510 Causes and Course of Organic Evolution 



But by retention of the cilia over the body surface, by con- 

 tinued elongation of the body, and correlatively by elongation 

 of the nerve fibres in it, also by formation of an increasingly 

 evident body cavity, organisms must have evolved that dif- 

 fered little from typical rhabdocoel turhellarians; while, by 

 restriction of the cilia to localized areas, by condensation of 

 the body substance into an oval active mass, and by formation 

 mainly of muscular body tissue, other organisms evolved into 

 typical rotifers. 



In subsequent history the turhellarians in large measure 

 branched off into a side line of development, in which some 

 offshoots by migration on to land or into the sea produced 

 there the increasingly complicated and enlarged triclad and 

 poly clad divisions of the series; while others, adopting a para- 

 sitic life mainly in fresh water or within land animals, have 

 evolved into the trematode and cestode series of organisms. 

 But most of the fresh- water as well as the marine and parasitic 

 derivatives from the primitive stock seem all to have failed 

 in the evolution of higher groups than themselves. 



One division of the Turbellaria however — the typical Rhab- 

 docoela — deserves special mention, for even living fresh-water 

 representatives of it like Dalyellia, Mesostoma, and Prorhynchus 

 conduct us, from small forms allied in structure to the Rotifera, 

 upward to organisms w^ith close affinity to the Nemertinea. 

 This last group has already been treated in detail as being in 

 the direct line of evolution with the Chordata (p. 450). 



A considerable number of the living Rhabdocoela are fresh- 

 water, and these we would regard as still inhabiting ancestral 

 environment. Others have assumed a marine habitat, and 

 have there ramified into diverse genera. So we would give 

 a totally different interpretation, than have some naturalists, 

 to the occurrence of species of Plagiostoma. The genus is 

 made up mainly of marine representatives, but the species 

 P. lemani occurs at varying depths in the Lake of Geneva, 

 as well as northward into Russia, along with species of allied 

 genera. These we would regard as very ancient and primi- 

 tively fresh-water species, from which the marine species have 

 been derived. 



As to the affinities then of the Rhabdocoela, we would regard 

 them as intermediate in structure between the more elongate 

 Rotifera like Notommata and Taphrocampa on the one hand, 

 and the Nemertinea on the other. Thus, in the oval to linear 

 shape, in the dorso-ventral orientation, in the secretion of 

 mucus from ventral cement glands, in the simple alimentary 

 canal, in the sucking protrusible pharynx, in the presence of 



