THE SPONGES (PORIFERA) 303 



when spring comes or when, in dry regions, the occasional floods 

 reach them, the gemmules in the pavement layers are supposed to 

 germinate where they were deposited; the floating kinds, set adrift, 

 lodge upon any suitable surface and begin their seasonal growth. 



Each gemmule is provided with a foramen, or a foraminal aperture, 

 sometimes plain, but often more or less tubular, through which 

 the growing cells usually escape by amoeboid action and appear 

 as a delicate creamy film surrounding the gemmule. Sometimes, 

 however, they escape by the rupture of the whole gemmule. Where 

 this is part of a pavement-layer or one of a group of detached gem- 

 mules the escaping currents flow together as a filmy mass, sometimes 

 rounded up like a small pea, otherwise as a spreading film or like 

 the wandering trail of slime left by one of the larger snails. The 

 appearance of the young spicules is nearly coincident with the 

 escape of the cells and they at once begin to arrange themselves 

 according to the habit of the species, forming a network over the 

 supporting surface, upon which is built a superstructure suggesting 

 that of our modern steel-framed buildings. Special interspaces 

 become the chambers lined with the collared flagellate cells already 

 mentioned. The action of these flagella creates currents of water 

 flowing in through myriads of almost invisible pores in the cover- 

 ing, bearing food particles to nourish the growing sponge and then 

 carrying off and discharging useless matter through the larger canals 

 by the efferent osteoles already mentioned. 



The study of fresh-water sponges should begin here and follow 

 the cycle of growth from gemmule to gemmule, watching, if it be 

 possible, even the development within their own especial cells of 

 the various classes of spicules, observing in the autumn the gradual 

 gathering together of the germ cells before they are shrouded in 

 chitin or committed to the waters within their floating crusts. 

 Under favorable conditions and constant as well as careful control 

 much of this work may be made independent of the seasons, after 

 germinating the fresh gemmules in shallow glass dishes at home, 

 and in a small way afford excellent opportunity for study; but it 

 will not be found practicable to grow sponges in aquaria excepting as 

 small fragments in very large bodies of water or in vessels in which 

 the water is constantly or frequently renewed. 



