604 



COLLEGE ZOOLOGY 



Figure 427. Electron micrograph of virus particles, which are the small spherical bodies scat- 

 tered around the large oblong bacterial cell. The viruses are the smallest and simplest of living 

 things. They appear to be on the borderline between living and nonliving states. Electron 

 micrograph magniScation 48,000 times. (Courtesy of A.P. Jucnker, Michigan Department of 

 Health.) 



which appears to be an ancestral form that 

 gave rise to coelenterates, and, as a side 

 branch, to ctenophores. From planulae also 

 evolved primitive bilateral animals with an- 

 teroposterior and dorsoventral axes — ances- 

 tral flatworms. From such an ancestral type, 

 the flatworms may have arisen. Certain 

 other phyla of invertebrates seem to be 

 closely related through a common type of 

 larval stage, the trochophore; these are the 

 annelids, arthropods, and mollusks. Adult 

 echinoderms are radially symmetrical, but 

 their larvae are bilateral; and the phylum 

 apparently also arose from a bilateral type 

 of ancestor (Fig. 194). 



Relations between 

 vertebrates and invertebrates 



Many eminent zoologists have attempted 

 to trace the ancestry of the vertebrates to 

 some invertebrate form. These investigations 

 have resulted in a number of theories, but 

 it is impossible in this place to give an ac- 

 count of each. That their differences are 

 considerable may be inferred from the fact 

 that arguments have been advanced in favor 

 of annelids, arachnids, and echinoderms as 

 invertebrate ancestors of the vertebrates. 



The origin of vertebrates from an echino- 

 derm-chordate line seems to have so many 



