Arthropoda

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Annelid/Arthropod Similarities
Metamerism → Body is composed of a series of basically identical units called metameres, somites or segments One pair of appendages per somite in primitive forms. Ex: legs, antennae, mouth parts...Annelids → parapodia Central Nervous System → Solid ventral nerve cord with segmental ganglia Embryology

Phylum Arthropoda

-Chitinous exoskeleton poses a limit on size and gets heavy. It can't stretch... -Ecdysis (Molting)
-Serial, jointed appendages (except on acron and telson; aka annelid prostonium) -Small size. Range: 1mm<Arthropod<3m
-Special sensory and respiratory surfaces
-Tagmatization → fusion or modification of segments to form a head, thorax, etc -NO cilia
-NO nephridia
-Coelom largely replaced by haemocoel (blood seeps)
→Open regulatory system
→ Triploblastic coelomates with greatly reduced coelom
-All insects have mouth parts, they just use them in different ways (can be seriously modified) -Blood feeders are responsible for a lot of human diseases → vectors of malaria, sleeping sickness, chagas, etc

Ecdysis
-Forebrain secretes Brain Hormone (BH)
-BH stimulates prothoracic gland to produce ecdysone
-Ecdysone stimulates the hypodermis to:
Detach from old cuticle
Lay down new epicuticle
Secrete proteases and chitinases
Resorb old endocuticle
Secrete new procuticle
-Swells → stretches and breaks old cuticle. New stretches and becomes tanned -Ecdysial lines

Crayfish appendages on tagmata

Head (5 somites):
Atennae 1&2
Mandibles
Maxillae 1&2
Thorax (8 somites):
Maxillipeds 1-3
Walking legs 1-4
Chelae
Abdomen (6 somites):
Pleopods 1-5
Uropods
(+telson=tail fan)

Crayfish
-Gas exchange through gills
-Respiratory current is produced by vibration of max 2 (bailer) -Blood contains haemocyanin (blue when oxygenated)
-Gap at base of each leg
-Open circulatory system, capillaries only around brain
-Arteries: opthalmic,...
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