JELLY-FISH ; ETC. 
487 
ment or alternation of generations. Bougainvillea ramosa is another form in 
which, owing to the stock being branched, the division of labour is even more 
clearly seen; some of the individuals are feeding and some are reproductive, these 
latter turning into swimming-bells, and breaking loose. Both these forms are 
small, as indeed are the great majority of the hydroid stocks, but whole forests 
of hydroid-polyp stocks may be seen on the reefs in the Pelew Islands, almost as 
tall as a man, and with roots three or four inches in diameter. A bather, entering 
such a forest, is terribly stung, the pain lasting for hours. A solitary form 
{Monocaulus imperator )—the upper portion of which is here figured—nearly related 
to Corymorpha , and 
found in the Northern 
Pacific, attains still 
larger proportions. 
These animals, brought 
up during the Chal¬ 
lenger expedition from 
a great depth, were 
more than two yards 
in length, with a pro¬ 
portionate diameter. 
As examples of 
stocks of which the re¬ 
productive individuals 
do not swim away as 
jelly-fish, we may select 
the pretty, feathered, 
plant - like creatures 
found along the sea¬ 
shore, which are often 
thought to be plants 
but are really animal colonies, well-known types being Sertularia and Plumularia. 
In these cases, in addition to the nutritive individuals, there are the egg-bearing 
individuals which never turn into free-swimming medusae. In saying this, it must 
be left undecided whether these colonies are degenerate, that is, were once capable of 
producing medusae, but have now lost the power; or whether they are in a lower 
stage of development, above which they have never risen. One small form which is 
not branched and feathered is Hydractinia echinata, found in the North Sea and 
on the English and Norwegian coasts, where it attaches itself to gastropod shells, 
inhabited by hermit-crabs. The polyp probably profits by changes of place for 
feeding, or else for some other reason adapts itself to the restless life of the crab. 
The part of the stock common to all the individuals is the skin-like portion which 
adheres to the surface of the shell or other object to which it is attached. This 
skin is raised up into spiny prominences, as shown in the figure on p. 488. A horny 
layer occurs in this integument, similar to that of which the single tubes consist. 
The nutritive canals running down the stems of the polyps are continued into 
this membrane, promoting its life and growth. In such a stock there are uever 
Monocaulus , upper third (much reduced). 
