INTRODUCTION. 



XV 



enclosed in horny or calcareous cases, yet they 

 do not grow indiscriminately in any situation, 

 but exercise a faculty of selection. This is very 

 different from the selection of soils by plants, 

 which imbibe their nourishment through their 

 roots ; for though rooted, all nourishment in 

 these creatures is taken through the polype 

 mouths. Among the Hydroida the Sea Beard, 

 ( Antennularia antennina ) prefers oysters and 

 other bivalves, but is also found on stones and 

 sand ; the form of the whole varying according 

 to the locality. The Sea Oak, ( Sertularia 

 pumila) prefers the fronds of fuci growing near 

 the tide marks, or the declivities of rocks in 

 sheltered situations; the Sea Threads, the fronds 

 of the larger fuci and the margins of pools, 

 but they have been found abundantly on the 

 fins of a shark ; the Sea Bristles, dead muscle 

 shells and horny corallines. This is the case 

 also with the Sea Anemonies, but as they are 

 naked and locomotive it is not so much a 

 matter of surprise. Some species prefer one 

 locality, and others another ; some the smaller 

 fuci, and others the larger; but the situations 

 sometimes selected are exceedingly curious ; 

 thus the small climbing Coralline (Campanularia 

 volubulis,) has a prediliction for the antennae 

 of crabs, where it can enjoy the advantages of 

 locomotion in catching its prey. 



This power of selection is also to be found in 

 the Ascidian Zoophytes ; thus the Flustra lineata 

 prefers flat stones between tide marks, while 

 the Membranipora Peachii most commonly se- 

 lects a dead muscle valve and sometimes the 

 dead oyster and great Pinna; the purple Tubuli- 

 pora ( T. Serpens) prefers corallines, while the 

 Sea Mat ( Flustra membranacea ) always encrusts 

 the frond of the great sea weed ( Lamiuaria digi- 



