822 PROFESSOR J. STEPHENSON ON 



It is commonly found, when any element of structure becomes split up in the course 

 of evolution, that the resulting portions become differentiated anatomically, and take 

 on different physiological functions. This has happened to the original mouth ; but 

 the specialisation of the two apertures which have resulted did not take place primarily 

 in the Avay that at first sight appears most probable. The differentiation was not into 

 an aperture for entrance and an aperture for exit ; but into an aperture for the entrance 

 of food in mass, and an aperture for the introduction of the fluid medium carrying 

 oxygen and food-matters in solution. 



A second line of thought is suggested by the statement on a previous page (p. 816), 

 — that, so far as my observations go, it is only among the Nereidiformia (Rapacia) that 

 families are to be found in which ascending ciliary action in the intestine certainly does 

 not occur ; in other words, this phenomenon is more universally prevalent among the 

 sedentary and tubicolous than among the errant families. 



I should be sorry to trust too far to this statement. My observations do not 

 embrace all the families of Polychseta ; it is very possible, therefore, that ascending 

 ciliary action may be absent in some of the families of Sedentaria in which I have not 

 been able to make any, or any satisfactory, observations ; and, on the other hand, a 

 more exhaustive investigation might have resulted in showing its occurrence among the 

 Errantia in families in which I have not so far discovered it. 



Proceeding, then, with this qualification in mind, I would observe that, according 

 to the reasoning given above, ascending ciliary action is to be considered as a primitive 

 character ; and the question therefore arises. Are the Sedentaria — the division of the 

 Polychseta in which this phenomenon occurs most markedly — the primitive Polychsetes ? 



I believe that in all probability there exists no such thing as a primitive animal. Those 

 that commonly go by this name are, if primitive in one or in some respects, much 

 modified — either highly specialised or degenerate — in others. All that can be said is 

 that certain animals are primitive in certain respects ; but no animal of any group can be 

 put down as primitive in all respects, just as no animal of a group can be said to 

 be in all respects the " highest." Still, some animals do undoubtedly show a larger 

 number of primitive features than others ; and in considering whether the sedentary 

 Polychseta have not a better right to the title of primitive than the errant forms, all 

 that we have to decide is whether or not, in the greater number of their more important 

 features, they retain characters which we judge to be ancestral. No one, for example, 

 would maintain that the tuft of feather-like gills which ornaments the anterior end of 

 the sedentary Serpulimorpha was a primitive character, any more than a similar claim 

 could be made for the perfectly shaped glass-like tube of the errant Hyalinoecia. 



The view is perhaps generally current, that active free-living forms are more likely 

 to be primitive, sedentary and especially fixed forms to be modified ; and so far as I 

 can gather, opinion seems rather against the view that the sedentary, especially the 

 tubicolous, forms rejjresent the primitive Polychseta. The usual representative of the 



