HYDRA. 351 



inhabit the shallower and even deeper parts of all seas ; 

 Alcyonidce and Gorgonidce are found at considerable 

 depths; Oorallium and Gorgonia abound both in cold 

 and warm latitudes. The Perforata and Tabulata almost 

 exclusively haunt the shallower parts of warm seas, but 

 the Turbinolidce extend into very cold regions. The whole 

 group of the JKugosa is now extinct, only one genus, 

 Holocystis, having survived even the palaeozoic period. Not 

 only heat, but light, and probably rapid and effectual 

 seration, are essential conditions for the activity of the 

 reef- building Actinozoa. Different species of corals exhibit 

 great differences as to the rapidity of growth, and the 

 depth at which they flourish best ; and no one must be 

 taken as evidence for another in these respects. Certain 

 species of Perforata, Madreporidce, and Poritidce, appear 

 to be at once the fastest growers, and those which delight 

 in the shallowest waters. The Astrceidce among the 

 Aporosa, and Seriatopora among the Tabulata, live at 

 greater depths, and are probably slower of increase. The 

 most careful accounts of the structure of corals extant, are 

 from the pens of M. M. Milne Edwards and Haime, pub- 

 lished at various times in the Annales des Sciences Natu- 

 relles, in the Memoires du Museum, and in the publications 

 of the Palseontographical Society ; and by Mr. Darwin, 

 whose beautiful work on Coral Reefs will amply repay 

 a careful perusal. 



With these brief introductory observations, the principal 

 part of which have been derived from Professor Huxley's 

 lectures, we proceed to direct the reader's attention to 

 some of the more interesting generic forms. 



HYDRA, FRESH- WATER POLYPE. In polypes of this 

 family the body generally consists of a homogeneous 

 aggregation of vesicular granules, held together by a sort 

 of glairy intercellular substance, and capable of great 

 extension and contraction; so that the creature can at 

 pleasure assume a great variety of forms, extending its 

 body and tentacles until the latter become so fine as to be 

 almost invisible, and again retracting itself until it acquires 

 the appearance of a small gelatinous mass. The tentacles 

 which surround the anterior extremity are irregular in 

 number; they are capable of extension to a very great 



