44 THE AMERICAN MONTHLY [March, 



bound together by an almost invisible quantity of firmer sarcode or perhaps 

 of colloidal silica. 



To form the main lines of this skeleton structure the spicules, averaging 

 about y^iyth of an inch in length, are fasciculated in bands made up of 

 several spicules, lying side by side, and somewhat overlapping at their ex- 

 tremities ; the crossing lines being formed of more slender fascicles, or even 

 of single spicules. In the different species these ' skeleton ' spicules vary 

 in size, in the shape of their terminations, and in their more or less spinous 

 character ; but w^hile these differences serve, in some degree, as specific 

 guides, they are not sufficiently constant or positive to form a basis for generic 

 arrangement. 



Besides the skeleton spicules, a second class, knov\'n as ' dermal ' or flesh 

 spicules is found only in some species And in greater or less numbers, either 

 lying upon the outer ' dermal ' film or lining the canals in the deeper por- 

 tions of the sponge. They 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 embedded 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 divided the single genus Spong-illa of 

 the earlier- authors. His method 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 confervte 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 efferent or discharging apertures, like those of the com- 

 mercial sponge, if it is really a sponge, ma}' serve to dispel them, and still 

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

 the points of multitudinous spicules thickh' studding the surface. When, in 

 addition to these guiding features, the spherical gemmules just described are 

 found v^nthin or under it, there should be no further hesitation. 



The green color spoken of is coinmon and characteristic ; yet it is not 

 universal, but closely dependent upon the quantity or quality of the light re- 

 ceived. 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 ci'eeps around the edge and up into 

 the full sunlight it assumes a delicate shade of green, deepening as the ex- 

 posure increases, till it attains a bright vegetable hue. Even in the sunlight, 

 however, some species are never ^reen. 



These organisms hav^e 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 ' rifts' 

 or shallow places in streams ; 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 ' tailrace ' beneath an old mill; — in all these 

 places they have been found in great abundance and of a very lusty growth. 

 Of all discouraging situations it is almost hopeless to look for them in shallow 

 water having a mud bottom. Mud is their great enemy, as gravity aids their 

 natural currents to fill the cavities with earthy matters that soon suffocate 

 them, because the latter are too feeble to throw them oft'. Of course in any 

 body of water liable to be charged with sedimentary material, the principle 



