acoelomates

ZOOLOGY NOTES—ACOELOMATE ANIMALS THIS GROUP CONSISTS OF THREE PHYLA: PLATYHELMINTES (FLATWORMS), NEMERTEA (RIBBON WORMS), AND GNATHOSTOMUDIDA CHARACTERISTICS PHYLUM PLATYHELMINTHES CHARACTERISTICS ü TRIPLOBLASTIC, BILATERAL SYMMETRY, BODY FLATTENED DORSOVENTRALLY ü EPIDERMIS MAY BE CELLULAR OR SYNCYTIAL (CELLS WHICH ARE MULTINUCLEATE); MAY HAVE CILIA ü THE EPIDERMIS OF SOME CONTAIN RHABDITES, WHICH SECRETE A PROTECTIVE MUCUS COVERING FOR THE BODY ü MUSCLE LAYERS FOUND BENEATH THE EPIDERMIS, ACOELOMATE (DIGESTIVE TUBE IS THE ONLY BODY SPACE), SPACES BETWEEN THE ORGANS FILLED WITH MESENCHYME (CONNECTIVE TISSUE) ü NERVOUS SYSTEM CONTAINS SIMPLE SENSE ORGANS; SOME WITH EYESPOTS ü EXCRETORY SYSTEM CONTAINS FLAME CELLS, WHICH HELP REGULATE WATER BALANCE ü RESPIRATORY, CIRCULATORY, AND SKELETAL SYSTEMS LACKING ü MOST ARE MONOECIOUS (HAVING BOTH MALE AND FEMALE GONADS); FERTILIZATION IS INTERNAL; SOME PARASITES HAVE A COMPLICATED LIFE CYCLE WITH SEVERAL HOSTS CLASS TURBELLARIA MOSTLY FREE LIVING; ALL OTHER CLASSES are parasitic
 * THESE ANIMALS LACK A COELOM, OR BODY CAVITY, AND ARE CALLED ACOELOMATE
 * BILATERAL SYMMETRY: THESE ARE THE SIMPLEST ANIMALS TO HAVE BILATERAL SYMMETRY, IN WHICH THEIR BODIES CAN BE DIVIDED INTO TWO HALVES WHICH ARE MIRROR IMAGES OF EACH OTHER. BECAUSE THEY HAVE DIRECTED MOVEMENT, THEY HAVE AN ELONGATED BODY WITH A HEAD (ANTERIOR) AND TAIL (POSTERIOR) END. ONE SIDE OF THE BODY IS KEPT UP (DORSAL), WHILE THE OTHER IS SPECIALIZED FOR LOCOMOTION AND KEPT DOWN (VENTRAL).
 * TRIPLOBLASTIC: THESE ANIMALS HAVE THREE TISSUE LAYERS—ECTODERM (OUTER LAYER), ENDODERM (INNER LAYER) AND MESODERM (A MIDDLE LAYER CONSISTING OF MUSCLE FIBERS AND CONNECTIVE TISSUE. THE ENDODERM LINES AN INTERNAL SPACE, THE DIGESTIVE CAVITY
 * ORGAN SYSTEMS: THESE ANIMALS HAVE MORE SPECIALIZATION THAN THE RADIATE ANIMALS (CNIDARIANS) AND ARE SAID TO HAVE REACHED THE ORGAN-SYSTEM LEVEL OF ORGANIZATION. CEPHALIZATION DEVELOPED ALONG WITH THE FORMATION OF A HEAD END, ALLOWING THE DEVELOPMENT OF A PRIMITIVE CENTRAL NERVOUS SYSTEM. THEY ARE THE SIMPLEST ANIMALS WITH AN EXCRETORY SYSTEM. NEMERTEANS ARE THE SIMPLEST ANIMALS WITH A CIRCULATORY SYSTEM. THEY HAVE A ONE-WAY ALIMENTARY CANAL (DIGESTIVE SYSTEM WITH ONLY ONE OPENING)