648 APPENDIX B 



About 8,000 species of Annelida have been described. The Annelida are closely 

 related to and presumably the ancestors of the Arthropoda. 



Phylum X. ONYCHOPHORA (on' i kof o ra; Greek, onyx, "claw," 

 and phoros, "bearing"). Soft-bodied, wormlike creatures up to a few 

 inches long, discontinuously distributed in the tropics. They show some 

 features like those of Annelida, such as the paired excretory nephridia 

 and the flexible cuticle covering the body ; in other respects they suggest 

 Arthropoda, having respiratory organs resembling the tracheae of insects, 

 and a series paired legs showing weak segmentation. The most ancient 

 known type was found in Cambrian marine sediments and had a pair of 

 gills near the head; all living species are terrestrial. Example: Peripatus. 



Phylum XL ARTHROPODA (ar throp' o da; Greek, arthron, "joint," 

 and podos, "foot"). 



Crustaceans, centipedes, insects, spiders, and their allies. Complex 

 Metazoa, typically exhibiting the following characters: 



Triploblastic ; bilaterally symmetrical; metameric. Although the body 

 cavity is continuous and without transverse septa, metamerism is shown 

 internally in the arrangement of the nervous system, muscles, heart, 

 and other organs. Groups of segments tend to fuse or specialize into 

 larger regions, such as head, thorax, and abdomen, or into combinations 

 of these (cephalothorax). With the exception of the vertebrates, arthro- 

 pods are the most highly developed metameric animals. Coelom present 

 but much reduced, its place being taken by an extensive blood space, the 

 hemocoele. 



Jointed appendages. Usually a hard exoskeleton. Ladder type of 

 nervous system (as in Annelida), with a tendency toward cephalization 

 (concentration of ganglia at anterior end). Usually with well-developed 

 and segmented head appendages (antennae, mouth parts, etc.). A main 

 longitudinal blood vessel ("heart") dorsal to the alimentary canal. 

 Respiration by gills or tracheae. 



The arthropods comprise about four-fifths of all the known species of animals. 

 Many occur in marine and fresh-water habitats, but they are best represented 

 on land. They may be regarded as the dominant animals of today, if numbers of 

 different species and numbers of individuals are accepted as criteria of dominance. 

 Approximately 650,000 living species are known, of which 600,000 are insects. 

 This huge assemblage of animals may be divided into classes, as follows: 



Class 1. The Trilobites (*Trilobita). Primitive marine arthropods of the Pale- 

 ozoic era (Figs. 25.2, 27.2 and B.17), with flattened, segmented bodies, covered 

 above with a carapace; with an anterior head region, a segmented thorax, and a 

 tail plate or telson; and with a pair of longitudinal grooves dividing the body 

 into a central and two lateral lobes. Head region with a pair of slender antennae, a 

 pair of compound eyes, and paired ventral appendages; thoracic segments each 



