DIFFERENTIATION OF STRUCTURE 31 



escape directly through an opening in the body wall, 

 but this stage is soon passed, and we find special tubes 

 formed which arise singly and are not connected with 

 one another. The earthworm, for instance, has a 

 separate pair of these tubes, opening directly to the 

 exterior, for practically every segment or ring in its 

 body. For a considerable time in the animal kingdom 

 there was no great advance on this, but among the 

 vertebrates the tubules were restricted to certain 

 parts of the body, and were grouped together into de- 

 finite groups, and also came into direct communication 

 with the blood or circulatory system. At the same 

 time the tubules, instead of continuing to open directly 

 to the exterior, opened into a common duct, which 

 became the only means of communication with the 

 exterior. The ultimate result of this was the develop- 

 ment of the kidney or excretory organ as we find it in 

 mammals. A hint of this union of ducts may be said 

 to be seen in one of the worms, Lanice conchilega. 



In a similar way we might trace the gradual increase 

 in complexity in the reproductive and respiratory 

 systems, but it will be more interesting to devote our 

 attention for a short time to the nervous system. 



As structural specialisation increases, some means 

 must be taken to place the inner cells in communication 

 with the outer, as it is important that the whole body of 

 cells must work together. From the forms where there 

 is a general irritability and power of response, we pass 

 gradually upwards to such forms as Hydra and its allies, 

 where some of the cells are modified to transmit the 

 impulses to their neighbours, and in this modification 

 we have the laying down of the foundations of the 



