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a. 环节动物的

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  • Annelid | Definition, Characteristics, Habitat, Reproduction, Diet . . .
    annelid, any member of a phylum of invertebrate animals that are characterized by the possession of a body cavity (or coelom), movable bristles (or setae), and a body divided into segments by transverse rings, or annulations, from which they take their name The coelom is reduced in leeches, and setae are lacking a few specialized forms, including leeches
  • Annelid - Segmentation, Metamorphosis, Reproduction | Britannica
    Typically, the first three segments form almost simultaneously in the lower half of the trochophore Development of the parapodial lobe and the appearance of larval bristles (setae) follow shortly thereafter The body grows in length by the addition of new segments from the preanal segment (pygidium or tail), which is the site of all additional segment formation
  • Annelid - Nerves, Segmentation, Cephalization | Britannica
    Annelid - Nerves, Segmentation, Cephalization: The nervous system of free-moving polychaetes consists of a dorsal brain, the ventral subesophageal ganglion, and paired nerve cords with one ganglion per segment The polychaete digestive system is generally a straight tube; a mouth leads into an esophagus, which is followed by the intestine and the anus
  • Structure and classes of annelid | Britannica
    annelid , Any member of a phylum (Annelida) of invertebrate animals that possess a body cavity (coelom), movable bristles (setae), and a body divided into segments by crosswise rings
  • Nervous system - Annelids, Reflexes, Sensory | Britannica
    Nervous system - Annelids, Reflexes, Sensory: The brain of most annelids (phylum Annelida; segmented worms, including the leeches and terrestrial earthworms) is relatively simple in structure The earthworm brain is a bilobed mass lying above the pharynx in the third body segment Sensory nerves leave the brain and run forward into the prostomium (extreme anterior end) and first segment
  • Animal reproductive system - Annelids, Mollusks, Reproduction | Britannica
    Animal reproductive system - Annelids, Mollusks, Reproduction: Annelids have a well-developed body cavity (coelom), a part of the lining of which gives rise to gonads In some annelids, gonads occur in several successive body segments This is true, for example, in polychaetes, most of which are dioecious Testes and ovaries usually develop, though not invariably, in many body segments; and
  • Declaration of Independence | Summary, Definition, Date, Text . . .
    Declaration of Independence, in U S history, document that was approved by the Continental Congress on July 4, 1776, and that announced the separation of 13 North American British colonies from Great Britain It explained why the Congress on July 2 “unanimously” by the votes of 12 colonies (with New York abstaining) had resolved that “these United Colonies are, and of right ought to be
  • Lobopod | Ancient, Extinct Unique Animal | Britannica
    The groups are of interest chiefly because they bear strong resemblances to primitive annelidan forms as well as to the phylum Arthropoda (e g , insects, crustaceans)—the most highly evolved invertebrates Like arthropods, both onychophorans and tardigrades periodically molt an external cuticle, and lobopods and arthropods are sometimes





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