14 



THE II^ORGANIC CONSTITUENTS OF MARINE INVERTEBRATES. 



would be raised. Variations are also to be expected because of cold or warm currents and 

 different depths of water. Very deep water, even under the Equator, is always cold, whereas 

 shallow bays farther north may be relatively warm. Possibly, also, the alcyonarians may form 

 two or more distinct series that are not perfectly comparable in chemical composition. Corallium 

 and Tubipora, for example, are compact forms, with little organic matter and lower magnesia 

 than the genera with horny, organic axes, such as those whose names appear at the end of the 

 table. It is also noteworthy that the highest proportions of calcium phosphate are commonly 

 associated with a high content of magnesia. 



HYDROIDS. 



In the course of this investigation six analyses have been made of coralline hydroids belong- 

 ing to the genera Millepora and Distichopora. The species and localities are as follows: 



1. Millepora alcicomis Linn^. Shoal water, Tortugas, Fla. 



2. Millepora alcicomis. Bermuda. 



3. Millepora braziliensis Verrill. Candeas, Pernambuco, Brazil. 



4. Distichopora nitida Verrill. Micronesia, exact locality unknown. 



5. Distichopora coccinea Gray. South Sea Islands, exact locality unknown. 



6. Distichopora sulcata Pourtales. Off Habana, Cuba; depth of water, 143 to 179 meters. 



Analyses of hydroids. 



Reduced analyses of hydroids. 



SiOj 



(Al,Fe)A- 

 MgCOj.... 

 CaCOj .... 

 CaSO,. 



Ca,P,08 Trace. 



100.00 100.00 100.00 100.00 100.00 100.00 



A few partial analyses and one fairly complete analysis of millepores have already been 

 published, which in all important particulars agree with ours. The coralline structures consist 

 essentially of calcium carbonate, with minor impurities, and resemble chemically the true 

 corals. In two millepores from Bermuda A. G. Hogbom '* found respectively 95.86 and 94.39 

 per cent of calcium carbonate, -with 0.41 and 0.97 of magnesium carbonate. In Millepora 

 alcicomis from the Gulf Stream S. P. Sharpies '^ found 97.45 per cent of calcium carbonate, 0.27 

 of calcium phosphate, 2.54 of organic matter and water, and only traces of iron and magnesia. 

 B. Silliman, jr.,'" in one analysis of the same species reported no magnesia, which evidently 



" Hogbom, A. G., Neues Jahrb., 1894, Band 1, p. 262. 



'» Sharpies, S. P., .\ni. Jour. Sci., 3d ser., vol. 1, p. 168, 1871. 



' eilllman, B., jr.. Am. Jour. Sci., 2d ser., vol. 1, p. 189, 1846. 



