264 ALTERNATE GENERATIONS. 
than these Ctenophore as they move through the 
water.* 
I trust I have succeeded in showing that the 
three Orders of the Acalephs are, like the five 
Orders of the Echinoderms, different degrees of 
complication of the same structure. In the Hy- 
droids, the organization does not rise above the 
simple digestive cavity enclosed by the double 
body-wall; and we might not suspect their rela- 
tion to the Acalephs, did we not see the Jelly- 
Fish born from the Hydroid stock. In the Hy- 
droid-Meduse and Discophore, instead of a sim- 
ple digestive sac, as in the Hydroids, we have a 
cavity sending off tubes toward the periphery, 
which ramify more or less in their course. Now 
whether there are four tubes or eight, whether 
they ramify extensively or not, whether there are 
more or less complicated appendages around the 
margin or the mouth, makes no difference in the 
essential structure of these bodies. They are all 
disk-like in outline, they all have tentacles hang- 
ing from the margin, and a central cavity from 
which tubes diverge that divide the body into a 
certain number of segments, bearing in all the 
same relation to each other and to the central 
cavity. In the Ctenophore, another complication 
of structure is introduced in the combination of 
* For more details concerning the Ctenophore, see the third 
volume of my Contributions. 
