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ArthropodaEcdysozoaParthenogenesis A form of asexual reproduction where viable offspring develop from unfertilized eggs that depending on the organism may be either haploid or diploid and a variety of other unusual reproductive strategies are seen in some insectsnot surprising considering the diversity of the groupSuphylumChelicerataSpiders scorpions mites ticks and horseshoe crabsPresence of chelicera on the first body segment its how they got their formal name of Chelicerataanimals with chelicera Chelicerates have six pairs of uniramous appendages and pedipalps behind the chelicera are usually involved in feeding locomotion or sometimes reproduction and are followed by four pairs of walking legsA chelicerates body consists of two tagmata including an anterior prosoma cephalothorax with the six pairs of appendages and a posterior opisthosoma abdomen formed from the fusion of twelve or less segmentsThe opisthosoma may be divided into a mesosoma and metasoma or may have a terminal telson with the anus at its baseWith the exception of the horseshoe crabs chelicerates lack the compound eye found in most arthropods If eyes are present they are simple eyes No antennaePredatory animals and consume their food in liquid form and extracorporeal where digestive enzymes are regurgitated into the food Once it has been broken down the resulting product is consumed digestion is not uncommon especially in the arachnidsDigestive track is complete including digestive ceca or diverticulaThe main body cavity is a hemocoel and a dorsal heart that fills through ostia in its wall and pumps hemolymph that bathes the internal organs The respiratory pigment hemocyanin is used by some chelicerates especially those that use book lungs or book gills for gas exchange pigments are absent in spiders that use trachea to deliver air directly to the tissuesExcretory organs include coxal glands the produce guanine or Malpighian tubules that excrete uric acidThe nervous system follows the usual arthropod plan but the brain located above the esophagus doesnt have a deuterocerebrum a subesophageal ganglion is connected to a paired ventral nerve cord that ancestrally had paired ganglia in each segment of the bodyChelicerates are dioecious and depending on the species fertilization may be internal with sperm transferred either directly by the male or in a spermatophore Development of the fertilized egg is usually direct in terrestrial species and indirect in marine cheliceratesSubphylumCrustaceaLobster crab shrimp or crayfish which are carnivores or omnivoresAncestrally Fivesegmented head and a multisegmented trunk he head included compound eyes and two preoral appendages antennules and antennae Behind the mouth were paired mandibles and two pairs of maxilla Primitively the trunk appendages were all the same and attached to the body by the protopodite consisting of the coxa and basis and had two branches an inner endopodite and outer exopodite which together created the inverted Yshaped biramous appendage characteristic of the subphylum Each leg was involved in locomotion feeding and respiration Presently Legs found on the trunk now assisting the maxillae in processing food and became maxillipeds The head and the trunk segments with the maxillipeds became the cephalothorax The second tagma is the abdomen which is part of the remaining trunk often became
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