302 THE AMERICAN NATURALIST. [VoL. XXXV. 
will be made to the Scyphomedusz in considering possible 
relationships or phylogeny among the Hydromedusz. 
The Hydromedusze may be distinguished by the following 
somewhat broad characteristics. In typical cases there is a 
more or less well marked alternation of generations, z.¢., a non- 
sexual, hydroid stage and a sexual, medusoid, stage. The latter 
are derived by a process of budding from the stem or hydranth 
of the hydroid as gonophores which may become free as 
medusze or may exhibit varying phases of degeneration as 
medusoids or mere sporosacs, as in Clava, Campanularia, etc. 
In many cases there may be exhibited proliferous medusz 
from various portions of the parent medusa, as in Hybocodon. 
In rare cases one or other of these phases may be entirely 
lacking, as in Hydra, in which the medusa phase is wholly 
absent, or as in Rhegmatodes and many others the hydroid 
stage is apparently lacking. 
Perhaps in no phylum of the animal world is there a more 
striking exhibition of polymorphism than among the Hydro- 
medusz. This seems to reach its climax in the Siphonophora, 
though in such forms as Hydractinia it is also evident. 
In general the hydroid exhibits a sedentary habit quite in 
contrast with the free-swimming habit of the medusa. But 
here again are numerous exceptions. Hydra is capable of 
locomotion, as are also other hydroid forms, while as already 
indicated many medusz are sessile and degenerate, and in the 
Siphonophora the entire polymorphic colony is free-swimming. 
In general the hydroids are colonial though with notable 
exceptions, as in Hydra and many others. While in general 
form the hydroid and medusoid present rather striking mor- 
phological differences, they may yet be reduced to a common 
and fundamental likeness. Both are of diploblastic structure, 
having a definite ectoblast and entoblast separated by a middle 
lamella, or mesogloea, which is a delicate, structureless mem- 
brane in the hydroid and in the medusa a rather massive, gelati- 
nous structure, making up the bulk of the body and giving it 
the characteristic glassy appearance. 
In the absence of definite knowledge concerning details of 
the life history of many of the Hydromeduse, it is as yet 
