264 ALTERNATE GENERATIONS. 



than these Ctenophorae 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 tlie 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-Medusae and Discophorae, instead of a sim- 

 ple digestive sac, &s 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 Ctenophorae, another complication 

 of structure is introduced in the combination of 



* For more details concerning the Ctenophorae, see the third 

 volume of my Contributions. 



