142 INVERTEBHATK MORPHOLOGY. 



and more oblique until wliat were originally the posterior 

 edges of the* lappets meet ou the dorsal surface. The edges 

 of the lobes of the lappets are fringed with long cilia, and 

 consequently a lobed prseoral band of cilia is produced. 

 These larvae pass into the adult form by gradually becoming 

 more and more flattened dorsoventrally, the ciliated lappets 

 or lobes at the same time growing smaller and smaller until 

 they finally disappear. 



Relationships of the Turbellaria. A relationship of the Turbellaria, 

 especially of the Polyclads, with the Ctenophores has been advocated 

 within recent years, and through this relationship genetic affinities with 

 the Cnidaria have been sought. The question of the affinities of the Cteno- 

 phores has already been discussed, and it has been pointed out that it is 

 probable that, instead of being a connecting link between the Cnidaria and 

 the Turbellaria, they are rather highly modified Turbellaria adapted to a 

 pelagic life. In this sense the idea of a genetic affinity between the Turbel- 

 laria and Ctenophores may be correct , though it seems probable that the 

 Polyclad affinity should be given up and the relationship sought for among 

 Alloioccelan forms. 



The Ctenophore-Polyclad theory necessarily viewed the Polyclads as the 

 most primitive Turbellaria, and came into contact in this way with the 

 more simple organization of the Accela, Alloiocoela, and Rhabdocoela, a 

 difficulty which was avoided by assuming that these were degenerate 

 groups derived from Polycladan ancestors. No good grounds for such an 

 assumption exist however, nothing in the mode of life suggesting a cause 

 for degeneration ; and until embryological evidence of degeneration is 

 obtained, it is preferable to consider their simplicity primitive. 



This latter view is strengthened if it harmonizes with a probable phy- 

 logeny. It has already been pointed out that the solid embryo or sterrula 

 is to be recognized as an ancestral form of the Cnidaria, With such an 

 ancestral form the Accela show affinities in the absence of a differentiation 

 of the central mass into well-defined endoderm and mesoderm. The local- 

 ization of a definite region for the ingestion of nutrition would lead to the 

 formation of a mouth in. the Sterrula, just as it has done in the riayellata. 

 The differentiation of muscle-fibres from the mesendodermal cells would 

 naturally f >llow the assumption of a creeping habit, so that it is only the 

 possession of a definite nervous system imbedded in the mesoglcea (in which 

 tissue, however, Cnidarian characteristics are yet discernible, as already 

 pointed out) and the occurrence of a complicated reproductive apparatus 

 that render a close comparison witli the Sterrula difficult; but even the 

 explanation of the presence of these structures makes fewer demands upon 

 our ideas of developmental possibilities than does the assumption that the 

 Acoela owe their peculiarities to degeneration. 



Upon this view of the phylogeny the Accela are united with the Coelcn- 



