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Chapter 33 The Ecdysoans: The Molting Animals
Biology 102
Tri-County Technical College
Pendleton, SC
Distinguishing Features
Exoskeleton—firm, nonliving covering which cannot grow as animal inside it does– molted (shed) and new, larger one made
Cannot use cilia for locomotion so new forms evolved
Hard exoskeleton impede passage of oxygen into animal so new respiration mechanisms evolved
Features, cont.
Exoskeleton may be thin and flexible Thin, flexible exoskeleton called a cuticle Provides some protection, but does NOT
support the body Such animals rely on a hydrostatic skeleton “Incompressible internal fluids that transfer
forces from one part of body to another when acted upon by surrounding muscles”
Nematode Characteristics Commonly called roundworms ( one of must
abundant and universally distributed) Have thick, multilayered cuticle secreted by
underlying epidermis Exchange O2 and nutrients through cuticle
and intestine (one cell layer thick) Move materials through gut by contracting
pharynx (highly muscular organ at worm’s anterior end)
Predators, parasites, and free-living
Nematode Visual
Arthropod Characteristics
Regional segmentation, jointed appendages, and exoskeleton (chitin and protein)
Possess hemocoel – Circulating fluid called hemolymph (no
distinction made between blood and interstitial fluid surround body cells)
– Hemolymph exits vessels and percolates through body tissues into large cavity called the HEMOCOEL
Arthropod Visual
Pros and Cons of Exoskeleton
Advantages include protection and points of attachment for muscles to move appendages
Relatively impermeable to water Disadvantage is the old exoskeleton
must be shed (molted) for an arthropod to grow
Phylum Arachnida
Scorpions, spiders, ticks, and mites Cephalothorax with 6 pairs of appendages
– Chelicarae (either of first pair of appendages used for grasping and crushing), Pedipalps (sensing or feeling) and 4 pairs of walking legs
Gas exchange by book lungs (stacked plates in internal chamber)
Ability to make silk
Phylum Crustacea
Crabs, lobsters, shrimps, barnacles, & others
Two pair antenna, 3 or more pairs of mouthparts (including mandibles), walking legs of thorax, appendages present on abdomen
Gas exchange across thin areas of cuticle or by gills
Crustacea, cont.
Open circulatory system with hemolymph
Salt balance regulated by pair of specialized antennal or maxillary glands
Most are dioecious Some males have specialized pair of
appendages to transfer sperm to female (intromittent organ)
Crustacea Visual
Phylum Uniramia/SP Insecta
Insecta has greater species divesity than all other forms of life combined
Flight is key to insect success 1 or 2 pair of wings emerge from dorsal side
of thorax in most species Wings are extension of cuticle (not modified
appendages) Complete digestive system with specialized
regions
Insecta, cont.
Excretory organs (Malphighian tubules) are outpockets of the gut
Gas exchange by tracheal system with openings via spiracles
Dorsal brain with pair of ventral nerve cords Most undergo some type of metamorphosis Are dioecious and reproduce sexually
Insecta Visual
Comparing the cavities…
Coelom: fluid-filled body cavity completely lined by mesodermally derived tissues; inner and outer layers of tissue surrounding cavity connect dorsally & ventrally to form mesenteries which suspend internal organs
Hemocoel: Open-circulatory system in which fluid called hemolymph propelled by heart(s) through short arteries into spaces called sinuses surrounding tissues and organs
Comparing, cont.
Term “blood” best reserved for fluid in closed circulatory system
Body sinuses collectively called the hemocoel which is NOT part of coelom
In most arthropods, coelom that forms during embryo stage becomes greatly reduced
Hemocoel becomes main body cavity in adults
Metamorphosis
Incomplete: type of development during which young resemble adults but are smaller and have different body proportions– grasshoppers
Complete: type of development characterized by larval stages (maggot, grub, or caterpillar) which are VERY different in appearance from adults