242 Proceedings of the Royal Physical Society. 



It is very usual for ciliated larvae to have at first a uniform coat of cilia 

 all over, and it is very generally admitted that this is, in all probability, 

 the primitive condition. Along what lines then should we expect such 

 larva? to evolve ? 



The first step we should probably expect to be the establishment of a 

 definite axis along which movement takes place, i.e. the larva would take to 

 moving in a particular direction so that an anterior and a posterior pole 

 would be distinguishable. Such development of polarity is one of the first 

 steps in the evolution of the great majority of animals. Now it will carry 

 with it certain consequences. If we start from a larva having the simplest 

 possible shape — that approximating to a sphere — the establishment of a 

 definite axis of movement will mean that the cilia placed round the equator 

 will be most favourably placed for producing onward movement. We should 

 therefore expect that the process of evolution would be in the direction 

 of exaggerating the equatorial cilia and reducing the others. Though 

 comparatively useless for propulsion the cilia at the posterior pole would, on 

 the other hand, be effective for steering. We might therefore confidently 

 expect the evolution of a larva ciliated only round the equator, with possibly a 

 ring or tuft of cilia close to the posterior pole. If an alimentary canal were 

 present we should obviously expect the anus to come to be situated at the 

 posterior pole so that faecal matter might be left behind. We might, perhaps, 

 also expect the mouth to come to be placed in the region of eddies just behind 

 the equatorial ring of cilia. If the skin were to develop sensitiveness, we 

 should expect this to take place especially at the anterior or apical pole. If 

 excretory organs were developed, we should expect these to open towards the 

 hind end. 



As is well known one of the best known types of larva?, the Trochosphere, 

 answers exactly to these expectations. Much weight is attached to the 

 occurrence of such a larva in different groups as evidence of genetic affinity. 

 My object is rather to emphasise the fact that the very features which serve 

 to characterise the Trochosphere larva, and which are often interpreted as 

 expressive of genetic affinity, are quite satisfactorily explicable as simply 

 adaptations to the pelagic mode of life. 



Now in what direction might we expect further evolutionary progress to 

 take place ? Clearly one of the most natural directions would be towards 

 increased efficiency of the locomotor apparatus. But how can this be brought 

 about? Obviously not, at least beyond narrow limits, by mere increase in 

 the size and power of the individual cilia. The only other possible method 

 is by increase in the number of cilia. This necessarily involves the increase 

 in length of the ciliated band, and this in turn involves the ciliated band 



