J23^ ERNST HAECREL. 



natural theory in its place, and without their having made 

 any attempt to explain the origin of the Echinodermata. On 

 the other hand, my theory, which fully explains its origin, 

 has received the full sanction of two zoologists of the first rank 

 upon Avhose judgment I lay the greatest weight, Gegenbaur 

 and M. Sars (senior), the last recognised as one of the natu- 

 ralists most thoroughly acquainted with the Echinodermata.^ 



The organs of sense of the different groups of animals are 

 for the most part (perhaps entirely, with the exception of the 

 skin as the organ of touch) not homologous ; moreover, the 

 homology is often not to be proved even within one of these 

 groups, or is even positively negatived within a given class, as, 

 for instance, in the organs of hearing of different insects. All 

 point to these as of polyphyletic origin, and as having 

 originated at different times from different portions of the 

 upper germ-lamellge. This manifoldly different and inde- 

 pendent origin of the organs of sense is also very well con- 

 ceivable phylogenetically. 



The primordial kidneys have probably also originated from 

 the upper germ-lamellse, and these organs are probably homo- 

 logous in all the Bilateria (in all the members of the five 

 higher animal groups). The simplest form would be repre- 

 sented by the so-called " excretory organs " or " water- 

 vascular system " of the Plathelminthes, which are originally 

 nothing more than strongly developed tube-shaped dermal 

 glands (like the sweat-glands). Comparative anatomy will 

 perhaps later be in a position to prove that these primary 

 kidneys of the unarticulated Plathelminthes, which reappear 

 in each metamer of the articulated Vermes as so-called 

 looped canals or segmental organs, have given rise both to 

 the kidneys of the Mollusca and to the primary kidneys of 

 the Vertebrata.- Gegenbaur has already proved the homo- 



^ The origin of the central nervous system from the original outer layer of 

 the body of the animal, the horny layer, is one of the most striking examples 

 of the value of the phylogenetic view, and its signification for the compre- 

 hension of the ontogenetic process. Hitherto this origination of the " in- 

 ternal" nervous system from the outer germ-lamellae has been almost 

 universally considered wonderful and paradoxical. But as soon as the pro- 

 blem is thus stated : " How can the nervous system generally have originated 

 at first (phyletically) P" only the one answer, after ripe reflection, will be 

 given to it : " From the most superficial parts of the body, which were con- 

 stantly in communication witli the outer world." Only from this constant 

 communication could the first " sensation" develop itself. The nervous 

 system has then withdrawn itself secondarily into the protected interior of 

 the body, " separated from the horny layer." I do not consider the idea of 

 a special "nervous layer," which many embryologists separate from the 

 cuticular sensitive layer, to be confirmed. 



* In Amphioxus the broad caual discovered by von Rathke, and more fully 



