Origin of the Segmented Worms. 513 



We merely assume, then, that a certain number of forms 

 remained free-swimming long enough to feed and to grow and 

 to put out buds. Now, without going so far as to say that 

 there is only one place where such a free-swimming form 

 could bud and yet continue to pursue its active life, viz. at its 

 hinder end, it is certainly clear that the most likely place for 

 a bud to appear would be at the posterior end. Not only 

 would any other place make forward locomotion impossible, 

 but at the hinder end the conditions seem to be more favour- 

 able than anywhere else. The external pressure would here, 

 in the wake of the animal, be less than anywhere else in the 

 body — indeed, during rapid forward swimming there would 

 be something like suction at this spot. Now if this is true 

 for the first bud, it is surely more true for the second, and it 

 seems clear that we get all the conditions for rapid serial 

 budding : for the addition of the first bud, by increasing the 

 number of available cilia, would propel the animal faster 

 through the water ; this greater speed would enable more food 

 to be swallowed, part of which would find its way into the 

 bud, while at the same time it would still further diminish 

 the external pressure of the water at the extreme hinder end, 

 and we have still more favourable conditions for the forma- 

 tion of a second bud behind that already formed, and so on. 

 If there is any probability in this argument at all, it would 

 seem that the more buds there were the faster would buds be 

 produced, until some limit, presently to be discovered, wa3 

 reached. 



But apart from this argument, and merely bearing in mind 

 the wealth of colony formation developed among the Cceleu- 

 terates, is there not full justification for believing that round 

 the shores of the ancient seas, soon after the primitive forms 

 appeared, there would be free-swimming individuals trailing 

 behind them longer or shorter strings of buds diminishing in 

 size progressively backwards ? Of these, some, we may fairly 

 assume, would sooner or later again give up more or less com- 

 pletely the laborious free-swimming life and become creeping 

 forms. It is in this direction that I should be inclined to 

 look for the ancestors of some of the other members of the 

 ancient class " Vermes " *. 



* The Turbellarian Microstomum lineare, Ehr., still swims about trailing 

 short strings of buds after it. I was once fortunate enough to get a 

 glimpse of it living, under a high power, and saw very clearly what 

 appeared to be tvpical nematocysts scattered here and there in its skin. 

 The derivation of the Phitihelminthes from primitive Coelenterates, which 

 gave up a free-swimming for a creeping manner of life, has much to be 



