628 APPENDIX 8 



parts. Spherical symmetry is that of a ball, any section through the center 

 dividing the organism into symmetrical or mirrored halves. Radial sym- 

 metry is that of a wheel or cylinder, any one of several sections through the 

 main axis dividing the organism into symmetrical halves. Bilateral 

 symmetry is that of a boat or wagon, there being only one section, a 

 vertical one that passes through the longitudinal axis, that will divide 

 the organism into mirrored halves. 1 



4. Metamerism. Several phyla are characterized by the fact that their 

 members have bodies composed of linear series of segments, each segment 

 being built on the same basic structural plan and repeating, in more or 

 less modified form, the structure of the segments anterior and posterior 

 to it. This condition is known as metamerism, and each segment is a 

 metamere or somite. 



5. Body Cavities or Internal Spaces. Any metazoan has either one 

 or two body cavities. In the lower Metazoa the body is saclike in general 

 plan; i.e., the two-layered body wall surrounds a single large space that 

 has but one opening. The cavity may be a true digestive cavity and 

 the single opening a mouth (as in the Coelenterata), or it may be a cloaca 

 or water cavity and the single opening an excurrent pore (as in the 

 Porifera). In the higher Metazoa the body is constructed according to 

 the tube-within-tube plan. The alimentary canal is a tube open at both 

 ends (mouth and anus), lined with digestive cells (entoderm). In addition, 

 a new cavity lined with mesoderm separates the body wall from the 

 digestive tube; this is the coelom. 



6. Special Features and Combinations of Features. A number of phyla 

 and certain subphyla are characterized by the possession of structures 

 that are not found in any other group. Among examples that might 

 be cited are the pores and canal system of the Porifera, the stinging cells 

 (nematocysts) of the Coelenterata, the water-vascular system of the 

 Echinodermata, the hollow dorsal nervous system and notochord of the 

 Chordata, and the vertebral column of the subphylum Vertebrata. 



Even when structures are shared by more than one phylum (for in- 

 stance, metamerism by Annelida, Arthropoda, and Chordata; a ventral 

 ladderlike nervous system by Annelida and Arthropoda), they occur in 

 combinations that are distinctive and diagnostic of each phylum. Thus 

 the combination of metamerism, ventral ladderlike nervous system, soft 

 skin, and unsegmented appendages, with other features, is peculiar to 



1 Just as there may be minor departures from complete bilateral symmetry in a 

 boat or wagon, bilaterally symmetrical animals may show imperfect symmetry in 

 details of their structure. Thus in man the intestine, though essentially a bilaterally 

 symmetrical structure, is so long that it is assymetrically coiled in the abdomen; 

 and the originally symmetrically paired aortic arches have been reduced in number to 

 a single unpaired arch that lies on the left side of the median line of the body. 



