162 PEOCEEDINGS OF THE ACADEMY OF [1887. 



are almost always much smaller than those of the skeleton and are 

 never fasciculated or bound together in any way. A third class of 

 spicules is composed of those before mentioned as imbedded in the 

 "crust" of the gemmules, and form what may be regarded as their 

 armor or defensive coating. These gemmule-spicules represent two 

 principal and several subordinate types, which have been selected 

 by Mr. Carter to define the different genera into which he has divi- 

 ded the single genus Spongilla of the earlier authors. His metliod 

 of classification will be given later. 



The sponge in its entirety as a growing organism can generally 

 be easily recognized by the collector, after he has escaped from the 

 thraldom of the idea that any fixed growth, of a more or less vivid 

 green color, must be a plant of some kind. Of course the mosses 

 and confervse will be rejected after examination, upon the evidence 

 given by the leaves of the one and the smooth slender threads of the 

 other. If doubts remain as to any specimen, the presence in it of 

 efl^erent or discharging apertures, like those of the commercial sponge, 

 if it is really a sponge, may serve to dispel them, and still more con- 

 vincing proof will be given by the use of a pocket lens, in detecting 

 the points of multitudinous spicules thickly studding the surface. 

 When, in addition to these guiding features, the spherical gemmules 

 just described are found within or under it, there should be no fur- 

 ther hesitation. 



The green color spoken of, is common and characteristic ; yet it 

 is not universal, but closely dependent upon the quantity or quality 

 of the light received. When a sponge has germinated away from 

 the light and has grown upon the lower side of a plank or stone, it will 

 be found nearly white, gray or cream colored. As it enlarges and 

 creeps around the edge and up into the full sun light it assumes a 

 delicate shade of green, deepening as the exposure increases, till it 

 attains a bright vegetable hue. Even in the sunlight, however, 

 some species are never green. (See description of Meyenia leidyi.} 



These organisms have occasionally been discovered growing in 

 water unfit for domestic uses ; but as a rule they prefer pure water, 

 and in my experience the finest specimens have always been found 

 where they were subjected to the most rapid currents. The lower 

 side of large, loose stones at the "riffs" or shallow places in stre'ams : 

 the rocks amid the foaming water at the foot of a mill-dam fall ; the 

 timbers of a sluice-way, the casing of a turbine waterwheel, or 

 the walls of a "tail race" beneath an old mill; — in all these places 



