128 ANATOMY AND PHYSIOLOGY 



THE CILIA AND CILIARY ACTION. 



Our knowledge of the cilia of the Spongiadae is, compa- 

 ratively speaking, very small. Dr. Grant is, I believe, the 

 first author who has seen and described these organs in situ. 

 This learned and accurate observer, in his paper ' Observa- 

 tions on the Structure and Functions of the Sponge,' has 

 described the origin and gradual development of the ova or 

 gemmules of Spongia panicea {Halichondria incrustans, 

 Johnston). After the liberation of these bodies from the 

 sponge, he writes, " The most remarkable appearance exhi- 

 bited by these ova, is their continuing to swim about by 

 their own spontaneous motions for two or three days after 

 their detachment from the parent, when they are placed 

 separately in vessels of sea-water, at perfect rest. During 

 their progressive motions they always carry their rounded 

 broad extremity forward, and when we examine them under 

 a powerful microscope, we perceive that these motions are 

 produced by the rapid vibration of cilia, which completely 

 cover over the anterior two thirds of their surface." And 

 he further states that they are "longest and exhibit the 

 most distinct motions on the anterior part," and that they 

 " are very minute transparent filaments, broadest at their 

 base, and tapering to invisible points at their free extre- 

 mities; they have no perceptible order of succession in 

 their motions, nor are they synchronous, but strike the 

 water by constantly and rapidly extending and inflecting 

 themselves." The author describes the attachment and 

 spreading out into a thin disk of the ovum or gemmule, 

 and the cessation of action and gradual disappearance of the 

 cilia ; and he further observes, " although all visible cilia 

 have ceased to move, we still perceive a clear space round 

 the ovum, and a halo of accumulated sediment at a little 

 distance from the margin." This observation is important, 

 as tending to prove the existence of ciliary action, although 

 the organs themselves were too minute to be detected. 



Dujardin, in his work on the Infusoria, in Plate III, 19, b, 



