Rise of the Animal Kingdom and Epigenetic Mechanisms of Evolution - Building the Most Complex Structure on Earth - page 275

Figure 5.12 Fossilized specimen of Fuxianhuia protensa , representative of an arthropod
group found in Chengjiang Lagerstätte. Abbreviations : A1, antenna; Ab, abdomen; Es, eye
stalk; Ey, eye; Hs, head shield; Oc, optic capsule; Th, thorax. Scale bar, 1cm.
Source : Ma et al. (2012) .
blood circulation system with a number of vessels and annelid blood contains
hemoglobin for oxygen transport. The excretory system in lower annelids has a
metanephridium in each segment and a duct that opens at the body surface. In other
annelids, the excretory system consists of a single duct. Many aquatic annelids
evolved gills for exchanging gases with the environment. Most of them have separate
sexes and produce free-swimming larvae. A number of species reproduce asexually.
Arthropods
Arthropod-like animals evolved during the Cambrian period. A fossilized specimen
of Fuxianhuia protensa is 520 Mya old ( Figure 5.12 ). Today they represent the most
successful phylum. With more than 1 million species, arthropods encompass 90% of
animal species on Earth. Insects are the largest arthropod group.