90 MEMOIR V. 



and often raise themselves to its very jsurface, is scarcely to 

 be conceived by those who have not visited the warmer 

 regions of the globe, where they appear at present to be in 

 most active operation, at times extending themselves in an 

 almost continuous sheet over the midulations and anfrae- 

 tuosities of the bottom of the sea for many leagues, and 

 hundreds of leagues, forming submarine meadows and 

 thickets of vast extent, that yield protection and cover to 

 myriads of animals which lurk between their folds and 

 contortions, or sport amongst their branch-like forms. — 

 In texture some are fleshy, others fibrous, horny, or appa- 

 rently lapidescent, and in form more various and dissimilar 

 than the Lichens, these being the vegetables to which they 

 most approximate in this respect ; indeed until the beginning 

 of tlie Eighteenth Century they were generally considered 

 by Naturalists as Plants, yet Gesner in 1565, Imperati 

 in 1599, Bocconc in 1674, Shaw in 1638 and 1646, 

 and some other Naturalists, had observed and described 

 the animal inhabitant of several. To Peysonnel however 

 must be given the chief merit of having drawn the attention 

 of Naturalists to their real nature in his Memoirs presented 

 to the Royal Academy of Sciences in 1727? and to the Royal 

 Society of London and published in their Transactions for 

 the year 1752 — in these memoirs he has described the 

 animals of many Madrepores, Millepores, Gorgonias &c. 

 To these interesting discoveries of Peysonilel, followed the 

 no less important one of the naked Polypi by Trembley [See 

 Trans. Roy. Soc. Vol 42, 43 and 44.] These were subse- 

 quently named Hydrai by Linnaeus and have been considered 

 as the animal inhabitant (under certain modifications) of 

 the greater part of the Zoophites. In order to be enabled 

 to appreciate tlie value of this opinion it will be necessary 

 to understand tlic structure of these Hydrse, which are 

 eminently simple, soft, dilatable, and extensile, furnished 

 with but a single central moutli or opening above, thrtuigh 



