242 Description of the Plates. 



as in tlie Dendrocoela, and less markedly in E/hynehocoela, 

 are not wanting- to careful inspection. 

 c. Orifice of female organs. The penis, and a pear-sliaped organ 

 v\hich is of doubtful function, but which may be supposed 

 to be concerned in the formation of the shell of the ova, 

 have their openings distinct from, and placed posteriorly to, 

 this orifice instead of within it, as in the genus Planaria, 

 which is on tliis account separated from the genus Dendro- 

 coelum. For detailed descriptions and for figures of the 

 reproductive organs, see O. Schmidt, Zeitschrift fiir Wiss. 

 Zoologie, X., 1859, P- 24; Tafs. iii. and iv., xi., 1861, p. 11, 

 Tafs. i., ii., iii., iv. 



The Turbellarian Worms show in various parts of their organisation 

 points of affinity to several other orders of animals besides the Trematodes 

 and Taeniadae, with which they have been here classed as Platyelminthes. 



In some of the smaller and almost microscopic forms of Rhabdocoelous 

 Turbellarians the oesophagus opens into the general cavity of the body, 

 no distinct intestinal wall being developed ; the resemblance of such forms 

 to the larger Infusoria, and indeed also to the Coelenterata, is very close, 

 so far as the digestive and tegumentary systems are concerned. Still even 

 in a young Turbellarian, in which the digestive tract might not be distin- 

 guishable within, nor the generative organs differentiated from the general 

 parenchyma of the body, the presence of such organs as the otolithic cap- 

 sule Avoukl point to the real character of the animal. The presence of ' thread 

 cells' in the integument is a point common to the Turbellarians, with 

 the Coelenterata and many of the Holotrichous Infusoria {Paramaeciv/m 

 hursaria), but it has been noted also in certain Polychaetous Vermes, and 

 in the Nudibranchiate Mollusca, which by their dendritic digestive tract 

 present a real resemblance to the Dendrocoelous Turbellarians ; as also 

 to some extent by their complex reproductive organs. The singular 

 aproctous parasitic Mollusc Entoconcha mirabilis, which appears to be 

 nearly allied to the Nudibranchiate Molluscs, is, as figured and described 

 by Baur, Nova Acta, 1864, p. 35, by no means unlike, in the general rela- 

 tions of its various systems, to an aproctous Rhabdocoelous Turbellarian. 



It is of more importance perhaps to note that the order Turbellarians 

 comprises forms which, whilst inseparably connected with each other by 

 connecting links, ai-e yet so various as to present, at either end of the 

 series they make up, close approximations both to the highest and to the 

 lowest of the Vermes, to the unity and real compactness of which sub- 

 kingdom they thereby appear to speak very distinctly. Dinophilus, for 

 example, has 1 eon ranked by Schmarda with the Naidina (see Nene Wir- 



