THE PRESIDENT'S ADDRESS. 471 



inany cases that the process of strobilation assumes much greater 

 importance, and finally leads to a new type of structure char- 

 acterised by what zoologists term metameric segmentation, or 

 serial metamerism. The earthworm is, of course, a typical 

 example of such a metamerically segmented animal, the body 

 consisting of a number of distinct segments or metameres arranged 

 in linear series one behind the other, and each one, to a certain 

 extent, repeating the structure of all the others, each with its 

 own division of the alimentary canal, its own division of the 

 vascular system, its own division of the excretory system, its own 

 division of the nervous system, and so on, but all united together 

 in mutual dependence and incapable of separate existence. 



Differentiation and integration have, indeed, gone so far in 

 the case of the earthworm that we can no longer regard the 

 animal as a mere strobila or linear colony. It is undoubtedly a 

 single individual of the third order. 



In some other groups of worms, however, the process of 

 integration has hardly commenced, and the different segments 

 sooner or later separate from one another as distinct individuals. 

 We see a good example of this in the Planarian Microstoma 

 lineare, where transverse division, frequently repeated, results in 

 the formation of a strobila or chain of perfect individuals that 

 only remain temporarily associated with one another. We see 

 something of the same sort in the tape- worm, which consists of 

 a chain of so-called proglottides attached to a head or scolex, and 

 each containing, amongst other things, a complete set of repro- 

 ductive organs. 



Even in some of the highly organised chaetopod annelids, the 

 group to which the earthworm belongs, we sometimes find new 

 segments being added to the chain throughout life, by a kind of 

 linear budding or transverse division, and in many cases groups 

 of segments separate off from time to time as independent 

 individuals. 



The earthworm, however, has lost the power of reproducing 

 independent individuals in this fashion. The process of integra- 

 tion has gone too far, for certain essential organs have become 

 restricted to special segments and separation into constituent 

 units is no longer possible. 



The same phenomenon of metameric segmentation is exhibited 

 throughout the whole of the great group Arthropoda, which 



