ALTERNATE GENERATIONS. 267 



Polyps sc much as to contract the chambers 

 between them, till they form narrow alleys in- 

 stead of wide spaces, and then we have the tubes 

 of the Jelly-Fish. In the Jelly-Fish there is a 

 circular tube around the margin, into which all 

 the radiating tubes open. What have we to 

 compare with this in the Polyps ? The outer 

 edge of each partition in the Polyp is pierced by 

 a hole near the margin. Of course when the 

 partition is thickened, this hole, remaining open, 

 becomes a tube ; for what is a tube but an 

 elongated hole ? The comparison of the Aca- 

 lephs with the Echinoderms is still easier, for 

 they both have tubes ; but in the latter the tubes 

 are enclosed in walls of their own, instead of 

 traversing the mass of the body, as in Aca- 

 lephs, etc. 



In preparing these chapters on the homologies 

 of Radiates, I have felt the difficulty of divesting 

 my subject of the technicalities which cling to all 

 scientific results, until they are woven into the 

 tissue of our every-day knowledge and assume 

 the familiar ga,rb of our common intellectual 

 property. When the forms of animals are as 

 familiar to children as their ABC, and the 

 intelligent study of Natural History, from the 

 objects themselves, and not from text-books 

 alone, is introduced into all our schools, we 



