PHYLOGENY 



phylum Platyhelminthes. But the majority of zoologists believe that the 

 appearance of homology is misleading. Haeckel believed that a primitive 

 hydrozoan was the ancestor of the bilateral phyla, basing his opinion as 

 usual upon the Biogenetic Law. The evidence is insufBcient, and it seems 

 at least as probable that flatworms were first derived from the same 

 planula-like stock which gave rise to the coelenterates. But in the flat- 

 worms, the middle cell layer ( mesoderm ) became more highly developed, 

 with organized muscle layers, reproductive system, and an excretory sys- 

 tem. Yet no coelom, or body cavity, developed within the mesoderm, as it 

 does in most higher groups. The nervous system is formed from the ecto- 

 derm, although it is imbedded in the mesoderm. It is not a diffuse nerve 

 net as in the coelenterates, but rather it is somewhat centralized, being 

 organized about cerebral ganglia at the head end and longitudinal cords. 

 There are organized sense organs, including eyes. Food is still distributed 

 to the various parts of the body by the branches of the digestive tract, 

 which is now called an intestine rather than a gastrovascular cavity. Thus 

 the flatworms are much advanced beyond the tissue grade of construction 

 which characterizes the radiate phyla, for definite organ systems are pres- 

 ent, principally in the mesoderm. 



Hadzi has urged a radically different phylogeny. He believes that multi- 

 nuclear ciliates became acoelous flatworms by formation of cell mem- 

 branes. In evidence, he points out that both are ciliated; hermaphroditism 

 of flatworms he finds homologous with conjugation of ciliates; trichocysts 

 of ciliates he believes are represented by sagittocysts, rod-like inclusions 

 in some of the epidermal cells of acoelous flatworms. He believes that the 

 Anthozoa were derived from flatworms by adoption of the sedentary life, 

 and that the other coelenterates were then derived from anthozoans. The 

 higher Metazoa were also derived from flatworms, and are grouped into 

 only four phyla, a procedure which unites some highly diverse groups. 



At this point it may be well to recall the admonition of Hyman * that 

 "the exact steps in the evolution of the various grades of invertebrate struc- 

 ture are not and presumably never can be known. Statements about them 

 are inferred from anatomical and embryological evidence and in no case 

 should be regarded as established facts." Although the phylum Platyhel- 

 minthes, and especially the most primitive class of this phylum, the Tur- 

 bellaria, is commonly treated as the stem group from which the higher 

 phyla arose, this is by no means established. Closely related to the Platy- 

 helminthes, and on the same general level of organization, is the phylum 

 Nemertinea, a small group of marine flatworms. This group is less well 

 known than the former because it is predominantly marine, and because 

 it is a difficult group to study. But the Nemertinea show some characters 

 which qualify them for special consideration as potential forerunners of 

 the higher invertebrates. For the first time, there is an anus present, so 

 that the digestive system is said to be complete. Also, there is a simple 

 blood circulatory system, and the blood contains hemoglobin. Particularly 



* By permission from Hyman, L. H., "The Invertebrates," Vol. I, McGraw-Hill Book 

 Co., Inc., 1940. 



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