Significant evolutionary events during the Cambrian explosion z1 - evolution of multicellularity - animals can therefore become large z2 - evolution of true tissues yThe most primitive animals with true tissues (the Cnidaria) are diploblastic - yThey have two embryonic (or germ) layers: ectoderm and endoderm

Cnidarian cell types

Significant evolutionary events during the Cambrian explosion z3 - evolution of triploblastic animals - a third germ layer, the mesoderm, appears yMesoderm is used to make muscles, gonads, circulatory systems, some excretory structures z4 - evolution of HOX genes yThese are the homeotic genes we read about in Ridley earlier yThese genes are found in many animals and build similar structures yThe HOX genes are sometimes referred to as a “genetic tool kit”

HOX genes

Sponge Flatworm Earthworm Velvet worm Crustacean Centipede Fruit fly Amphioxus Mouse Zebrafish Figure 24.8

Hoxd-11 transcripts in hind part of limb bud Head Tail Later in development Early in development FISH LIMB BUD Few Hoxd-11 transcripts, only in hind part Figure 24.9a,b left

Hoxd-11 transcripts in hind part of limb bud Head Tail MOUSE LIMB BUD Hoxd-11 transcripts along head-tail axis Figure 24.9a,b right

Foot Fin Hoxd-11 expression along long axis of limb Hoxd-11 expression as in fish, followed by expression toward head Head Tail Head Tail (c) Hypothesis (b) Later in development (a) Early in development FISH LIMB BUD MOUSE LIMB BUD Figure 24.9

Significant evolutionary events during the Cambrian explosion z5 - the evolution of radial cleavage

Significant evolutionary events during the Cambrian explosion z6 - the evolution of deuterostomatous development yThe mouth formed early in development becomes the adult anus yThe adult mouth is formed secondarily (deutero - second, stoma - mouth)

Significant evolutionary events during the Cambrian explosion z7 - the evolution of a coelom (body cavity lined with mesoderm) yThis cavity can serve as a means of circulation yThis cavity can also serve as a hydrostatic skeleton to permit burrowing through sediments

Significant evolutionary events during the Cambrian explosion z8 - the evolution of spiral cleavage

Significant evolutionary events during the Cambrian explosion z9 - the evolution of protostomatous development yThe mouth formed early in development becomes the adult mouth yThe adult anus is formed secondarily (proto - first, stoma - mouth)

zThe lineage to which arthropods, annelids and other related phyla belong is called the Protostomia

Significant evolutionary events during the Cambrian explosion z10 - the evolution of a coelom (body cavity lined with mesoderm) yThe coelom of protostomes forms in a different way from that of deuterostomes and is hence convergent yThis cavity also serves as a means of circulation yThis cavity can also serve as a hydrostatic skeleton to permit burrowing through sediments

Significant evolutionary events during the Cambrian explosion z11 - the evolution of segmentation yDifferent segments are often specialized for particular functions (e.g., reproduction, excretion) yIn annelids, segmentation permits localized shape changes and hence efficient burrowing

Significant evolutionary events during the Cambrian explosion z12 - the evolution of a calcified exoskeleton yA pair of jointed appendages is associated with each segment yMuscles attaching to the inside of each joint allow great flexibility yThe appendages vary in form and function along the length of the body

Arthropod appendages