve cord and a pair of ganglia with lateral nerves in each segment; the brain is a pair of dorsal cerebral ganglia which has connections to ventral nerve cord. • They have a sensory system which consists of tactile organs, taste buds, statocysts, photoreceptor cells, and eyes with lenses; the specialization of head region into differentiated organs like tentacles, palps, and eyespots of polychaetes is also seen. • Asexual reproduction occurs by fission and fragmentation; the fragmented parts are capable of complete regeneration. • Their Excretory system consists of a pair of nephridia for each segment which remove waste from blood as well as from coelom. • Respiration occurs through skin, gills, or parapodia. • They possess a closed Circulatory system with muscular blood vessels and aortic arches for pumping blood which is segmentally arranged. The respiratory pigments (hemoglobin, hemerythrin, or chlorocruorin) are often present; amebocytes are present in blood plasma.

• Annelid body plan: The annelid body has a two-part head composed of a prostomium and a peristomium followed by a segmented body and a terminal part called the pygidium having an anus. • Body wall: The body wall surrounds the peritoneum and coelom. It contains strong circular and longitudinal muscles adapted for swimming, crawling, and burrowing. • Segments: Segments are arranged in linear series and externally marked by circular rings called annuli. Body segmentation (metamerism) is a division of the body into a series of segments. Each segment contains circulatory, respiratory, nervous, and excretory structures, as well as a coelom. • Coelom and its compartments: In most annelids the coelom develops embryonically as a split in the mesoderm on each side of the gut (Schizocoel), thus forming a pair of coelomic compartments in each segment. • Coelomic lining: Peritoneum lines the body wall of each coelomic compartment, forming dorsal and ventral mesenteries that cover all organs. Peritonea of adjacent segments meet to from septa, which are perforated by the gut and longitudinal blood vessels.

The coelom of annelids is filled with fluid and serves as a hydrostatic skeleton. This hydrostatic skeleton of annelids helps them to burrow in the following way: As the volume of fluid in a coelomic compartment is essentially constant, contraction of the longitudinal body-wall muscles causes a segment to shorten and to become larger in diameter, whereas contraction of the circular muscles causes it to lengthen and become thinner. The presence of septa means that widening and elongation occur in restricted areas. In this way, crawling motions are produced by alternating waves of contraction by longitudinal and circular muscles passing down the body (peristaltic contractions). This helps them to burrow. The efficiency of burrowing is increased by segmentation in the following way: Segments in which longitudinal muscles are contracted widen and anchor themselves against the substrate while other segments, in which circular muscles are contracted, elongate and stretch forward. Thus, powerful forces for rapid burrowing as well as locomotion can be generated.