However, cross-fertilization is preferred in hermaphroditic animals. The nerve cord is ventral in position and bears enlarged nodes or ganglia in each segment.Īnnelids may be either monoecious with permanent gonads (as in earthworms and leeches) or dioecious with temporary or seasonal gonads that develop (as in polychaetes). Annelids show well-developed nervous systems with a nerve ring of fused ganglia present around the pharynx. ![]() Excretion is facilitated by a pair of metanephridia (a type of primitive “kidney” that consists of a convoluted tubule and an open, ciliated funnel) that is present in every segment towards the ventral side. These animals lack a well-developed respiratory system, and gas exchange occurs across the moist body surface. In addition, these vessels are connected by transverse loops in every segment. This schematic drawing shows the basic anatomy of annelids in a cross-sectional view.Īnnelids possess a closed circulatory system of dorsal and ventral blood vessels that run parallel to the alimentary canal as well as capillaries that service individual tissues. A cross-sectional view of a body segment of an earthworm (a terrestrial type of annelid) is shown in Figure 2 each segment is limited by a membranous septum that divides the coelomic cavity into a series of compartments.įigure 2. The gizzard leads to the intestine and ends in an anal opening. A well-developed and complete digestive system is present in earthworms (oligochaetes) with a mouth, muscular pharynx, esophagus, crop, and gizzard being present. Annelids show the presence of a true coelom, derived from embryonic mesoderm and protostomy. Chitinous hairlike extensions, anchored in the epidermis and projecting from the cuticle, called setae/ chaetae are present in every segment. Circular as well as longitudinal muscles are located interior to the epidermis. 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. ![]() 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 overall body can be divided into head, body, and pygidium (or tail). ![]() This metamerism is thought to arise from identical teloblast cells in the embryonic stage, which give rise to identical mesodermal structures. Metamerism allows animals to become bigger by adding “compartments” while making their movement more efficient. Annelids have a segmented body plan wherein the internal and external morphological features are repeated in each body segment. (credit: Rob Hille)Īnnelids display bilateral symmetry and are worm-like in overall morphology. 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.
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