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