20 A NATURAL HISTORY OF THE SFAS 



To the casual observer many of the Hydroids will not 

 be recognisable as animals at all since the bulk suggest 

 tangled tufts of weed. The lens, however, will reveal 

 them all as branching tubular growths. The animals — 

 flowerlike in form, are built much on the plan of a Sea 

 Anemone, and, like the Sea Anemone, feed on living 

 organisms which they capture with their petals or tentacles. 

 Each separate animal is known as a Polyp and each is 

 connected to its fellow by living tissues. These colonies 

 of Polyps increase by the mode of reproduction known as 

 " an alternation of generations, by budding off, or by 

 laying eggs." In the first method certain of the Polyps 

 give rise to a colony of buds which presently develop into 

 a series of minute jellyfish piled like saucers one upon 

 another. These in due season break away and swimming 

 off on their own account lay eggs which eventually give 

 rise to other branching colonies of fixed Polyps. Thus 

 a fixed generation gives rise to a swimming one, and the 

 swimming one to a fixed one. 



A large number of these quaintly-formed, exquisitely- 

 coloured Hydroids are found throughout the summer in 

 home waters. A common species is sometimes found on 

 whelk shells tenanted by the common hermit crab and 

 owing to the spiny nature of the colony's horny skeleton 

 is known as the Hedgehog Hydroid (Hydractinid). In this 

 form the expanding Polyps are about half an inch long 

 and may be observed twisting from side to side and snatch- 

 ing at scraps of food in the liveliest manner. 



In some tropic seas Hydroids reach a huge size. In the 

 British Museum may be seen a Japanese species dredged 

 from deep water which consists of a single solitary Polyp 



