74 THE AMERICAN MONTHLY [April, 



the method of microchemical reactions ; of Leydolt, Rose, Haushofer, and 

 Baumhauer, on the etched figures on crystal planes ; of Thoulet, Goldsmidt, 

 Klein, and others on the separation of mineral components by means of heavy 

 solutions : all these are the offspring of microscopic petrography, and represent 

 some of the methods of which the modern petrographer makes use to confirm 

 his optical study of minerals. 



Biology of fresh-water Sponges.— II.* 



By EDWARD POTTS, 



PHILADELPHIA, PA. 



My own first experience in the propagation of fresh-water sponges may 

 prove instructive in various ways. Late in the autumn of the year 1879, in 

 a pond within the ' Centennial Grounds,' Philadelphia, I found for the first 

 time a living sponge. It was a vigorous, branching specimen of Spongilla 

 lacustris. charged with gemmules in all parts of its structure.' A fragment 

 firmly attached to a stone was taken home and placed in a gallon ' specie- 

 jar' with water, in the hope, begotten of inexperience, that it would continue 

 to grow, exhibit its inflowing and exhalent currents, etc. On the contrary, 

 and as I now know, almost necessarily, it died, and in a few days the water 

 became insupportably foul. It was changed and another trial made, which 

 resulted as before. This time the jar was thoroughly cleansed ; the stone 

 with the attached sponge was taken out and held long under a flowing hy- 

 drant before it was replaced in the jar, which was now left in an outer shed 

 and, very naturally, forgotten. Weeks passed and winter came on, and one 

 severe night the water in my jar was frozen solid and the vessel fractured. I 

 supposed that the low temperature to which it had been subjected would 

 pro\e fatal to the germs, but, as the specimen was a fine one, it seemed well 

 to save it. even in its skeletonized condition. So, when its icy envelope had 

 been melted off', the sponge was again thoroughly washed until all the sar- 

 code was removed, when, in a fresh jar, it again became a parlor specimen, 



I do not clearly remember when signs of germination were first observed. 

 It was probably in January, as during that month I find that artificial con- 

 ditions veiy frequently bring about the hatching of such animal germs as 

 those of the polyzoa, etc. I detected first a filmy, grayish-white growth 

 that seemed associated with the detached gemmules which lay in the groove 

 around the bottom of the jar. A grav, featureless growth at first, — then 

 spicules were seen, in slightly fasciculated lines, attached to the glass and 

 reaching upward, then spreading out fan-like and branching. These were, 

 of course, covered with sarcode, nearly transparent at first, and through the 

 filmy surface pores and ostoles could be detected with a pocket lens. The 

 latter was surmounted by the so-called ' chimneys ' or cone-shaped exten- 

 sions of the dermal film ; and through the apertures at their summits effete 

 particles could almost constantly be seen, puffed out, as if thrown from a 

 volcano and then blown oft' by the wind. 



These products of single gemmules did not, as time passed on, greatly in- 

 crease in size ; possibly, because of deficient nutriment in the unchanged 

 water of the jar ; but, crawling upward along the glass to an average height 

 of an inch or less, left the naked spicules in place behind them as so many 

 ladders or ' stepping-stones of their dead selves ' by which they had reached 

 to ' higher things.' Near the summit, one or more new gemmules would 

 sometimes be formed, after which the mother mass entirely disappeared. 



* Reprint of the introduction to a monograph of the fresh-water sponges read before the Philadelphia 

 Academy, May 31, '87. 



