ALTFRNATE GENERATIONS. 267 
Polyps so 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 garb of our common intellectual 
property. When the forms of animals are as 
familiar to children as their A B C, and the 
intelligent study of Natural History, from the 
objects themselves, and not from _ text-books 
alone, is introduced into all our schools, we 
