THE GROUPS AND ANCESTRY OF ORGANISMS 



103 



usually are not believed to display an independent 

 organ basis of structure. However, there is suf- 

 ficient peculiarity of flatworm (Platyhelminthes) 

 structure for some zoologists to describe it as a 

 complex of tissue, organ, and organ system organiza- 

 tion, often termed "tissue-organ." 



EMBRYOLOGY 



Many features of embryonic development are im- 

 portant for understanding of evolutionary history, 

 and for classification of living phyla. Here only the 

 germ layers found in multicellular animals are re- 

 viewed. In development, the fertilized egg divides to 

 forma ball of many cells, the blaslula, which has a cav- 

 ity, the blastocoel. Ultimately these many cells dif- 

 ferentiate into two or three layers, germ layen, in a 

 so-called gastrula stage (Figure 7.3). The germ lay- 

 ers probably are not strictly comparable in all phyla; 

 however, each germ layer forms certain body struc- 

 tures. The outer germ layer, the ectoderm, forms the 

 nervous system and outer coverings of the body; the 

 inner layer, the endoderm, forms the lining of the or- 

 gans of the digestive system; the middle layer, the 

 mesoderm (a layer sometimes absent), forms the skele- 

 ton, muscles, and circulatory system. 



asymmetrica I 



spherical 



radial 



biradia I 



Sv^ 



bilateral 



SYMMETRY 



Many sponges are asymmetrical, as indicated by 

 one's inability to divide them into two equal, or 

 mirror-image, halves. Other Porifera are essentially 

 spherical in their symmetry. A test of this is the ability 

 to divide one of these animals into halves by any cut 

 passing through the center of the ball-like form (Fig- 

 ure 7.4). The Coelenterata, Ctenophora, and Echino- 

 dermata have radial or biradial symmetry; however, 

 that of echinoderms is a secondary evolutionary ac- 

 quisition from bilateral forms as a result of a 

 sedentary life. Radial symmetry is found in a pie, 

 because an infinite number of cuts through the center 

 and in one plane will produce two mirror images. 

 Biradial symmetry can be likened to a pie with two 

 cherries put on a diameter of the pie and at equal 

 distances from the center. In biradial symmetry 

 there are only two ways of cutting to produce equal 

 halves, one through the centers of the pie and two 

 cherries and the other through the center of the pie 

 and perpendicular to the first possible cut. Bilateral 



Figure 7.4 The kinds of symmetry. 



symmetry is the commonest and most advanced type. 

 It is found in most of the phyla. Only a single cut is 

 possible if one is to form equal halves, or right and left 

 mirror images. 



BODY CAVITIES 



Body cavities will be discussed in more detail 

 later. The Ectoprocta through Chordata have true 

 body cavities, or coeloms, of two types, schizocoels and 

 enterocoels. The Mollusca and Arthropoda have re- 

 duced coeloms; their body cavity, the hemocoel, is so 

 named because blood circulates through it. The 

 Acanthocephala, Aschelminthes, and Entoprocta 

 have so-called false body cavities, pseudocoels. Those 

 animals lacking a body cavity are said to be acoelomate. 



