EXISTING ORGANISMS— CLASSIFICATION 35 



The Echinodcrms have returned ahnost completely to the older 

 form, although they begin life as bilaterally symmetrical larvae. 



The Body Cavity. Triploblastic animals, even in the lower phyla, 

 develop another characteristic, the body cavity, or coelom, formed 

 by a splitting of the middle layer. This cavity is important in 

 many animals in connection with circulation and excretion, but 

 in the higher phyla is littk^ more than a cradle for the visceral 

 organs. It occurs in a modified form in roundworms, and is found 

 in all succeeding phyla (Fig. 17). 



Metameric Structure. The Annelids, particularly the common 

 earthworm, illustrate the development of repeated similar parts, 

 called metameres. These are indicated by the ring-like sub- 

 divisions on the outside of the earthworm, but involve also a sub- 

 division of the coelom and the arrangement of parts of various 

 organic systems. This arrangement is modified in »^^any ways, 

 but in the earthworm not to such an extent that the b accession of 

 similar organs is obscured. Many segments show a portion of the 

 alimentary tract, a ganglion of the nerve cord, lateral nerves, 

 portions of longitudinal blood vessels, a pair of transverse blood 

 vessels, a pair of nephridia, and other structures. Such a plan is 

 the basis of development in all higher phyla, even in man, al- 

 though in the adult there is little visible evidence of it save in the 

 spinal column, ribs, and associated muscles and nerves. In the 

 fish, one of the lowest vertebrates, everyone is familiar with the 

 V-shaped bands of muscle which lie along the sides, each repre- 

 senting a metamere. 



Appendages. While none of the preceding phyla have paired 

 jointed appendages for walking, grasping and other functions, the 

 Arthropods develop a long scries, so characteristic that they have 

 given the name to the order from the Greek apdpov, a joint, and 

 TTovs, TToSos, a foot. Metameric structure is conspicuous in the 

 phylum, although modified by the division of the body into three 

 distinct regions, head, thorax and abdomen, and each metamere 

 appears to have been fundamentally capable of producing a single 

 pair of appendages. In the nineteen pairs present on the lobster 

 or crayfish, there is a fine lesson in the possibilities of such struc- 

 tures (Fig. 18). Developed on a common plan of structure, they 

 are formed for swimming, walking, grasping, accessory organs 

 of generation, accessory mouth parts, mandibles, and sensory 

 organs. 



