345 



inhabited by true reef-bnilders. In all seas, liowever, which 

 are subject to freezing, or are regularly invaded by floating- 

 ice, reef-building corals cannot thi'ive, and hence the occur- 

 rence of modern or ancient coral reefs is a reliable indication 

 of a minimum winter temperature above freezing. 



The reproduction of the Anthozoa is both asexual and 

 sexual. The asexual method is carried on by fission and 

 budding, the new-formed corallites commonl^^ remaining 

 united with their parents, thus producing colonial forms. 

 In some cases, however, the buds will become free and begin 

 an independent life. ( Fungia, Balanophyllia, etc.) New 

 colonies, however, are chiefly begun by sexually generated 

 individuals. Fi-om the fertilized ei^g develops a mero- 

 planktonic ciliated embryo, not unlike in appearance to 

 the planula of the Hydrozoa. After attachment, this 

 develops into the polyp, which early begins to secrete its 

 horny or calcareous corallum. 



Crinoiden . — The crinoids are without exception marine or- 

 ganisms, though Antedon rosacea has l)een taken in water 

 containing only 2.5 per cent, of salts, or nearly a third less 

 than in normal sea water. The majority of ci"inoids belong- 

 to the sedentary benthos, being anchored or attached to the 

 sea bottom either by a stem or by the base of the calyx. 

 Antedon must be classed with the vagrant benthos, for 

 although it ordinarily rests on the sea bottom or other sta- 

 tionary objects of support, it is able to walk about on the 

 bottom b}^ means of its arms ; and also to swim with grace- 

 ful movements through the w^ater. Planktonic crinoids ap- 

 pear to have existed in the Mesozoic seas (Saccocoma, 

 Uintacrinus). and, as already noted, Walther has furnished 

 evidence which indicates that some of the stemmed Penta- 

 crini of the Lias lead a pseudo-planktonic life, growing at- 

 tached to floating timbers with which they were carried 

 about, calyx downward. The bathymetrical distribution of 

 the modern crinoids ranges from shallow water to 2,000 

 fathoms, rarely more. One species of Antedon (.4. ahyssicolR 

 Carp.) has been obtained at a depth of 2,900 fathoms, but 



