Mouth is small median aperture situated in the anterior sucker. Most polychaetes live in the ocean, where they either float, burrow, wander on the bottom, or live in tubes they construct; their colours range from brilliant to dull, and some species can produce light., AILSA ALLABY and MICHAEL ALLABY "Annelida Aquatic Aelosoma crawls with the ventral cilia of the head lobe. The tendency of ‘Head’ forma­tion is distinct. Some of the fami­lies with examples are: Family Aphroditidae (e.g., Aphrodita = Aphrodite)—sea mouse. Leeches are almost all ectoparasites , which attach to the external surface of the host (as opposed to endoparasites , which live within their hosts). The Class Hirudinea consists of the leeches. In oligochaetes locomotion is caused by the setae which are implanted directly in the body muscles and are mostly oriented in the central region of the body segments. AILSA ALLABY and MICHAEL ALLABY "Annelida In other species, there is a larval stage. Family Polynoidae (e.g., Polynoe)—scale worms. (ii) Branchial heart—which is situ­ated near the gills and pump the blood through the gills. Excretory organs are segmentally paired nephridia. Most of the pelagic and benthic forms swim for the purpose of food-collection and reproduction. Each body segment bears a pair of fleshy, lateral, paddle-like outgrowths, called parapodia, bearing numerous long setae in setigerous sacs. Class 1. While the posterior half develops a new head and becomes an independent male or female individual. ." Bilateral symmetry is well represented in annelida and this is an evolutionary advance­ment over more primitive radial symmetry. Ruppert and Barnes (1994) have suggested that the members of Oligochaeta which are the inhabitants of freshwater or terrestrial as well as marine environs may have stemmed from some early polychaetes but likely the oligochaetes evolved independently from some ancestral annelids. By the action of the segmentally arranged parapodia they pad­dle through water. Similarly, the contraction of longitudinal muscles produces pressure in the coelomic fluid. 7. Alimentary canal is tube-like, complete, extending from mouth to anus. The polychaetes are believed to be the most primitive of the annelid classes. Branchellion is found attached to skates and rays. Reproductive System of Phylum Annelida: Definition and Fossil Record of Phylum Annelida, Generalised Characters of Phylum Annelida, Specialised Characters of Phylum Annelida, Trend in Classification of Phylum Annelida, Phylogenetic Relationship of Phylum Annelida, The best answers are voted up and rise to the top. Annelid Structure. . On the basis of initiating contraction of heart muscle (pacemaker mechanism) in all animals the hearts are divided into two groups: The hearts where the initiating impulses for contraction arise within the muscles itself (intrinsic). Many minute vessels that arise from the vessels anastomose beneath the integument and organs. The closed, circulatory system is re­duced or absent in leeches. Most species are found in fresh­water or terrestrial habitats, a few species are marine. 5. (vi) Coelom reduced to sinuses without botryoidal tissues. Species lacking a pelagic trochophore stage show special adaptive features—e.g., protection by a parent, formation of an egg capsule, the discharge of eggs within one of the parent’s tubes, or viviparity (live birth rather than hatching from eggs). The ventral vessel cannot contract while lateral vessels pulsate. Nephridial opening situated on the surface between the segments. Anterior sucker is absent but posterior sucker is well developed and composed of four segments. Mostly terrestrial or some freshwater forms. First the wound seals over; then a structure (blastema) forms on the surface of the wound. In sand or mud burrowers and tube-dwellers the para­podia are poorly developed or absent espe­cially those of the posterior part of the body. Anterior five segments are provided with double rows of setae. When some commissural blood vessels are enlarged and achieve the function of contraction and relaxation, it is called pulsatile hearts. The members of the phylum are modified for sedentary, active, or ectoparasitic life, they occur on land, in freshwater, or in the sea. Cuvier first separated it into an independ­ent segmented higher group. These, combined with their segmentation and hydrostatic skeleton, allow for great flexibility in movement. The soft bodied animals including para­sitic arachnids and crustaceans were grouped under a phylum vermes by Linnaeus (1768). 6. Male reproductive system consists of 4- 17 pairs of metamerically arranged testis sacs. Most polychaetes on being transferred to freshwater die, while a few species like N. diversicolor, Troglochaetus can withstand and live in freshwater. (vi) Fertilization by the insertion of penis.