JELLY-FISH, ETC. 



487 



merit 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 Coryniorpha, 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 medusas, 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 never 



/) 



u 



Monocaulus, upper third (much reduced). 



