Invertebrates

Page
8

trilobite fossil

Slide 71

General Characteristics of Arthropods

The appendages of some living arthropods are modified for many different functions.

The body of an arthropod is completely covered by the cuticle, an exoskeleton made of layers of protein and the polysaccharide chitin.

When an arthropod grows, it molts its exoskeleton.

Slide 72

Arthropod External Anatomy

Cephalothorax

Antennae

(sensory

reception)

Head

Thorax

Abdomen

Swimming appendages

(one pair located

under each

abdominal segment)

Walking legs

Mouthparts (feeding)

Pincer (defense)

Slide 73

Arthropods have an open circulatory system in which fluid called hemolymph is circulated into the spaces surrounding the tissues and organs.

A variety of organs specialized for gas exchange have evolved in arthropods.

Slide 74

Molecular evidence suggests that living arthropods consist of four major lineages that diverged early in the phylum’s evolution:

Cheliceriforms (sea spiders, horseshoe crabs, scorpions, ticks, mites, and spiders)

Myriapods (centipedes and millipedes)

Hexapods (insects and relatives)

Crustaceans (crabs, lobsters, shrimps, barnacles, and many others).

Slide 75

Horseshoe crabs

Slide 76

Most modern cheliceriforms are arachnids, which include spiders, scorpions, ticks, and mites.

Slide 77

Arachnids

Scorpion

Dust mite

Web-building spider

50 µm

Slide 78

Arachnids have an abdomen and a cephalothorax, which has six pairs of appendages, the most anterior of which are the chelicerae.

Gas exchange in spiders occurs in respiratory organs called book lungs.

Many spiders produce silk, a liquid protein, from specialized abdominal glands.

Slide 79

Anatomy of a spider

Intestine

Heart

Digestive

gland

Ovary

Anus

Spinnerets

Silk gland

Gonopore

(exit for eggs)

Sperm

receptacle

Book lung

Chelicera

Pedipalp

Poison

gland

Eyes

Brain

Stomach

Slide 80

Myriapods