820 PROFESSOE J. STEPHENSON ON 



it is not unreasonable to suppose that similarity or community of origin may indicate a 

 primitive similarity or community of function. 



(4) An argument against the view that the primary function of the anus was to 

 act as a passage for undigested food, was that, holding this view, it is impossible to 

 give any satisfactory account of its origin. Conversely, it will be an argument in 

 favour of the opposite view — that the anus is primarily inhalant — if, on the basis of 

 this view of its function, we can give a reasonable account of its first appearance. 



Can we conceive that in an animal — for example, a Turbellarian — possessing a mouth 

 and alimentary tract, in which the mouth serves for the intake of food both in mass 

 and in solution, a new aperture was formed in a remote region of the body, for the 

 purpose of introducing into another part of the alimentary tract a certain portion — let 

 us suppose the fluid portion — of what previously entered at the mouth ? The difliculties 

 in the way of such a separate origin seem insuperable ; indeed, it seems to be impossible 

 adequately to explain the anus on any view, so long as it is considered to be a relatively 

 recent formation, separate in origin from and arising at a later phylogenetic stage than 

 the mouth. 



We have, however, in Sedgwick's theory (43) an explanation of the origin of the 

 anus which satisfies our requirements. The theory views the anus as coeval in origin 

 with the mouth, and as being an equally fundamental and essential fact of structure ; 

 it thus escapes the insuperable difliculties we encounter if we attempt to consider the 

 anus as a later acquisition. While recognising the primarily inhalant character of the 

 anus, it has moreover from my present point of view this advantage, that it was not 

 evolved to meet the present case, but was independently enunciated in an altogether 

 different connection, — in connection, namely, with the consideration of the origin of 

 segmentation. 



Sedgwick, as is well known, derived both the mouth and anus of higher forms from 

 a common aperture, which is represented in embryology by the blastopore, in compara- 

 tive anatomy by the mouth-opening of the Actinozoa. Among the facts which 

 Sedgwick brought forward in support of the theory two may be briefly mentioned in the 

 present connection : (a) that the mouth, or the anus, or both, are in the great majority 

 of cases derived embryologically from the blastopore ; (6) that even in the Actinozoa 

 the mouth-aperture is, physiologically if not anatomically, divided into two by the 

 approximation of the lips and locking of the cilia. 



Briefly expressed, then, the theory supposes that the ancestor of the segmented 

 Bilateria was of the Actinozoan type ; that an elongation of the body took place in the 

 direction of the long axis of the already oval or dumbbell-shaped mouth ; that the 

 mouth, now still more drawn out and slit-like, became divided into two apertures by 

 the fusion of the lips across the middle, as occurs, for example, in the blastopore of 

 Peripatus during development ; and that in this way an elongated form resulted, 

 with an oral aperture near one end and an anal aperture near the other, both 

 apertures opening into a common gastrocoel. The original oral surface became the 



