36 THE AQUARIAN NATURALIST. 



almost imperceptible dimensions. The word cilia, 

 whereby these miraculous organs are now universally 

 designated, is, as we need scarcely say, derived from 

 their resemblance to rows of eye-lashes (cilium, an 

 eye-lash) ; but the most rapid wink will not express 

 the quickness of their motion. 



The gemmules of the sponge are somewhat egg- 

 shaped (PL I. fig. 1, b), and cilia cover every part of 

 their surface, excepting their posterior tapering extre- 

 mity. In swimming, they always carry their broadest 

 end foremost. They have a granular structure, and 

 in some species minute spicula, resembling those that 

 constitute the framework of the adult sponge, are 

 distinctly discernible at the time of their expulsion. 

 They do not change their form while swimming, like 

 those of many other zoophytes, but glide along with a 

 regular smooth motion. After remaining some time 

 in the water, they generally come to the surface, and 

 collect round the margin of the vessel. These gem- 

 mules often continue to swim about, by their own 

 spontaneous movements, for two or three days after 

 their detachment from their parent, as though in 

 search of a suitable locality whereon to rest ; at length 

 they are to be observed beginning to fix themselves on 

 the sides and bottom of the tank, and some of them 

 are found spread out like a transparent membrane on 

 the surface of the water. When examined through 

 a microscope, while in the act of attachment to the 

 glass, each gemmule may be seen to extend itself 

 into a thin circular film ; still the cilia may be ob- 

 served in rapid motion on the upper part, propelling 

 the floating particles in their neighbourhood to a 



