578 THE ANATOMY OF INVERTEBRATED ANIMALS. 



Beneden insists as a distinction between the Dicyemida and 

 the Metazoa, the manner in which the contents of the axial 

 cell give rise to germs is so completely unlike anything which 

 is known to obtain in the Metazoa, as, to my mind, to justify 

 the separation of the Dicyemida from the whole of this divi- 

 sion. On the other hand, the similarity of their development 

 to the formation of metazoic embryos by epiboly, as com- 

 pletely divides the Dicyemida from all the Protozoa. It 

 must be recollected that the changes which are undergone by 

 the ciliated embryos are still to be discovered ; but, provision- 

 ally, I am disposed to agree with Van Beneden, that the Di- 

 cyemida should be regarded as the representatives of a dis- 

 tinct division, the Mesozoa, intermediate between the Pro- 

 tozoa and the Metazoa. And without distinctly j^ledging 

 myself to any such view, I yet think it is worth while to 

 throw out the suggestion that the Cestoidea, if not the 

 Acanthocephala, may be modifications of the same type, 

 differing from the Dicyemida in the development of a meso- 

 derm, but resembling them in the total absence of an alimen- 

 tary apparatus. 



The Serial Relations of the Inveetebeata. — When 

 the various groups of invertebrate animals are compared, it is 

 obvious that they present very different degrees of morpho- 

 logical complexity ; whence they may be considered as terms 

 in a graduated progression, in which the place of each group 

 corresponds broadly with the degree of its differentiation. 

 The lowest Protozoa will occupy one extreme of such a pro- 

 gression, the Arthropoda and the Mollusca the other, while 

 the remaining groups fall into intermediate places. On at- 

 tempting to carry out this serial arrangement into detail, 

 however, it will be found that no single series will suffice to 

 express the facts, but that, starting from the lowest Protozoa, 

 we are led along various lines, none of which, as far as our 

 present knowledge enables us to judge, can be traced, with- 

 out interruption, throughout the whole length of the scale. 



If we assume, in the absence of proof to the contrary, that 

 the Monera have the simplicity of structure ascribed to them 

 by Haeckel, then, on comparing the Endoplastica with the 

 Monera, the different groups of the former appear to be re- 

 lated to those of the latter division, as if they were similar 

 forms complicated by the addition of one or many nuclei. 

 Protofjenes may thus be considered as the root of the Foram- 

 iniferal series, Protamoeba of the Protoplasta, Myxastrum 



