THE MAIN LINES OF ANIMAL EVOLUTION 



important according to Kofoid is the fact that the nervous system is based 

 upon cerebral ganglia and eight longitudinal nerve cords, two dorsal, two 

 ventral, and two on each side. He has pointed out that this lends itself 

 to the formation of the principal invertebrate nervous systems by the de- 

 velopment of the ventral cords and the suppression of the others; and to 

 the formation of the chordate nervous system by the development of the 

 dorsal cords and the suppression of the others. But the fact that the ne- 

 merteans capture their food by means of an extensible proboscis, a mecha- 

 nism found in no other group, suggests that they are a terminal group. 



Yet another possibility is that a third group, an unknown phylum of 

 very primitive flatworms derived from the primitive planula by the devel- 

 opment of the mesoderm may have been ancestral to both the Platyhel- 

 minthes and the Nemertinea. Whatever their origin, once formed, the 

 platyhelminths diverged along three main Hues of descent. The first of 

 these is represented by the class Turbellaria, comprising the free-living 

 flatworms, of which Pla7ioria is the best known example. The other two 

 classes have become greatly modified for parasitism, and most of them 

 have developed complicated life cycles to permit their transfer from one 

 host to another. These are the class Trematoda, the flukes, which have 

 retained all of the organ systems of the Turbellaria, and are generally 

 internal parasites of vertebrates; and the class Cestoda, the tapeworms, 

 which have undergone an extreme degenerative evolution. They are all 

 intestinal parasites of vertebrates. 



On the same general level of organization as the above discussed acoe- 

 lomate groups, there are a number of minor phyla, of very uncertain re- 

 lationships, which are characterized by the possession of a pseudocoel. 

 A coelom is, by definition, a body cavity formed \vithin mesoderm. A pseu- 

 docoel, on the other hand, is a remnant of the cavity of the blastula, and 

 it may be partially filled by large, vacuolated cells. The taxonomy and 

 relationships of these pseudocoelomate organisms are very uncertain. It 

 seems best here simply to acknowledge that they exist, and that they are 

 among the most primitive of bilaterally symmetrical animals, without at- 

 tempting to unravel their relationships. These are the Rotifera, a phylum 

 of microscopic animals which are known to every student of elementary 

 biology as fascinating contaminants of most protozoan cultures; the phy- 

 lum Gastrotricha, which are small, worm-like animals of wide distribution; 

 the phylum Kinorhyncha, comprising a group of minute, marine, worm- 

 like animals; the phylum Nematoda, a very important group, the so-called 

 round worms, which includes many little-known free-living worms as well 

 as some of the best known parasites of both plants and animals; the phy- 

 lum Nematomorpha, or horsehair worms, consists of long, slender worms 

 which are parasitic during larval life, but are generally free-living when 

 mature; and the phylum Priapuloidea, comprising a few species of small, 

 externally segmented marine worms. These six phyla are sometimes 

 grouped together as classes of a single phylum, Aschelminthes. The phy- 

 lum Acanthocephala, or spiny-headed worms, are generally small worms 

 the young of which are parasitic on invertebrates, while the adults are 

 parasitic on vertebrates, including man. A final pseudocoelomate phylum 



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