THE GASTKAEA-THEOltV, ETC. 23 1 



The question of the homology of the central nervous system 

 is much more difficult. This has, doubtless, arisen from the 

 exoderm in all six stem-groups of the Metazoa, but the central 

 nervous system of the zoophytes has certainly arisen inde- 

 pendently of that of the worms, and is in no respect to be 

 compared to it. On the other hand, the simplest form of the 

 central nervous system, which is found in the worms, espe- 

 cially the simple pair of ganglia lying over the oesophagus, the 

 so-called upper pair of ganglia or primitive brain, is to be 

 regarded as homologous, firstly, in all classes of the group of 

 Avorms, and, secondly, is to be compared also to the corre- 

 sponding parts in the MoUusca and Arthropoda, as well as to 

 the original medullary tube of the Vertebrata (from which the 

 brain of the latter is only the furthest differentiated division^). 

 This original central organ has been lost in the Echinodermata, 

 and their oesophageal ring is only a secondary commissure 

 between the five radial nervous threads, Avhich appear in the 

 Asterida in their most original form. Each of these five radial 

 threads of the Echinodermata is homologous to the jointed ven- 

 tral cord of the Annelida and Arthropoda. It is necessary to 

 accept the correctness of my theory of the origin of the 

 Echinodermata for the conception of this apparently para- 

 doxical comparison, according to which the root-form of the 

 phylon of the form of the Asterida is to be regarded as a 

 stem composed of five-jointed Avorms united into a star-shape. 

 This theory has, indeed, been rejected by Glaus, Leuckart, 

 Semper, and others, but without their putting any other 



phylogenetically by moultings (or " Mauserungen") which the ancestors of 

 the organism in question have suiTered in earlier periods of tiie earth's 

 history. So is, especially, to be explained the larval form of many of tlie 

 higher Crustacea, which originates within the egg-shell, and is itself fre- 

 quently changed, upon repeated moullings of the root-form of the Crustacea, 

 the Nauplius, and other old root-forms which have arisen from this. (Com- 

 pare the statements and explanations relating to this in the detailed works 

 of Eritz Miiller, Edouard von Beneden, A. Dohrn, &c.) This is, perhaps, 

 also the explanation of the so-called Amnion in many animals. On the other 

 Jiand, the amnion of the vertebrata is certainly of a different origin. As for 

 the special homology of this amuion in Vertebrata and Arthropoda, as main- 

 tained by Kowalevsky and others, it is already contradicted, independently 

 of other reasons, by the fact that the amuion only occurs in the three higher 

 classes of Vertebrata (Amniota). This has, therefore, apparently liist 

 developed itself iiere, duriug the origination of the root-form of the Amniota 

 from the Amphibia, and is entirely unconnected with the amnion of the 

 Arthropoda. The latter is only analogous (and homomorphous) to the 

 former, but not truly homologous (homophylous). 



^ The spinal marrow of the Vertebrata, and the ventral nervous cord of 

 the Annulosa, are of course not analogous from this point of view, and these 

 can just as little be compared as the sympathetic marginal cord of the former 

 and the ventral nervous cord of the latter. 



