108 THE ANATOMY OF INVERTEBRATED ANIMALS. 



craters. Dilatations of the canals occur at intervals, and are 

 lined by the characteristic monadiform endodermic cells, 

 which are restricted to the walls of these ciliated chambers. 

 It is by the working of the cilia of these cells that currents 

 of water are made continually to enter by the inhalent pores 

 and to pass out by the exhalent craters. The whole fabric 

 is supported and strengthened by a skeleton, which consists, 

 in the first place, of bands and filaments of keratose, and, 

 secondly, of silicious spicula, the majority of which resemble 

 needles pointed at each end, and contain a fine central canal 

 filled with an unsilicified substance. The individuality of 

 these animals is so little marked that two Sjjongillce, when 

 brought into contact, before long fuse into one; while they may 

 divide spontaneously, or be separated artificially into different 

 portions each of which will maintain an independent existence. 



A process analogous to the formation of cysts, which is so 

 common among the Protozoa, takes place in the deeper sub- 

 stance of the body, especially in the autumn. A number of 

 adjacent sponge-corpuscles, losing their granular appearance, 

 become filled with clear, strongly refracting granules, the nu- 

 cleus ceasing to be visible. The sponge-corpuscles which 

 surround these become closely applied together, and secrete 

 coats of keratose, which fuse with those of the adjacent cor- 

 puscles. In the interior of each a singular silicious spiculum 

 is formed, consisting of two toothed disks, like cogged wheels, 

 united by an axis. As this " amphidiscus " enlarges, the proto- 

 plasm of the corpuscle disappears, and at length nothing is left 

 but the envelope of keratose, with the imbedded amphidisks, 

 disposed perpendicularly to its surface. At one point of the 

 spheroidal envelope a small opening is left, and the so-called 

 " seed " of the Spongllla is complete. It remains throughout 

 the winter unchanged ; but, with the return of warmth, the 

 sponge-corpuscles inclosed within the coat of the " seed," or 

 more properly cyst, slowly escape through the pore, become 

 perforated with inhalent and exhalent apertures and canals, and 

 develop the characteristic spicula of a young Spongilla. 



This process of encystment, which may be regarded as a 

 kind of budding, akin to propagation by bulbs among plants, 

 has not been observed among marine sponges. 



Sexual propagation takes place in the same way as in the 

 Calcispongiw, and the embryo passes through morula and 

 planula stages. But the ciliated cells which form the outer 

 wall of the latter, and constitute its locomotive apparatus, 

 seem to vanish when the embryo fixes itself, and the body of 



