ZOOGENESIS 



into quadrants, the quadrants diagonally opposite each other 

 being alike, and reversed or mirror images of the two with which 

 they are paired. This cross symmetry permits the development 

 of a greatly elongated body, such as we see in the Venus' girdle 

 (Cestui) and to a lesser degree in the creeping forms. But no 

 matter what the form of the body may be, the center of the animal 

 is always the axis passing through the intersection of the crossed 

 planes, and all the radii are always alike on either side of this 

 axis. So the elongated ctenophores are always double ended 

 with the head, so to speak, in the center of the body. This 

 cross symmetry is not adaptable to the requirements of a fixed 

 existence, so the ctenophores are all free swimming or creeping 

 creatures. But their locomotor powers are limited because of 

 the similarity of the two sides of the body on either side of the 

 median plane. Because of the mechanical limitations imposed 

 by their unique symmetry the ctenophores are much less numerous 

 in species than the coelenterates, sponges or protozoans. 



The several animal types here grouped under the Vermiformes 

 are usually distributed among several different phyla because 

 of the great differences in bodily structure that they show. There 

 is no denying the fact that these differences are important. At 

 the same time there is no denying the equally obvious fact that 

 all these creatures agree among themselves and differ from all 

 other animals in possessing a symmetry which is in part radial 

 and in part — generally for the most part — bilateral. Thus in 

 the tapeworms the head or scolex is commonly (though not 

 alwaysj) radially symmetrical with four equal sectors. In the 

 spiny-headed worms (Acanthocephala) which have by some 

 authors been associated with the tapeworms, by others associated 

 with the nematodes, and by still others regarded as quite without 

 close relatives, the anterior end is radially symmetrical. In the 

 turbellarians the mouth is on the ventral surface at, in front of, 

 or behind the middle, and the digestive cavities commonly radiate 

 from the base of the pharynx. Various transition forms unite 

 the turbellarians with the wholly parasitic flukes or trematodes. 

 In the trematodes the mouth is always at the anterior end and 

 usually leads into a forked intestine. The nervous system, 

 however, is radially arranged, consisting of six longitudinal 

 cords, two ventral, two dorsal and two lateral, all of which are 

 connected by transverse anastomoses. There are various types 



^73] 



