818 PROFESSOR J. STEPHENSON ON 



intestine, the original condition of expulsion by the mouth is restored. The same 

 thing is also seen in ruminants, where undigested food is returned to the mouth for 

 purposes of mastication and reduction to a digestible form ; and a similar condition is 

 known pathologically in man under the name of mericism. 



(2) If it is thus unnecessary to suppose that an anus was developed merely in 

 connection with the expulsion of food-remains, it is also difficult to conjecture in what 

 manner such an assumed development took place. The original condition would be 

 that already indicated, in which the digestive juices act on the solid food in the 

 anterior part of the tract, the soluble matters being extracted and passed on for 

 absorption, the insoluble rejected by the mouth ; and it is not easy to suppose that 

 the undigested matters, originally retained, until expelled, in the anterior portion 

 of the tract, gradually travelled further and further along the tract, and at last 

 broke through at the posterior end, producing a mutilation which became hereditary 

 in the race. 



Indeed it is not easy, — in discussions, for example, of the origin of Annelids from 

 Turbellarians, — to find any adequate description of the origin of the anus. Meyer, in 

 his well-known paper on the origin of the Annelids (31), expressly begins with 

 Turbellarians, that is, forms without an anus ("Die Vorfahren der Eingelwiirmer stelle 

 ich mir als kraftige, niuberische Turbellarien vor, welche pelagisch lebend seiner Zeit die 

 Meere beherrschteu "') ; but in the passage where one would expect to find described 

 the origin of the anus, the ancestral forms have become " Turbellarian-/;X-e " animals 

 which already possess one (" die turbellarienartigen Vorfahren der Anneliden . . . 

 ahnlich den Nemertinen ein einfaches, hinten mit einem After endendes Darmrohr 

 hatten ") ; the point is therefore altogether evaded. We have, in the anus, to do with 

 what I have called " a fundamental fact of structure," with an aperture which is compar- 

 able in anatomical importance to the mouth, is equally definite in position and relations, 

 and arises by a similar process and frequently equally early in the ontogeny ; and 

 therefore it is not, I think, satisfactory to adduce, as the origin of the anus, the pores 

 which in many Ccelenterata and Turbellaria place the alimentary tract in communica- 

 tion with the exterior. Nor does Lang regard a similar suggestion of his own with 

 any degree of confidence : he can only say that an anus must have been formed (27) : — 

 " Es ist zur Zeit noch unmoorlich fiir den Enddarm und den After der Hirudineen bei 

 den Plathelminthen Homologa aufzufinden " (he is deriving Annelids by way of the 

 Hirudinea from Platyhehninth forms resembling (runda segmentata ; the introduction 

 of the Hirudinea in this connection lias since been given up). " Vielleicht — es ist dies 

 eine blosse Conjectur — hat sich einer der Excretionscanale, die wir bei Polycladen 

 vorfinden, specifisch entwickelt und zur Bildung des Enddarmes und der Afteroffnung 

 Veranlassung gegeben. Jedenfalls darf das Vorhandensein eines Afters bei den 

 Hirudineen uns nicht bestimmen, sie von den Plathelminthen weit zu entfernen. 

 Einmal muss er sich doch gebildet haben I Auch die Nemertinen haben einen After, 

 und doch fallt es Niemanden ein, sie aus dem Verbande der Plathelminthen loszulosen I " 



