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Chapter 18: 11-16 Complex Invertebrates

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Complex Invertebrates: Annelids, Arthropods

WebLecture Topics

Animals with Complex Organ Systems

Animals with a complete body cavity can support a higher level of organization, which means that different organ systems can specialize on different functions. In general, animals with a coelom have more complete and complex digestive, circulatory, respiratory, nervous, and muscle systems, and many species have complex skeletal systems as well that have developed to protect internal organs from damage and (in land animals) provide support against the pull of gravity. We'll discuss vertebrate animals in the next chapter.

Even within the invertebrate animal groups, the diversity of forms among those with a coelom show up int the many classes within the mollusks, annelids, and arthropods phyla. While mollusks provide food and recycle organic wastes on the ocean floor, and earthworms make agriculture possible, arthropods are often considered the most successful animal phyla, because this group contains not only the greatest number of individual organisms, but also the most diversity in forms and the widest range of habitats of any animal phyla. Arthropods are divided into groups according to the number of antennae (none, one pair, or two pairs) and legs (3 pair, 4 pair, one or two pairs per segment). The largest group of arthropods, the insects, are further classified according to their life stages in passing from juvenile to adult forms (no metamorphosis, partial metamorphosis, complete metamorphosis). The variety of adaptations and forms means some members of the insect order fit into nearly every habitat possible on Earth, from the Antarctic chironomid midge, a flightless bug to the diving beetle living in temperate rivers, to the Death Valley giant tarantula.

Animals without a coelom


or with a pseudocoelomPseudocoelom

Simple body plans; Porifera and Cnidarians are radially symmetric. Primitive nervous systems and sensory organs, simple to complex digestive and reproductive systems. No skeletal structure.

See detailed breakdown in Web Lecture Animals Without a Coelom.

Protostomes with a coelom Coelom At this point, differences become very difficult to track, so the following summeries are more general.
Mollusks Common characteristics: Mantle (which often produces a shell), radula. Open circulatory system.


Small marine animals covered with shells or plates like armor


Snails (shelled); slugs and nudibranchs (no shells). Most gastropods are twisted since one side grows faster than the other.


Clams, oysters and other burrowing shellfish. These are filter feeders: they get their food by sucking in water and filtering it over their gill system and into their mouths.


Shell-less mollusks like the octopus and squid: large brains and eyes.

Annelids Common characteristics: Segmented body, tubular, divided into sections and possessing a primitive brain and sense organs.



Polychaete (like the Christmas tree worm)

Arthropods Common characteristics: Paired, jointed appendages, exoskeletons, segmented bodies, specialized respiratory systems (gills, trachea or air tubes through the skeleton into the body, or book lungs), and a complex nervous system with a brain.

Note that this three-group system based on the number of antennae differs from the looser, five-group system of your text. As we discussed, classification is not a fixed area of biological study!

Chelicerata No antennae; instead, first appendages are chelicerae, which are used to manipulate food, in defense, for movement, and in mating, depending on the species. Represented in the fossil record by trilobites.

Horseshoe Crabs Longtail for locomotion; marine.
Arachnids Spiders. Fused head and thorax, separate abdomen. 6 pairs of appendages: chelicerae, pedipalps (hold food for chewing), and 4 pairs of legs.
Crustaceans Marine animals with mandibles (jaws), two antennae, gills, compound eyes, specialized organs for reproduction. Often bottom feeders (harvesting bits of organic materials which would otherwise decompose).

Lobsters, crabs, crayfish, shrimp
Uniramia One set of antennae.
Centipedes Segmented, one pair of appendages per segment
Millipedes Segmented, two pair of appendages per segment
Insects Three major body segments: head, thorax, abdomen. 6 legs from abdomen. Undergoes no, partial, or complete metamorphosis.
Metamorphosis Orders
None (egg → immature form → adult)
silverfish and bristletails
Partial (egg → nymph → adult)
Sucking lice
True Bugs
Aphids, cicadas
Complete (egg → larva → pupa → adult)
Moths and butterflies
Houseflies and mosquitoes
Beetles and weevils
Ants, bees, and wasps