THE SEA-ANEMONES AND THEIR ALLIES 16$ 



Each polyp has a general resemblance to a sea-anemone, so that 

 the whole is a colony of sea-anemones, analogous to those colonies 

 which form coral reefs in warm seas. 



On the shore rocks there occur also a considerable number 

 of sea-firs, which are colonies of polyps of much simpler structure 

 than those in dead men's fingers. The polyps are supported 

 by a horny substance, which has a general resemblance to sea- 

 weed, so that the sea-firs are often mistaken for plants. The 

 individual polyps are usually, though not always, very small, 

 so that it is not possible to make much of their structure without 

 a microscope. One cannot, however, omit on this account all 

 mention of the sea-firs, because their life-history includes a pheno- 

 menon of such interest that a few words must necessarily be 

 said about it. We have spoken above of the frequent occur- 

 rence in the life-history of shore animals of free-swimming young, 

 which, like the winged seeds of the great forest trees, carry the 

 species to fresh localities. The same effect is produced in the 

 sea-firs by a totally different method. Look over the edge of 

 the rocks or the side of a boat on a clear summer's day, and you 

 will see floating in the water crystal bells, which swim with a 

 pulsating movement, not strong enough to propel them against 

 the current, so that they are swept to and fro with the tide. These 

 swimming-bells are buds which have been produced by the sea- 

 firs of the pools. From their sedentary brothers they differ in 

 that they reproduce themselves by eggs, and not by budding 

 like the former. They float seawards with their load of minute 

 eggs, and being carried by the tidal stream as it sweeps along 

 the coast, they serve as a means of distributing the sea-firs. Each 

 little egg, if it be fertilised, is capable of producing not a swimming- 

 bell like its parent, but a sea-fir like its grandfather. To this 

 very curious phenomenon of alternative inheritance we give the 

 name of alternation of generations ; it is known among plants 

 as well as among animals. 



In speaking of gasteropod molluscs, we noted that while 

 some produce enormous numbers of delicate, free-swimming 

 young, others produce relatively few young of greater complexity 

 and more sedentary habit. If the whole reason for the difference 

 is still unknown, we saw that one clue may be found in the fearful 



